General characteristics, classification and identification of yeasts, molds

Unit I General characteristics, classification and identification of yeasts, molds and group of bacteria important in food industry, sources of contaminations: air, water, sewage, post processing contamination. Factors influencing growth of microorganism on foods, Intrinsic factors and Extrinsic factors

1. Dr. Sujeet Kumar Mrityunjay, PhD
Assistant Professor
Department of Life Science
School of Sciences
ITM University,Gwalior (Turari Campus)
Madhya Pradesh-474001 (India)
UnitIGeneralcharacteristics,classificationand
identificationofyeasts,moldsandgroupofbacteria
importantinfoodindustry,sourcesofcontaminations:
air,water,sewage,postprocessingcontamination.
Factorsinfluencinggrowthofmicroorganismonfoods,
IntrinsicfactorsandExtrinsicfactors

2. FoodMicrobiology Page 1 of28
UnitIGeneralcharacteristics,classificationandidentificationofyeasts,moldsandgroupof
bacteriaimportantinfoodindustry,sourcesofcontaminations:air,water,sewage,post
processingcontamination.Factorsinfluencinggrowthofmicroorganismonfoods,Intrinsic
factorsandExtrinsicfactors.
FoodMicrobiology:-Foodmicrobiologycomprehendsthestudyofmicroorganismsthatcolonise,
modify,andprocess,orcontaminateandspoilfood.Itisoneofthemostdiverseresearchareas
withinmicrobiology.Itcomprisesawidevarietyofmicroorganismsincludingspoilage,probiotic,
fermentative,andpathogenicbacteria,moulds,yeasts,viruses,prions,andparasites.Itdealswith
foodsandbeveragesofdiversecomposition,combiningabroadspectrumofenvironmentalfactors,
whichmayinfluencemicrobialsurvivalandgrowth.Foodmicrobiologyincludesmicroorganisms
thathavebeneficialordeleteriouseffectsonfoodqualityandsafetyandmaythereforebeofconcern
topublichealth.
YeastareSingle-celled,butwithCellularOrganizationSimilartoHigherOrganisms
YEASTS:-Yeastsaregenerallyunicellularanddifferfrombacteriaintheirlargecellsizeand
morphology,andbecausetheyproducebudsduringtheprocessofreproductionbydivisionor
budding.Yeastscanbespreadthroughtheair,orothermeans,andalightonthesurfaceoffoodstuffs.
Yeastcoloniesaregenerallymoistorslimyinappearanceandcreamywhitecolored.Yeastspreferan
Awof0.90-0.94,butcangrowbelow0.90.Thesemicro-organismsgrowbestintheintermediate
acidrange,pHfrom4.0to4.5.Foodthatishighlycontaminatedwithyeastswillfrequentlyhavea
slightlyfruityodour.Yeastsmaybeusefulorharmfulinfoods.Yeastfermentationsareinvolvedin
themanufactureoffoodssuchasbread,beer,wines,vinegar,andsurfaceripenedcheese,andyeasts
aregrownforenzymesandforfood.Yeastsareundesirablewhentheycausespoilageofsauerkraut,
fruitjuices,syrups;molasses,honey,jellies,meats,wine,beer,andotherfoods.Therearehundreds
ofeconomicallyimportantvarietiesofascomyceteyeasts;thetypescommonlyusedinthe
productionofbread,beer,andwineareselectedstrainsofSaccharomycescerevisiae.Someyeasts
aremildtodangerouspathogensofhumansandotheranimals,especiallyCandidaalbicans,
Histoplasma,andBlastomyces.
GENERALCHARACTERISTICS OFYEASTS:-Yeastsareclassifiedprimarilyontheir
morphologicalcharacteristics,whiletheirphysiologicalonesaremoreimportanttothefood
microbiologist.
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3. FoodMicrobiology Page 2 of28
Saccharomycescerevisiae,atypeofbuddingyeast,isabletofermentsugarinto
carbondioxideandalcoholandiscommonlyusedinthebakingandbrewing
industries.
MorphologicalCharacteristics:-Themorphologicalcharacteristicsofyeastsaredeterminedby
microscopicexamination.
FormandStructure:-Theformofyeastsmaybesphericaltoovoid,lemonshaped,pear-shaped,
cylindrical,triangular,orevenelongatedintoafalseortruemycelium.Theyalsodifferinsize.
Visiblepartsofthestructurearethecellwall,cytoplasm,watervacuoles,fatglobules,andgranules,
whichmaybemetachromatic,albuminous,orStarchy.Specialstainingisnecessarytodemonstrate
thenucleus.
Budding yeastcells
Reproduction:-Yeastsreproducebybudding(asexualreproduction),whenasmallbudformsand
splitstoformanewdaughtercell,butunderstressconditionstheycanproducespores(aformof
sexualreproduction).ThebreadyeastSaccharomycescerevisiaeusesthesugarsintheflourto
produceenergy,releasingthealcoholethanol(whichevaporates)andbubblesofthegascarbon
dioxide,whichmakesthebreaddoughrise.Thebreadyeastisalsousedtomakesometypesofbeer;
inthiscasetheyeastusesthesugarsfromcerealslikebarley,toproduceethanolandcarbondioxide.
Thebreadyeasthasbeenwidelyusedbyscientiststostudyimportantcellularprocesses.
Mostyeastsreproduceasexuallybymultilateralorpolarbudding,aprocessinwhichsomeof
theprotoplasmbulgesoutthecellwall;thebulgegrowsinsizeandfinallywallsoffasanewyeast
cell.Insomeyeasts,notablysomeofthefilmyeasts,thebudappearstogrowfromatubelike
projectionfromthemothercell.Replicatednuclearmaterialisdividedbetweenthemotherand
daughtercells.Afewspeciesofyeastsreproducebyfission,andonereproducesbyacombinationof
fissionandbudding.Sexualreproductionof"true"yeasts(Ascomycotina)resultsintheproduction
ofascospores,theyeastcellservingastheascus.Theformationofascosporesfollowsconjugationof
twocellsinmostspeciesoftrueyeasts,butsomemayproduceascosporeswithoutconjugation,
followedbyconjugationofascosporesorsmalldaughtercells.Theusualnumberofsporesperascus
andtheappearanceoftheascosporesarecharacteristicofthekindofyeast.Theascosporesmay
differincolor,insmoothnessorroughnessoftheirwalls,andintheirshape(round,oval,reniform,
Dr. Sujeet KumarMrityunjay

4. FoodMicrobiology Page 3 of28
bean-orsickle-shaped,Saturn-orhat~shaped,hemispherical,angular,fusiform,orneedle-shaped).
"False"yeasts,whichproducenoascosporesorothersexualspores,belongtotheFungiImperfecti.
Cellsofsomeyeastsbecomechlamydosporesbyformationofathickwallaboutthecell,for
example,Candida,Rhodotorula,andCryptococcus.
Trueyeasts:-Theseareunicellularroundorovalfungi.Reproductionisbybuddingfromtheparent
cell.Culturesinvitrocharacteristicallyshow'creamy'colonies,e.g.Cryptococcusneoformans.
Yeast-like/"False"fungi:-Thesearelikeyeastssincetheymayappearasroundorovalcellsand
growbybudding.Theymayalsoformlongnon-branchingfilamentsknownas'pseudohyphae',e.g.
Candidaalbicans.
CulturalCharacteristics:-Growthasafilmonthesurfaceofliquidmediasuggestsanoxidativeor
filmyeast,andproductionofacarotenoidpigmentindicatesthegenusRhodotorula.Theappearance
ofthegrowthisimportantwhenitcausescoloredspotsonfoods.Itisdifficulttotellyeastcolonies
frombacterialonesonagarplates;theonlycertainwayisbymeansofmicroscopicexaminationof
theorganisms.Mostyoungyeastcoloniesaremoistandsomewhatslimybutmayappearmealy;
mostcoloniesarewhitish,butsomearecream-coloredorpink.Somecolonieschangelittlewithage,
butothersbecomedryandwrinkled.Yeastsareoxidative,fermentative,orboth.Theoxidativeyeasts
maygrowasafilm,pellicle,orscumonthesurfaceofaliquidandthenaretermedfilmyeasts.
Fermentativeyeastsusuallygrowthroughouttheliquidandproducecarbondioxide.
Dr. Sujeet KumarMrityunjay

5. Dr. Sujeet KumarMrityunjay
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PhysiologicalCharacteristics:-Mostcommonyeastsgrowbestwithaplentifulsupplyofavailable
moisture.Butsincemanyyeastsgrowinthepresenceofgreaterconcentrationsofsolutes(suchas
sugarorsalt)thanmostbacteria.Mostyeastrequiresmoremoisturethanmolds,however.Onthe
basisofwateractivityoraw,yeastsmaybeclassifiedasordinaryiftheydonotgrowinhigh
concentrationsofsolutes,i.e.,inalowaw,andasOsmophiliciftheydo.Lowerlimitsofawfor
ordinaryyeastsrangefrom0.88to0.94.Osmophilicyeastshavebeenfoundgrowingslowlyin
mediawithawaslowas0.62to0.65insyrups,althoughsomeOsmophilicyeastsarestoppedat
about0.78inbothsaltbrineandsugarsyrup.Theawvalueswillvarywiththenutritivepropertiesof
thesubstrate,pH,temperature,availabilityofoxygen,andpresenceorabsenceofinhibitory
substances.Therangeoftemperatureforgrowthofmostyeastsis25ºCto30ºCandthemaximum
about35to47ºC.Somekindscangrowat0ºCorless.Thegrowthofmostyeastsisfavoredbyan
acidreactioninthevicinityofpH4to4.5,andtheywillnotgrowwellinanalkalinemediumunless
adaptedtoit.Yeastsgrowbestunderaerobicconditions,butthefermentativetypescangrow
anaerobically,althoughslowly.Yeastsmaychangeintheirphysiologicalcharacteristics,especially
thetrue,orascospore-forming,yeasts,whichhaveasexualmethodofreproduction.Theseyeastscan
bebredforcertaincharacteristicsormaymutatetonewforms.Mostyeastscanbeadaptedto
conditionswhichpreviouslywouldnotsupportgoodgrowth.Illustrativeofdifferentcharacteristics
withinaspeciesisthelargenumberofstrainsofSaccharomycescerevisiaesuitedtodifferentuses,
e.g.,breadstrains,beerstrains,winestrains,andhigh-alcohol-producingstrainsorvarieties.
CLASSIFICATIONANDIDENTIFICATIONOFYEASTS:-Yeasttaxonomybeganin1837
withMeyer'sassigningofthegenusSaccharomyces(fromtheGreeksakehar,meaningsugarand
mykes,meaningfungus)toyeast.ThetrueyeastsareinthesubdivisionAscomycotina,andthefalse,
orasporogennous,yeastsareinthesubdivisionFungiImperfectiorDeuteromycotina.Certain
yeastsareactuallyrepresentedintwodifferentgenerabasedonwhethertheyreproducesexually.
Theprincipalbasesfortheidentificationandclassificationofgeneraofyeastsareasfollows:-
1.Whetherascosporesareformed.
2.Iftheyarespore-forming:-
A.Themethodofproductionofascospores:-
i.Producedwithoutconjugationofyeast,cells(parthenogenetically).Spore
formationmaybefollowedby:-(a)Conjugationofascospores.(b)Conjugation
ofsmalldaughtercells.
ii.Producedafterisogamicconjugation(conjugatingcellsappearsimilar).

6. Dr. Sujeet KumarMrityunjay
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iii.Produced by heterogamic conjugation (conjugating cells differ inappearance).
B.Appearance of ascospores: shape, size, and color. Most spores are spheroidal or ovoid,
but some have odd shapes, e.g., most species of Hansenula, which look like derbyhats.
C.The usual number of ascospores per ascus: one, two, four, oreight.
2.Appearance of vegetative cells: shape, size, color,inclusions.
3.Method of asexual reproduction:-
i.Budding.
ii.Fission.
iii.Combined budding andfission.
iv.Arthrospores (oidia).
4.Production of a mycelium, pseudo mycelium, or nomycelium.
5.Growth as a film over surface of a liquid (film yeasts) or growth throughoutmedium.
6.Color of macroscopicgrowth.
7.Physiological characteristics (used primarily to differentiate species or strains within a
species):-
i.Nitrogen and carbonsources.
ii.Vitaminrequirements.
iii.Oxidative or fermentative: film yeasts are oxidative; other yeasts may be fermentative
or fermentative andoxidative.
iv.Lipolysis; urease activity, acid production, or formation of starch likecompounds.
GENERALCHARACTERISTICS, CLASSIFICATIONANDIDENTIFICATIONOFMOLDS
Molds:-Moldsaremulticellularmicro-organismswithmycelial(filamentous)morphology.These
microbesarealsocharacterizedbytheirdisplayofavarietyofcolorsandaregenerallyrecognized
bytheirmildewyorfuzzy,cottonlikeappearance.Moldscandevelopnumeroustinysporesthatare
foundintheairandcanbespreadbyaircurrents.Thesesporescanproducenewmoldgrowthifthey
aretransferredtoalocationthathasconditionsconducivetogermination.Moldsgenerallywithstand
greaterfluctuationinpHthanbacteriaandyeastsandcanfrequentlytoleratemoretemperature
fluctuation.AlthoughmoldsthrivebestatornearapHof7.0,apHrangeof2.0to8.0canbe
tolerated,eventhoughanacidtoneutralpHispreferred.Moldsthrivebetteratambienttemperature
thaninacolderenvironment,eventhoughgrowthcanoccurbelow0°C.Althoughmoldgrowthis
optimalatawateractivity(Aw)ofapproximately0.85,growthcananddoesoccurbelow0.80.Atan

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Awof0.90orhigher,bacteriaandyeastsgrowmoreeffectivelyandnormallyutilizeavailable
nutrientsforgrowthattheexpenseofmolds.WhentheAwgoesbelow0.90,moldsgrowmore
effectively.Thatiswhyfoodstuffs,suchaspastries,cheeses,andnuts,thatarelowinmoisture
contentaremorelikelytospoilfrommoldgrowth.
Generalcharacteristicsofmolds:-Theterm"mold"isacommononeappliedtocertain
multicellularfilamentousfungiwhosegrowthonfoodsusuallyisreadilyrecognizedbyitsfuzzyor
cottonyappearance.Themainpartofthegrowthcommonlyappearswhitebutmaybecoloredor
darkorsmokyduetotheproductionofpigmentIsomefilamentousfungi.Coloredsporesaretypical
ofmaturemoldofsomekindsandgivecolortopartorallofthegrowth.Thethallus,orvegetative
body,ischaracteristicofthallophytes,whichlacktrueroots,stems,andleaves,e.g.Trichophyton
mentagrophytes.
MorphologicalCharacteristics:-Themorphologicalcharacteristicsi.e.formandstructure,of
moldsaredeterminedbymacroscopicandmicroscopicexamination.
HyphaeandMycelium:-Themoldthallusconsistsofamassofbranching,intertwinedfilaments
calledhyphae(singularhypha),andthewholemassofthesehyphaeisknownasthemycelium.The
hyphaemaybesubmerged,orgrowingwithinthefood,oraerial,orgrowingintotheairabovethe
food.Moldsaredividedintotwogroups:septate,i.e.,withcrosswallsdividingthehyphaintocells;
andnoncoenocytic,septatewiththehyphaeapparentlyconsistingofcylinderswithoutcrosswalls.
Thenon-septatehyphaehavenucleiscatteredthroughouttheirlengthandareconsidered
multicellular.Special,mycelialstructuresorpartsaidintheidentificationofmolds.Examplesare
therhizoids,or"holdfasts,"ofRhizopusandAbsidia,thefootcellinAspergillus,andthe
dichotomous,orY-shaped,branchinginGeotrichum.
ReproductivePartsorStructures:-Reproductioninfungiiscomplexandinvolvesagreat
diversityofstructures.Atthemostfundamentallevelwecansaythatmostmouldsreproduceby
spores.Sporesarelikeseeds;theygerminatetoproduceanewmouldcolonywhentheylandina
suitableplace.Unlikeseeds,theyareverysimpleinstructureandnevercontainanembryoorany
sortofpreformedoffspring.Sporesareproducedinavarietyofwaysandoccurinabewildering
arrayofshapesandsizes.Inspiteofthisdiversity,sporesarequiteconstantinshape,size,colourand
formforanygivenmould,andarethusveryusefulformouldidentification.Themostbasic
differencebetweensporesliesintheirmethodofinitiation,whichcanbeeithersexualorasexual.

8. FoodMicrobiology Page 7 of28
Rhizopus Digrame of simple conidial head ofAspergillus
Geotrichum
AsexualSpores:-Theasexualsporesofmoldsareproducedinlargenumbersandaresmall,light,
andresistanttodrying.Theyarereadilyspreadthroughtheairtoalightandstartnewmoldthallus
whereconditionsarefavorable.Thethreeprincipaltypesofasexualsporesare(1)conidia(singular
conidium),(2)arthrosporesoroidia(singularoidium),and(3)sporangiospores.Conidiaarecutoff,
orbud,fromspecialfertilehyphaecalledconidiophoresandusuallyareintheopen,i.e.,not
enclosedinanycontainer,incontrasttothesporangiospores,whichareinsporangium(plural
sporangia),orsac,atthetipofafertilehypha,thesporangiophore.Arthrosporesareformedby
fragmentationofahypha,sothatthecellsofthehyphabecomearthrospores.Examplesofthese
threekindsofsporeswillbegiveninthediscussionofimportantgeneraofmolds.Afourthkindof
asexualspore,thechlamydospore,isformedbymanyspeciesofmoldswhenacellhereandtherein
themyceliumstoresupreservefood,swell,andformsathickerwallthanthatofsurroundingcells.
Thischlamydospore,orrestingcell,canwithstandun-favorableconditionsbetterthanordinarymold
myceliumcanandlater,underfavorableconditions,cangrowintoanewmold.
SexualSpores:-Sexuallyinitiatedsporesresultfromamatingbetweentwodifferentorganismsor
hyphae,whereasasexualsporesresultfromasimpleinternaldivisionorexternalmodificationofan
individualhypha.Therecognitionofamatingandsubsequentsporeformationisoftendifficultfor
Dr. Sujeet KumarMrityunjay

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anobserver,andisusuallyreservedforpatientspecialists.Themoldswhichcanproducesexual
sporesareclassifiedonthebasisofthemannerofformationofthesesporesandthetypeproduced,
namely,thefourkindsofsexuallydeterminedsporesthatappearinmouldfungi:-
A.
A.Oospores
B.Zygospores
C.Ascospores, and
D.Basidiospores.
Oospores:-OosporesaretermedOomycetes.Thesemoldsaremostlyaquatic;however,
includedinthisgroupareseveralimportantplantpathogens.Oosporesareproducedwhenmale
gametes(reproductivenuclei)enteralargesphericalcell(oogonium)andfertilizetheeggswithin.
Theresult,asseeninroutineexamination,isnumerousoogoniacontainingonetoseveralspherical
andoftenbrownisheggs.Theoogoniaareusuallypenetratedbyoneormorehyphae(antheridia)that
giverisetothemalenuclei.(Theoosporesareformedbytheunionofasmallmalegameteanda
largefemalegamete).
B.Zygospores:-Zygosporesdonotoccurinsideanykindofenclosingstructure,butareproduced
bythedirectfusionoftwohyphalprotrusions(suspensors)fromneighbouringfilaments.Usually
zygosporesarerecognizedaslarge,nearlyspherical,oftendarkbrownorblack,rough-walled
sporeswithtwoconnectinghyphae,representingthetwomatinggametangia.Sometimesthe
zygosporemaybesurroundedbyseveralfinger-likeextensionsfromthetwogametangia.
(Zygomycetesformzygosporesbytheunionofthetipsoftwohyphaewhichoftenappearsimilarand
whichmaycomefromthesamemyceliumorfromdifferentmycelia.BothOosporesandzygospores
arecoveredbyatoughwallandcansurvivedryingforlongperiods).
C.Ascospores:-Ascosporesareproducedwithinsphericaltocylindricalcellscalledasci,mostoften
ingroupsoffouroreight.Usuallytheasciareproducedwithinsomekindofenclosingstructureand
thusarenotfoundexposedonthehyphae.Inafewcasestheascimaybeborneamonghyphaeand
resembleoogoniawitheggs,buttheywillneverbepenetratedbyanysortoffertilizinghypha.
Fertilizationoccursearlyinthelifecycleandisnotevidentatthetimeascosporesareproduced.
D.Basidiospores:-Basidiosporesarealwaysproducedexternallyonastructurecalledabasidium.
Basidiacomeinavarietyofforms,butthosecommonlyencounteredonmouldswillbeclub-shaped
andbearfouroreightsporesonsharpprojectionsattheapex.Atfirstitmaybedifficultto
distinguishbetweenabasidiosporesandoneoftheasexuallyinitiatedsporetypes,butoneshould
alwayssuspectthepresenceofbasidiawhenexternallyproducedsporesconsistentlyoccuringroups

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offouroreight.Aswithascospores,basidiosporesaretheresultofanearlyfertilizationthatisnot
easilyobserved.
CULTURALCHARACTERISTICS :-Somemoldsarelooseandfluffy;othersarecompact.Some
lookvelvetyontheuppersurface,somedryandpowdery,andotherswetorgelatinous.Definite
zonesofgrowthinthethallusdistinguishsomemolds,e.g.Aspergillusniger.Pigmentsinthe
mycelium-red,purple,yellow,brown,gray,black,etc-arecharacteristic,asarethepigmentsof
massesofasexualspores;green,blue-green,yellow,orange,pink,lavender,brown,gray,black,etc.
Theappearanceofthereversesideofamoldonanagarplatemaybestriking,liketheopalescent
blue-blackorgreenish-blackcoloroftheundersideofCladosporium.
PHYSIOLOGICAL CHARACTERISTICS :-Thephysiologicalcharacteristicsofmoldswillbe
discussedbriefly.
Temperaturerequirement-Mostmoldsgrowwellatordinarytemperature.Anumberofmolds
growwellatrefrigerationtemperatures.Afewcangrowatahightemperature.Theoptimal
temperatureformostmoldsisaround25to30ºC,butsomegrowwellat35to37ºCorabove,e.g.,
Aspergillusspp.,andsomeatstillhighertemperatures.Anumberofmoldsarepsychrotrophic;i.e.,
theygrowfairlywellattemperaturesofrefrigeration,andsomecangrowslowlyattemperatures
belowfreezing.Growthhasbeenreportedataslowas-5to-10ºC.Afewarethermophilic;i.e.,they
haveahighoptimaltemperature.
MoistureRequirements:-Ingeneralmoldsrequirelessmoisturetogrowthanyeastandbacteria.If
driedfoodhasamoisturecontentbelow14to15%,itwillpreventordelaymoldgrowth.
OxygenandpHRequirements:-Moldsareaerobic,sotheyrequireoxygenfortheirgrowth.Most
moldscangrowoverawiderangeofhydrogen-ionconcentration(pH2to8.5),butthemajorityare
favoredbyanacidpH.
FoodRequirements:-Moldsingeneralcanutilizemanykindsoffoods,rangingfromsimpleto
complex.Mostofthecommonmoldspossessavarietyofhydrolyticenzymes,andsomearegrown
fortheiramylases,pectinases,proteinases,andlipases.
Inhibitors:-Compoundsinhibitorytootherorganismsareproducedbysomemolds,suchas
penicillinfromPenicilliumchrysogenumandclavacinfromAspergillusclavatus.Certainchemical
compoundsaremycostatic,inhibitingthegrowthofmolds(sorbicacid,propionates,andacetatesare
examples),orarespecificallyfungicidal,killingmolds.

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CLASSIFICATIONANDIDENTIFICATIONOFMOLDS:-Moldsarepartofthekingdom
Myceteae.Theyhavenoroots,stems,orleavesandaredevoidofchlorophyll.Theybelongtothe
Eumycetes,ortruefungi,andaresubdividedfurthertosubdivisions,classes,orders,families,and
genera.Thefollowingcriteriaareusedmainlyfordifferentiationandidentificationofmolds:-
1.Hyphae septate ornon-septate
2.Mycelium clear or dark(smoky)
3.Mycelium colored ofcolorless
4.Whether sexual spores are produced and the type: oospores, zygospores, orascospores
5.Characteristics of the sporehead
a)Sporangia: size, color, shape, andlocation
b)Spore heads bearing conidia: single conidia, chains, budding conidia, or masses;
shape and arrangement of sterigmata or phialides; gumming together ofconidia
6.Appearanceofsporangiophoresorconidiophores:simpleorbranched,andifbranchedthe
typeofbranching;sizeandshapeofcolumellaattipofsporangiophore;whether
conidiophoresaresingleorinbundles
7.Microscopicappearancesoftheasexualspores,especiallyofconidia:shape,size,color;
smoothorrough;one-,two-,ormany-celled
8.Presenceofspecialstructures(orspores):-stolons,rhizoids,footcells,apo-physis,
chlamydospores,sclerotia,etc.
GROUPSOFBACTERIAIMPORTANTINFOODINDUSTRY
1.Lacticacid–formingbacteriaorlactics:-ThelacticacidbacteriaareagroupofGram-
positivebacteria,non-respiringnon-spore-forming,cocciorrods,whichproducelacticacid
asthemajorendproductofthefermentationofcarbohydrates.Thesebacteriafermentsugars
tolacticacid.Thismaybedesirableinmakingproductssuchassauerkrautandcheese.But
undesirableintermsofspoilageofwinesbecausetheyusuallyformacidrapidly.Ex:
Leuconostoc,Lactobacillus,StreptococcusandPediococcus.
2.Aceticacidformingbacteriaoracetics:-Aceticacidbacteria(AAB)areagroupofGram-
negativebacteriawhichoxidizesugarsorethanolandproduceaceticacidduring
fermentation.Theaceticacidbacteriaconsistof10generainthefamilyAcetobacteraceae.
Severalspeciesofaceticacidbacteriaareusedinindustryforproductionofcertainfoodsand
chemicals.Allaceticacidbacteriaarerod-shapedandobligateaerobes.Aceticacidbacteria
areairborneandareubiquitousinnature.Theyareactivelypresentinenvironmentswhere

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ethanolisbeingformedasaproductofthefermentationofsugars.Aceticacidbacteriaare
someofthemostcommonwinespoilageorganismsand,assuch,theyrepresentagenuine
threattothewinemaker.Themostcommonreactionassociatedwithwinespoilageisthe
oxidationofethanoltoformaceticacid,whichleadstotheproductionofwinecommonly
referredtoas“pricked.”ThegrowthofAcetobacterinwinecanbesuppressedthrough
effectivesanitation,bycompleteexclusionofairfromwineinstorage,andbytheuseof
moderateamountsofsulfurdioxideinthewineasapreservative.Characteristicsthatmake
aceticacidbacteriaimportantare:-
iii.
i.Their ability to oxidize ethanol to aceticacid.
ii.Their strong oxidizing power, result in oxidation of desired product like acetic acid,
by desirable sps or undesirable sps under favorableconditions.
Excessive sliminese of some species Ex: Acetobacter acetic sub sp. suboxydans. This
bacteria clog vinegargenerators.
3.Butyricacidformingbacteriaorbutyrics:-Mostbacteriaofthisgrouparesporeforming
anaerobesofthegenusclostridium.
4.Propionicacid–formingbacteriaorpropionics:-Thebacteriathatproducepropionic
acidwerenamedPropionibacteriumbyOrla-Jensenin1898andtheirfermentationwas
studiedbyPasteurandFitz.PropionicacidbacteriaareusuallyfoundinSwiss-typecheeses
wheretheygrowduringripeningandcontributetothecharacteristicflavourandappearance
ofthesecheeses.Ex:Propionibacterium.(Propionibacteriumisagram-positive,anaerobic,
rod-shapedgenusofbacterianamedfortheiruniquemetabolism:Theyareabletosynthesize
propionicacidbyusingunusualtranscarboxylaseenzymes.)
5.LipolyticBacteria:-Thisbacteriaproducelipaseswhichcatalyzethehydrolysisoffatsto
fattyacidsandglycerol.Manyoftheaerobic,activelyproteolyticbacteriaalsoarelipolytic.
Pseudomonasfluorescens–Stronglylipolytic,Pseudomonas,Alcaligenes,Staphylococcus,
SerratiaandMicrococcusaregenerathatcontainlipolyticbacteria.
6.Saccharolyticbacteria:-Thesebacteriahydrolyzedisaccharidesorpolysaccharidesto
simplersugars.Amylolyticbacteriapossessamylasetobringaboutthehydrolysisofstarch
outsidethecell.AmylolyticbacteriaareBacillussubtilisandClostridiumbutyricum.
7.PectinolyticBacteria:-Pectinsarecomplexcarbohydratesthatareresponsibleforcellwall
rigidityinvegetablesandfruitspecticsubstancesderivedfromcitrusfruitscanbeusedin
commercialproductsasgellingagents.Ex:Erwinia,Bacillus,Clostridum,Achromobacter,
Aeromonas,Arthrobacter,Flavobacterium.

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8.ThermophilicBacteriaorThermophiles:-Optimumtemperaturerequiredforthesebacteria
45°C-55°C.Bacillusstearothermophilus–thermophilicflatsourspoilageoflowacid
cannedfoods.
9.ThermoduricBacteria:-Thermoduricbacteriaareusuallydefinedasthosewhichcan
surviveaheattreatmentsuchaspasteurization.Ex:Bacillussps,Micrococci,Enterococci
cansurvivepasteurizationofliquideggs.FungilikeByssochlamysfulva,Aspergillusand
Penicilliumarethermoduric.SomethermoduricbacterialikeBacillusandenterococcican
alsobepsychrotrophic.
10.PsychrotrophicBacteriaorpsychrotrophs:-Thesebacteriaareabletogrowat
commercialrefrigerationtemperatures.Unlikepsychrophiles,psychrotrophsdonothave
theiroptimaltemperatureforgrowthatrefrigerationtemperatureandtheiroptimumbetween
25°Cand30°C.Ex:Pseudomonas,Flavobacterium,AchromobacterandAlcaligenes,
Micrococcus,Lactobacillusetc.
11.HalophilicBacteriaorHalophiles:-Halophilesareorganismsthatthriveinhighsalt
concentrations.Theyareatypeofextremophileorganism.ThenamecomesfromtheGreek
wordfor"salt-loving".Halophilicbacteriaarecategorizedbasedontheirrequirementsto
salt.Theyapplysomestrategiesinadaptationtothehighsalineenvironment.Theirlow
nutritionalrequirementsandresistancetohighconcentrationsofsalt,introduceaspotent
agentsinwiderangeofbiotechnologicalapplications.Halophilicbacteriaareverydivergent
andmorethan150speciesintroducein70generaofhalophilicbacteriaarereported.
Halophilicbacteriaaresuitablefordevelopingopenandunsterilefermentationprocessas
theycanbegrownunderhighsaltconcentrationtopreventcontaminationfromnonhalophilic
microorganisms.HalophilicBacteriarequirecertainminimalconcentrationsofdissolved
sodiumchlorideforgrowth.Ex:-Pseudomonas,Moraxella,Acinetobacter,
Flavobacterium,Vibriosps.whichgrowbestinmediawith0.5–3.0percentsalt.These
microorganismsareisolatedfromfishshellfish.Theseareslightlyhalophilic.Extreme
halophilesgrowintheheavilybrinedfoods15–30%salt.Ex:-Halobacterium,
Halococcus.
12.OsmophilicorSaccharophilicBacteria:-Osmophilicorganismsaremicroorganisms
adaptedtoenvironmentswithhighosmoticpressures,suchashighsugarconcentrations.
Osmophilesaresimilartohalophilic(salt-loving)organismsbecauseacriticalaspectofboth
typesofenvironmentistheirlowwateractivity,aw.Highsugarconcentrationsrepresenta
growth-limitingfactorformanymicroorganisms,yetosmophilesprotectthemselvesagainst

14. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 13 of28
thishighosmoticpressurebythesynthesisofosmoprotectantssuchasalcoholsandamino
acids.Osmophilicbacteriaarethosewhichgrowinhighconcentrationsofsugar.Ex:-
Leuconostoc.Manyosmophilicmicroorganismsareyeasts(Zygosaccharomycesbailii,
Wallemiasebi,Debaryomycessp.).
13.PigmentedBacteria:-Colorsproducedbypigmentedbacteriagrowingonorinfoods.
Flavobacterium–Yellowtoorange;Serratia–Red;Halobacterium–Pink.
14.Slimeorropeformingbacteria:-Alcaligenesviscolactis,Enterobacteraerogenes&
KlebsiellaoxytocacausesropinessofmilkandLeuconostocspp.,producingslimeinsucrose
solutionsandslimysurfacegrowthofvariousbacteriaoccurringonfoods.Streptococcus&
Lactobacillusmakemilkslimyorropy.Micrococcusmakescuringsolutionsformeatsropy.
LactobacillusplantarumandLactobacillimaycauseropinessinvariousfruit,vegetableand
grainproductse.g.incider,sauerkrautandbeer.
15.GasformingBacteria:-Manykindsofbacteriaproducesmallamountsofgasandyieldit
slowly.Ex:Leuconostoc,Lactobacillus(heterofermentative),Propionicbacterium,
Escherichia,Enterobacter,Proteus,BacillusandClostridium.
16.Coliformandfecalcoliformgroup:-Coliformsareshortrodsthataredefinedasaerobic
andfacultativeanaerobic,gramnegative,non-sporeformingbacteria.Ex:Escherichiacoli,
Enterobacteraerogenes.Fecalcoliformgroupincludescoliformscapableofgrowingat44-
45°C.Geotrichumcandidumisthemachinerymoldandasanindicatorofplantsanitation
andcontaminatedequipment.Someofthecharacteristicsthatmakethecoliformbacteria
importantinfoodspoilageare:-
1.Their ability to grow well in a variety of substrates and synthesize most of the
necessaryvitamins.
2.Their ability of the group to grow well over a fairly wide range of temperatures from
below 10°C to about46°C.
3.Their ability to produce considerable amounts of acid and gas fromsugars.
4.Their ability to cause off –flavours often described as unclean orbarny.
5.Their ability of E. aerogenes to cause sliminess or ropiness offoods.
SOURCESOFCONTAMINATION :-Foodcontaminationistheintroductionoroccurrenceofa
contaminantinfood.Acontaminantisanybiologicalorchemicalagent,foreignmatter,orother
substanceunintentionallyaddedtofoodthatmaycompromisefoodsafetyorsuitability.Among
thesecontaminantsarebiological,chemicalorphysicalagentsin,orconditionof,foodwiththe
potentialtocauseanadversehealtheffect.Majorcontaminationsourcesarewater,air,dust,

15. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 14 of28
equipment,sewage,insects,rodents,andemployees.Contaminationofrawmaterialscanalsooccur
fromthesoil,sewage,liveanimals,externalsurface,andtheinternalorgansofmeatanimals.
Additionalcontaminationofanimalfoodsoriginatesfromdiseasedanimals,althoughadvancesin
healthcarehavenearlyeliminatedthissource.Contaminationfromchemicalsourcescanoccur
throughaccidentalmixingofchemicalsupplieswithfoods.Ingredientscancontributetoadditional
microbialorchemicalcontamination.Contaminationcanbereducedthrougheffectivehousekeeping
andsanitation,protectionoffoodduringstorage,properdisposalofgarbageandlitter,andprotection
againstcontactwithtoxicsubstances.Microorganismsfromvariousnaturalsourcesactassourceof
contamination.
1.From green plants andfruits
2.From animals
3.From sewage
4.From soil
5.From water
6.From air
7.During handling andprocessing.
1.Fromsewage:-Raw,untreatedsewagecancontainpathogensthathavebeeneliminated
fromthehumanbody,aswellasothermaterialsoftheenvironment.Examplesare
microorganismcausingtyphoidandparatyphoidfevers,dysentery,andinfectioushepatitis.
Sewagemaycontaminatefoodandequipmentthroughfaultyplumbing.Whenuntreated
domesticsewageisusedtofertilizeplantcrops,thereisachancethatrawplantfoodswillbe
contaminatedwithhumanpathogensespeciallythosecausinggastrointestinaldiseases.The
useof“nightsoil”asafertilizerstillpersistsinsomepartsoftheworld.Inadditiontothe
pathogens,coliformbacteria,anaerobes,enterococci,otherintestinalbacteriaandvirusescan
contaminatethefoodsfromthissource.Naturalwatercontaminatedwithsewagecontributes
theirmicroorganismstoshellfish,fish,andotherseafood.Ifrawsewagedrainsorflowsinto
potablewaterlines,wells,rivers,lakes,andoceanbays,thewaterandlivingorganismssuch
asseafoodarecontaminated.Topreventthiscontamination,priviesandseptictanksshould
besufficientlyseparatedfromwells,streams,andotherbodiesofwater.Rawsewageshould
notbeappliedtofieldswherefruitsandvegetablesaregrown.
2.Fromsoil:-Soilcontainsgreatestvarietyofmicroorganisms.Theyarereadytocontaminate
thesurfacesofplantsgrowingonorinthemandthesurfacesofanimalsroamingoverthe

16. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 15 of28
land.Soildustiswhippedupbyaircurrentsandsoilparticlesarecarriedbyrunningwaterto
getintoorontofoods.Soilisanimportantsourceofheatresistantsporeformingbacteria.
3.Fromwater:-Waterservesasacleaningmediumduringthecleaningoperationandisan
ingredientaddedintheformulationofvariousprocessedfoods.Itcanalsoserveasasource
ofcontamination.Naturalwatercontainnotonlytheirnaturalflorabutalsomicroorganisms
fromsoilandpossiblyfromanimalsorsewage.Surfacewatersinstreamsorpoolsandstored
watershavelowmicrobialcontentbecauseself-purificationofquietlakesandpondsorof
runningwater.Groundwatersfromspringsorwellshavepassedthroughlayersofrockand
soiltoadefinitelevelhencemostofthebacteria,suspendedmaterialhavebeenremoved.
KindsofbacteriainnaturalwatersarechieflyofinPseudomonas,Chromobacterium,
Proteus,Micrococcus,Bacillus,Streptococcus,EnterobacterandEscherichiacoli.
4.FromAir:-Contaminationcanresultfromairbornemicroorganismsinfoodprocessing,
packaging,storage,andpreparationareas.Thiscontaminationcanresultfromuncleanair
surroundingthefoodplantorfromcontaminationthroughimpropersanitarypractices.The
mosteffectivemethodsofreducingaircontaminationarethroughsanitarypractices,filtering
ofairenteringthefoodprocessingandpreparationareas,andprotectionfromairby
appropriatepackagingtechniquesandmaterials.Airdoesnotcontainanaturalfloraof
microorganisms,butaccidentallytheyarepresentonsuspendedsolidmaterialorinmoisture
droplets.Microorganismsgetintoairondustorlint,drysoil,sprayfromstream,lakesor
oceans,dropletsofmoisturefromcoughing,sneezingortalkingandgrowthofsporulating
moldsonfloors,etc.
Microorganismsinairhavenoopportunityforgrowthbutmerelypersistthereandthe
organismsresistanttodessicationswilllivelonger.Moldsporesbecauseoftheirsmallsize,
resistancetodryingandlargenumbersofpermoldplantareusuallypresentinair.Cocciare
morenumerousthanrodshapedbacteria.Yeastsespeciallyasporogenouschromogenicones
arefoundinmostsamplesofair.Numberofmicroorganismsinairatanygiventimedepend
onfactorslikeamountofmovement,sunshine,humidity,locationandtheamountof
suspendeddustorspray.No.ofmicroorganismsvaryfrommountainstodustyair.Lesson
mountainsandmoreindustyair.Directraysfromthesunkillmicroorganismssuspendedin
airandhencereducenumbers.Dryaircontainsmoreorganismsthanmoistair.Rainorsnow
removesorganismsfromtheair.Numberofmicroorganismsinairmaybereducedunder
naturalconditionsbysedimentation,sunshineandwashingbyrainorsnow.

17. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 16 of28
Filtersinventilatingorairconditioningsystemspreventthespreadoforganismsfromone
partofaplanttoanother.
5.Duringhandlingandprocessing:-Additionalcontaminationmaycomefromequipment
comingincontactwithfoods,frompackagingmaterialsandfrompersonnel.Asickperson
canpassongerms,rangingfromflutogastroenteritis.Achoppingboardusedformeatthatis
notwashedandthenusedforvegetablesisanothersourceofpossiblecontamination.
Unwashedhands,dirtykitchenspaces,insectsandrodentsinthekitchenetc.areallpossible
sourcesoffoodcontamination.
POSTHARVEST CONTAMINATION :-Postharvestcontaminationistheunintended
contaminationofafoodproductwithmicroorganismsorparticlesafteraprocessingstephasbeen
usedtoremoveorkillthem.
POST-HARVEST/SLAUGHTER CONTAMINATION/ POSTPROCESSINGCONTAMINATION
Foodprocessingandpreparationarebothpracticesusedtomakeachangetoafoodtoalteritseatingquality
orshelflife.Forexample,themillingofwheatintoflourorfermentationofmaizeinto‘ogi‘isfoodprocessing
whereascookingthewheatinthekitchenorcookingandmixingthe‘ogi‘withotheringredientsarepartof
theirpreparationforconsumption.However,thetermfoodprocessingisbroaderthanpreparingandcooking
foods.Itinvolvesapplyingscientificandtechnologicalprinciplestopreservefoods,byallowingchangestothe
eatingqualityoffoodstobemadeinapredictableandcontrolledway.Modernfoodprocessingissometimes
definedastakingplaceataplantorfactory.Thisisdistinctfromfoodpreparationwhichusuallytakesplacein
kitchens.However,manyactivities(suchascooking,washing,slicing,peeling,juicing,shredding,mixingetc.)
anduseofseveralequipmentarecommontobothprocessingandfoodpreparation.Foodpreparationor
processing,whetherinthehome,atarestaurantorquickservice,othertypesoffoodservicequality(suchas
cafeteria,nursinghomeorhospital)orfoodcompaniesorindustriescanintroducepathogensintoaproductif
notdoneproperly.Thisispossiblethroughaprocessofcross-contaminationdefinedasthephysical
movementortransferofharmfulmicroorganismsormicrobialtoxinsfromonefoodtoanothereitherdirectly
(foodtofood)orindirectly(equipment/utensilsorfoodcontactsurfacestofoodandpeopletofood).
Pathogenicmicroorganismscanbefoundmoreorlesseverywhereandconsequentlymaybefoundinraw
foods,e.g.meatorpoultrytobecookedpriortoconsumption.L.monocytogenescanbetransferredfrom
processingsurfacestofoods.Thoughthoroughcookingofsuchfreshfoodswillrenderthesmallnumbersof
microbesharmless,itthenconstitutesadangerwhenmicrobesarespreadfromraworcontaminatedfoodto
anotherfood,especiallyready-to-eatprepareddishes.
Foodcontactequipmentfactors:-Oneimportantfactorofmicrobialcontaminationduringfoodprocessing
andpreparationisfoodcontactequipmentwhichmayharborandintroducepathogensintofood.Processes
suchastrimming,slicing,milling,shredding,peelingmechanicalabrasionandvariousmethodsof

18. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 17 of28
disintegrationifdonewithcontaminatedequipmentmayintroducecontaminantsfromtheequipmentinvolved.
Whencuttingboardsusedforrawmeat,poultryorseafoodcomeincontactwithotherfoods,microorganisms
canbetransferred.Anotherexampleisslicingready-to-eatproduce,e.g.lettucewithaknifepreviouslyusedto
cutrawpoultrywithoutproperwashingandsanitationoftheknifeinbetweentheprocesses.Otherequipment
suchasconveyorbeltsandaprons,filters,blanchers,presses,screensandwoodensurfacesaredifficultto
cleanandsanitizeandthereforeareespeciallylikelytobesourcesofcontamination.Listeriamonocytogenes
hasbeenfoundonequipmentandprocesssurfaces,whicharedifficulttoclean.
Unhygienicpracticesoffoodhandlers:-Anotheravenuethroughwhichfoodsgetcontaminatedduring
processingandpreparationisinfectedfoodhandlersandtheirunhygienicpractices.Humans(theirskin,
mucousmembranesandcuts,opensoresoraskininfection)canserveasreservoirsofpathogens,e.g.S.
aureusandfromwherefoodsgetcontaminatedifhandledunderunhygieniccondition,especiallythrough
unwashedhands.Directcontaminationcanhappenbycontactbetweenthebodyandthefoodproduct.In
indirectcontamination,peopleactasvectorsandtransfercontaminationfromoneareaorsurfacetoanother.
Wheninfectedfoodhandlersfailtowashtheirhandsafterusingthetoilet,handlingrawmeatpoultryorfish,
takingoutgarbage,cleaningupspills,handlingmoney,touchingothercontaminatedsurfacessuchas
blemishes,pimples,thenose,boils,openwoundsandsoiledtissues,foodcontaminationtakesplace.Viruses
suchashepatitisAandNorwalkareeasilytransmittedtoshellfish,salads,vegetablesandfruitswheninfected
foodhandlersfailtowashtheirinfectedorcontaminatedhands.Failuretowearglovesortouching
contaminatedsurfaceevenwhilewearinggloves,sneezingorcoughingintoaglovedhand,food,foodcontact
equipmentorsurfacesarewaysfoodsalsogetcontaminatedduringprocessingandpreparation.Handlerswho
donotwearglovescommonlyspreadStaphbacteriatomeat,cream-filleddesserts,potatosaladsandegg
products.
Biofilmformation:-Duringfoodprocessing,microbialcommunitiesincludingpathogenscaninhabitor
accumulateoncriticalplacessuchasfoodcontactandenvironmentalsitesonequipmenttoformbiofilms
(microbialcellclusterswithanetworkofinternalchannelsorvoidsintheextracellularpolysaccharideand
glycoproteinmatrix,whichallowsnutrientsandoxygentobetransportedfromthebulkliquidtothecells).
Microbesinhabitingcontactandenvironmentalsitesinfoodprocessingaremostlyharmful.Thebiofilms
contributetofoodcontamination,especiallyinfoodspassingthroughthesameprocessinglinewherethe
processequipmentisnothygienicallyintegratedintheprocesslineorthecleaninganddisinfectionprocedures
arenotproperlydesignedtoremovetheorganicsoilfromtheprocesssurfaces.Anymicrobecanformbiofilm
undersuitableconditions.Howeversomemicrobesnaturallyhaveahighertendencytoproducebiofilmthan
others.FoodbornepathogensthatreadilyformbiofilmsincludeBacilluscereus,S.aureus,M.
paratuberculosis,C.perfringens,E.coli0157:H7,S.typhimurium,C.jejunii,YersiniaenterocoliticaandL.
monocytogenes.L.monocytogeneshasbeenfoundtoformbiofilmsoncommonfoodcontactsurfacese.g.

19. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 18 of28
plastic,polypropylene,rubber,stainlesssteelandglass.Alsoinclusionofdecayedpartsoffoodse.g.fruits
increasesthenumbersofmicroorganismsinsuchfoodproductssuchasfruitsjuices.
Useofpoorwashwater:-Priortoreachingtheconsumer,mostfoods,especiallyfreshproducearewashedat
leastonceandusuallyseveraltimesduringprocessing.Rawwatercontainsbacteria.Mostwatersuppliesare
sourcedfromriversandlakeswhichcanbecontaminatedbyrun-offfromland,sewageandslurry.Useofpoor
waterforwashinginfoodprocessingandpreparationcanintroducemicroorganismsintofood.
Packagingfactors:-Packagingservesasamajordefenseagainstexternalhazards.Howeverundesirable
interactionsbetweenpackagingmaterialandfoodcangiverisetopotentialproblems.Theriskconnectedto
packagingmaterialisthepotentialtransferoffoodspoilageorpathogenicorganismstothepackedfood.Most
packagingmaterialshaveproventobecompletelyimpervioustomicroorganisms.Alownumberofbacteriain
thepackagingmaterialcouldbeofconcernfore.g.asepticfoods;ifbacteriamigrateacrossthepackage.The
routesofcontaminationfromthepackagingmaterialtofoodincludethesurface,cuttingdustordirectcontact
withtherawedgeofthepaperboard.
Distribution,marketingandstoragefactors:-Thesealsoplayimportantrolesinmicrobialcontaminationof
food.Contaminationcanoccurviastorageinthemarketincontaminatedbinsandothercontainers,possible
contactwithdecayingproducts.Duringfoodstorageintherefrigerator,crosscontaminationtakesplacewhen,
fore.g.,juicefromrawmeat,poultryorfishdripsontovegetablesorotherready-to-eatfoodsontheshelf
below.Inclusionofrawfoodswithreadytoeatfoodsduringshoppingorstorageleadstocrosscontamination.
Contaminationalsooccurswhenfoodsarenotkeptattherighttemperature,thuspromotingthetemperature
dangerzone(i.e.thetemperatureinwhichbacteriaandsuchcanbethemostwidelyspread).Thisincludesthe
triphomefromthestore,thetimethefoodisonthetableandthetimeyouhaveitinyourbagforlunchat
work.Inaddition,handlingbysalespeopleorcustomersandsprayingwithcontaminatedwater(forfruitsand
vegetables)arealsoimportantfactors.Thesprayinggivesafreshappearancetothevegetablesbutalsoadds
organisms.Inclusionofdecayedfoodpartduringdistribution,marketingandstorageincreasesthenumberof
microorganismsinfoods.
FACTORSINFLUENCING GROWTH OFMICROORGANISM ONFOODS,INTRINSIC
FACTORSANDEXTRINSICFACTORS.
Foodsaremainlycomposedofbiochemicalcompoundswhicharederivedfromplantsandanimals.
Carbohydrates,proteinsandfatsarethemajorconstituentsoffood.Inaddition,minorconstituents
suchasminerals,vitamins,enzymes,acids,antioxidants,pigments,flavorsarepresent.Foodsare
subjecttophysical,chemical,andbiologicaldeterioration.Thefactorsthataffectmicrobialgrowth
infoods,anddeterminethenatureofspoilageandanyhealthriskscanbecauterizedinfourgroups.
Themajorfactorsaffectingmicrobialgrowthinfoodsareasfollows:-

20. Factors affecting the development of microbial associations infood
1.Nutrients
2.pH and bufferingcapacity
1IntrinsicFactors
3.
4.
Redoxpotential
Wateractivity
5.Antimicrobialconstituents
6.Antimicrobialstructures
FoodMicrobiology
Dr. Sujeet KumarMrityunjay
Page 19 of28
1.Intrinsic Factors (Physico-chemical properties of the fooditself)
2.Extrinsic Factors (conditions of the storageenvironment)
3.Implicit Factors (properties and interactions of the microorganisms present);and
4.Processing Factors.
2
ExtrinsicFactors/Environmental
factors
1.Relativehumidity
2.Temperature
3.Gaseousatmosphere
3
Implicitfactors
1.Specific growthrate
2.Mutualism
3.Antagonism
4.Commensalism
4
Processingfactors
1.Slicing
2.Washing
3.Packing
4.Irradiation
5.Pasteurization
ExtrinsicFactors/EnvironmentalfactorsThatInfluenceMicrobialGrowth:-Extrinsicfactors
areexternaltothefood.Temperatureandgascompositionarethemainextrinsicfactorsinfluencing
microbialgrowth.Theinfluenceoftemperatureonmicrobialgrowthandphysiologyishuge.While
theinfluenceoftemperatureonthegrowthrateisobviousandiscoveredinsomedetailhere,the
influenceoftemperatureongeneexpressionisequallyimportant.Cellsgrownatrefrigeration
temperaturedonotjustgrowmoreslowlythanthosegrownatroomtemperature,theyexpress
differentgenesandarephysiologicallydifferent.Aruleofthumbinchemistrysuggeststhatreaction
ratesdoublewithevery10°Cincreaseintemperature.Thissimplifyingassumptionisvalidfor
bacterialgrowthratesonlyoveralimitedrangeoftemperatures.
ExtrinsicFactors/Environmentalfactors:-Themainextrinsicparametersinfluencethefoods
are:
1.RelativeHumidity:-Humidityistheconcentrationofwatervapourintheatmosphere.Relative
humidityistheratioexpressedasthepercentageofmoistureinairtothemoisturepresentinfood
underthesaturationconditionattemperatureandpressure.Relativehumidityandwateractivity
areinter-relatedi.e.whenfoodwithlowwateractivityarestoredintheenvironmentofhigh

21. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 20 of28
humidity,waterwilltransferfromgasphase(air)tothefoodandthusincreaseswateractivityof
food,leadingtospoilagebymicroorganisms.Microorganismsalsoproducedwaterasanin
productofrespiration(metabolism).Thus,theyincreasesthewateractivity(aw)oftheirown
immediateenvironment.So,thateventuallymicroorganismsrequiringhighwateractivityareable
togrowandspoiledfood,whichwasinitiallyconsidertobemicrobiologicalsafe.Thestorageof
freshfruitsandvegetablesrequiredverycarefulcontrolofrelativehumidity.Ifitistoolowthan
manyvegetableswilllosewaterandbecomeflaccidorshrinkage.Ifitistoohighcondensation
mayoccurs.Hence,RHshouldbechosensuchthattherewillbeneithercondensationnor
desiccationi.e.foodandenvironmentshouldbeinequilibriumstate.Thereisalsorelationship
betweentemperatureandhumidity.i.e.higherthetemperaturelowerwillbetheRHandvice
versa.Thehighesthumiditynearsaturationisrequiredforbacterialgrowth,lessbyyeastandeven
lessbymold.Foodthatundergosurfacespoilagefrommold,yeastandcertainbacteriashouldbe
storedunderconditionoflowrelativehumidity(RH).Improperlywrappedfood(suchasmeat
item)sufferfromsurfacespoilageintherefrigeratorduetohigherRH.
2.Temperature:-Microbeshaveanoptimumtemperatureaswellasminimumandmaximum
temperaturesforgrowth.Theseareknownascardinaltemperaturesandare,toalargeextent,
characteristicofanorganism,althoughtheyareinfluencedbyotherenvironmentalfactorssuchas
nutrientavailability,pHandaw.Therefore,theenvironmentaltemperaturedeterminesnotonlythe
proliferationratebutalsothegeneraofmicro-organismsthatwillthriveandtheextentof
microbialactivitythatoccurs.Forexample,achangeofonlyafewdegreesintemperaturemay
favorthegrowthofentirelydifferentorganismsandresultinadifferenttypeoffoodspoilageand
foodpoisoning.Thesecharacteristicshavebeenresponsiblefortheuseoftemperatureasa
methodofcontrollingmicrobialactivity.Theoptimaltemperaturefortheproliferationofmost
micro-organismsisfrom15ºCto40ºC.However,manygeneraofmicrobesarecapableofgrowth
from0ºCto15ºCandotherevenmicro-organismswillgrowatsubzerotemperatures.Stillother
generawillgrowattemperaturesuptoandexceeding100ºC.Micro-organismscanbeclassified
intoseveralphysiologicalgroupsbasedontheircardinaltemperatures(accordingtotemperature
ofoptimalgrowthinclude):-
A.Thermophiles(high-temperature-lovingmicro-organisms),withgrowthoptimaat
temperaturesabove45ºC(e.g.,Bacillusstearothermophilus,Bacilluscoagulans,
andLactobacillusthermophilus).
B.Mesophiles(medium-temperature-lovingmicro-organisms),withgrowthoptima
between20ºand45ºC(e.g.,mostBactobacilliandStaphylococci).

22. FoodMicrobiology Page 21 of28
C. Psychrotrophs (cold-temperature-tolerant micro-organisms), which tolerate and
thrive at temperatures below 20ºC (e.g., Pseudomonas andAcinetobacter).
CardinalTemperaturesforMicrobialGrowth
Bacteria,moldsandyeastseachhavesomegenerawithtemperatureoptimaintherange
characteristicofthermophiles,mesophiles,andpsychrotrophs.Moldsandyeaststendtobeless
thermophilicthanbacteria.Astemperatureapproaches0ºC,fewermicro-organismscanthriveand
theirproliferationisslower.Astemperaturefallsbelowapproximately5ºC,proliferationofspoilage
micro-organismsisretardedasthegrowthofnearlyallpathogensceases.
3.GaseousAtmosphere:-Presenceofdifferentgasesanditsconcentrationmaysignificantlyaffects
thegrowthofmicroorganismsonthefoodi.e.surfacespoilageispreventedbyalteringgaseous
composition.Oxygenisoneofthemostimportantgaswhichaffectsbothfoodproductsaswellas
microorganisms.Oxygengaswhencomeincontactwithfood,influenceredoxpotentialoffood
andfinallythemicrobialgrowth.Carbondioxide(CO2)istheoneofmostimportantatmospheric
gasthatisusedtocontrolmicroorganismsinfoods.Thestorageoffoodinatmospherecontaining
increasedamountofcoupto10%isreferredas"controlledatmosphericstorage"(CAS).The
inhibitoreffectsofcoonmicrobialgrowthisappliedinmodifiedatmosphericpackingoffood
andfooddrinks.Moldandoxidativegramnegativebacteriaaremoresensitiveandgrampositive
bacterialikeLactobacillustendstomoreresistanttoCO2.TheinhibitoryeffectsofCO2increases
withdecreaseswithdecreaseintemperature.ThisisbecauseCO2ishighlysolubleatlower
temperature.SomemicroorganismsarekilledbyprolongexposuretoCO2.CO2whencomein
contactwithwater,formweakacidi.e.H2CO3(carbonicacid).H2CO3penetratetheplasma
membraneandaciditythecellsinterior.Ozone(O3)istheotheratmosphericgasthathas
antimicrobialproperties,andithasbeentriedoveranumberofdecadesasanagenttoextendthe
shelflifeofcertainfoods.Ithasbeenshowntobeeffectiveagainstavarietyofmicroorganisms,
butbecauseitisastrongoxidizingagent,itshouldnotbeusedonhigh-lipid-
Dr. Sujeet KumarMrityunjay

23. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 22 of28
contentfoodssinceitwouldcauseanincreaseinrancidity.Ozoneaddedtofoodasapreservative
actiononcertainfood.OzonehasgivenGRAS(generallyrecognizedassafe)statusinUS,
effectiverangeis1-5ppm.However,ithassomedemeritslikestrongoxidizingagents,causesthe
rancidityofhighlipidcontainingfood.Mouldsareaerobic,mostyeastgrowsbestaerobically,
andbacteriaofdifferentkindsmaybeaerobic,anaerobicorfacultative.Thebacteriawhichhave
theabilitytothriveintheabsenceofoxygenorfreeairareknownas'strict'or'obligate
anaerobes'.Theseorganismsdiewhenexposedtoairoroxygen.Butthereareonlyfewobligate
anaerobes.Manybacteriaarereferredthus,becausetheytolerateextremelylowlevelsofoxygen.
Thereisanothercategoryofbacteria,whichcansurviveeitherinthepresenceorabsenceof
oxygen.Theyareknownas“facultativeanaerobes”Theotherkindsofbacteriawhichcannot
surviveintheabsenceofoxygenarecalled'obligateaerobes'.Thereisyetanothercategoryof
bacteriaknownas'microaerophiles',whichrequireoxygen'oxygenforsurvivalalbeitatlow
concentrationsthanpresentinair.
IntrinsicFactorsThatInfluenceMicrobialGrowth:-Characteristicsofthefooditselfarecalled
intrinsicfactors.Theseincludenaturallyoccurringcompoundsthatinfluencemicrobialgrowth,
compoundsaddedaspreservatives,theoxidation-reductionpotential,wateractivity,andpH.
1.Nutrients
2.pH and bufferingcapacity
3.Redox potential
4.Wateractivity
5.Antimicrobialconstituents
6.Antimicrobialstructures
1.Nutrients:-Thekindsandproportionalofnutrientinfoodareallimportantindetermining
whichmicroorganismismostlikelytogrow.Microorganismscanusefoodsasasource
ofnutrientsandenergy.Fromthem,theyderivethechemicalelementsthatconstitute
microbialbiomass,thosemoleculesessentialforgrowththattheorganismcannot
synthesize,andasubstratethatcanbeusedasanenergysource.Theinabilityofan
organismtoutilizeamajorcomponentofafoodmaterialwilllimititsgrowthandputit
atacompetitivedisadvantagecomparedwiththosethatcan.Therefore,theabilityto
synthesizeAmylolytic(starchdegrading)enzymeswillfavorthegrowthofanorganism
oncerealsandotherfarinaceousproducts.Protein-richfoodlikemeat,egg,fishetc.are
alwaysspoiledbyproteolyticorganismbecausetheycanutilizeproteinasasourceof
energyifsugarisnotavailable.Infact,protein-richfoodpromotesmoregrowthof

24. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 23 of28
bacteriathenyeastandmould.Similarly,inthegeneralmouldcangrowinthehigher
concentrationofsugar,yeastinfairlyhighconcentrationbutmostbacteriagrowbestin
thelowconcentrationofsugar.
pHandbufferingcapacity:-Everyorganismhasaminimal,maximalandoptimalpH
forgrowth.SomeorganismcangrowbetteratlowpHoracidicpH,somecangrowin
alkalinepHandwhileothergrowatsomewhatneutralpH.pHinfluenceboththegrowth
rateandtypesoforganismthatwillpredominantthefood.Ingeneralyeastandmouldare
moreacidtolerantthanbacteria.Theacidityandalkalinityofanenvironmentaffects
growthandmetabolismofmicroorganismsastheactivityandstabilityof
macromolecules,enzymesandnutrienttransportisinfluencedbypH.Generally,bacteria
growfastatpH6-8.ButbacteriathatproduceacidshaveoptimumpHbetweenpH5and
6(Ex:LactobacillusandAceticacidbacteria).YeastgrowsbestatpH4.5-6.0andFungi
at3.5–4.0.InlowpHfoods(Ex.Fruits),spoilageismainlybyyeastsandfungithan
bacteria.FisheswithpHaroundneutrality(6.5-7.5)favorbacterialgrowthandspoil
rapidlythanmeat(pH:5.5–6.5).AbilityoflowpHtorestrictmicrobialgrowthhasbeen
employedasamethodoffoodpreservation(Ex:useofaceticandlacticacid).Buffering
capacityreferstotheabilityoffoodstowithstandpHchanges.Microorganismshave
abilitytochangepHofthesurroundingenvironmenttotheiroptimallevelbytheir
metabolicactivity.Decarboxylationofaminoacidsreleasesamineswhichincreases
surroundingpH.Deaminationofaminoacidsbyenzymedeaminizesreleaseorganic
acidscausingdeceaseinpH.Thus,proteinrichfoodslikefishandmeathavebetter
bufferingcapacitythancarbohydraterichfoods.
Redoxpotential(alsoknownasredoxpotential,oxidation/reductionpotential,ORP,pe,ε,
orEh):-Theoxidation–reduction(redox)potentialisanindicationoftheoxidizingand
reducingpowerofthesubstrate.Toattainoptimalgrowth,somemicroorganismsrequire
reducedconditionswhileothersneedoxidizedconditions.Thus,theimportanceofthe
oxidation-reductionpotentialisapparent.Aerobicmicro-organismsgrowmorereadily
underahighoxidation–reductionpotential(oxidizingreactivity).Alowpotential
(reducingreactivity)favorsthegrowthofanaerobes.Facultativemicro-organismsare
capableofgrowthundereithercondition.Micro-organismscanaltertheoxidation-
reductionpotentialoffoodtotheextentthattheactivityofothermicro-organismsis
restricted.Forexample,anaerobescandecreasetheoxidation–reductionpotentialto
suchalowlevelthatthegrowthofaerobescanbeinhibited.
2.
3.

25. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 24 of28
Redoxoroxidoreductionpotentialisdefinedasthesumofalltheoxidizing
(dissolvedoxygen,freeradicals,hydrogenperoxide,someoxidizedmetalions…)and
reducing(somevitamins,somereducedmetalions,thiol-containingmolecules,
hydrogen…)couplesfoundinthemedium.Thismeanswhentheconcentrationof
oxidizingmoleculesincreasestheredoxpotentialvalueofthemediumincreases
(determinedinmillivoltsorvolts)butwhentheconcentrationofthereducingmolecule
increasestheredoxvalueofthemediumdecreases.Forsimplifyingthetermofredox,we
cansaythatitexpressestheconcentrationofmobileelectrons(thatcanmovefrom
moleculetoanother)inthemedium,whereasthepHvalueexpressestheconcentrationof
protons(H+)inthemedium.
Howcantheredoxpotentialaffectthegrowthofmicroorganisms?
Torespondtothisquestionwemustkeepintheheadtherelationshipbetweenthe
oxygenasoxidantfoundintheatmosphereandtheredoxpotential.Whena
microorganismisfoundinaerobicmedium(containingdissolvedoxygen)itmeansthis
mediumisfavorableforthegrowthofaerobicmicroorganismswhichcanusetheoxygen
asafinalacceptorofelectronsproducedfromthesubstrateviathemetabolism.
Itisimportanttoindicateherealsothateverymicroorganismpossessesafavorable
valueofredoxforgrowth(thisbehaviorlikesthatfoundforpH,wateractivityand
temperature;andamicroorganismfoundinamedium“swims”toalocationwitha
favorableredoxvalue.Furthermore,itwasreportedthattheredoxpotentialvalueaffects
thepHintracellularofamicroorganism,whichformsacomponentoftheproton-motive
force.ThiseffectoftheredoxonthepHintracellularwasattributedtothechangeofthe
permeabilityofthecellularmembrane,whichincreasedbythedecreaseoftheredox
valueofthemedium.Otherstudiesreportedtheeffectofredoxpotentialonthesurface
propertiesofthecellularmembranesuchasthehydrophobicityandthecharacterof
donor/acceptorofelectrons.Theabilityofredoxpotentialofthemediumonthetoxin
productioncapacityandsporegerminationinsomemicrobialstrainswasalsoreported.It
isimportantheretoremindthatthebacterialmembraneshaveanessentialfunctionin
energyconservationasthelocationaboutwhichaprotonmotiveforceisgenerated.This
protonmotiveforce(alsocalledelectrochemicalgradient)formsasourceofenergyfor
bacteria.
Wecansummarizethemechanismoftheeffectofredoxpotentialonthegrowthof
microorganismsasfollows:

26. FoodMicrobiology Page 25 of28
4.
1)Itseffectonthestructuralcompositionofsomesensitive-components/molecules
foundonthesurfaceofthecell.Theseredox-sensitivecomponents/moleculescouldbe
enzymeslocatedinthesurfaceofthecell,whichitsproteinpart(Apoenzyme)couldbe
composedofsulfur-containingaminoacidsmakingitsensitivetotheredoxpotentialof
themedium.Furthermore,thecofactorsoftheseenzymessuchasFe,Zn,MgandCu
couldbefoundinoxidizedorreducedform,whichmeansthesusceptibilityofthese
metalionstotheredoxpotentialofthesubstrateor/andthemedium.
Bythesamemanner,wecandiscusstheeffectofredoxpotentialonseveralredox-
sensitivemoleculeslocatedinthesurfaceofthecellsuchasproteins,phospholipids,
saturatedandunsaturatedfattyacids,whichcouldbeaffecteddirectlybytheredox
potentialofthemedium.Themodificationinthestructuralorthecompositionofthese
moleculesaffectsdifferentcellularsystemssuchasthetransportandenergeticones.
2)Iftheenergeticsystemofthecell(protonmotiveforce)changestheATPcontentof
thecellwillchangewhichcanaffectmanyessentialfunctionsofthecell.
Moisturecontentorwateractivity(aw):-Microorganismhasanabsolutedemandfor
water,however,theexactamountofwaterneededforgrowthofmicroorganismsvaries.
Thisparameterhelpsustounderstandthemovementofwaterfromtheenvironmentto
thecytoplasmorfromcytoplasmtotheenvironment.Thewaterrequirementof
microorganismsisexpressedinphysicalform,calledwateractivity(aw).
Wateractivityistheratioofthevapourpressureofwaterpresentinfoodsubstrate(solution)to
thevapourpressureofpurewateratthesametemperature.Theunitofmeasurementfor
waterrequirementofmicroorganismisusuallyexpressedaswateractivity(aw).
Bacteria generally require higher value of aw than fungi
G –ve bacteria require higher aw than G +ve bacteria
Most spoilage bacteria do not grow at aw below 0.91
Spoilage molds grow at aw of0.80
Halophilic bacteria grow at aw of 0.75
Xerophilic and osmophilic yeasts grow at of0.61
Microorganismslikehalophiles,osmophilesandxerophilesgrowbetteratreduced
aw.Microorganismscannotgrowbelowaw0.60,andinsuchsituationsspoilage
offoodisnotmicrobiologicalbutduetochemicalreactions(Ex:oxidation).
Dr. Sujeet KumarMrityunjay
Relationship between aw, temperature andnutrition

27. FoodMicrobiology Page 26 of28
Growthofmicroorganismsdecreaseswithloweringofaw
Therangeofawatwhichthegrowthisgreatestoccursatoptimumtemperatureforgrowth
Thepresenceofnutrientsincreasestherangeofawoverwhichtheorganismscansurvive
Antimicrobialconstituents:-Thestabilityofsomefoodsagainstattackby
microorganismsisduetothepresenceofcertainnaturallyoccurringsubstancesthathave
beenshowntohaveantimicrobialactivity.Somespeciesareknowntocontainessential
oilsthatpossessantimicrobialactivity.Amongtheseareeugenolincloves,allicinin
garlic,cinnamicaldehydeandeugenolincinnamon,allylisothiocyanateinmustard,
eugenolandthymolinsage,andcarvacrol(isothymol)andthymolinoregano.Cow's
milkcontainsseveralantimicrobialsubstances,includinglactoferrin,conglutinin,andthe
lactoperoxidasesystem(seebelow).Rawmilkhasbeenreportedtocontainarotavirus
inhibitorthatcaninhibitupto10
6
pfu(plaqueformingunits)/mL.Itisdestroyedby
pasteurization.Milkcaseinaswellassomefreefattyacidshavebeenshowntobe
antimicrobialundercertainconditions.Eggscontainlysozyme,asdoesmilk,andthis
enzyme,alongwithconalbumin,providesfresheggswithafairlyefficientantimicrobial
system.Thehydroxycinnamicacidderivatives(p-coumaric,ferulic,caffeic,and
chlorogenicacids)foundinfruits,vegetables,tea,molasses,andotherplantsourcesall
showantibacterialandsomeantifungalactivity.Lactoferrinisanironbinding
glycoproteinthatisinhibitorytoanumberoffoodbornebacteria.Itsantimicrobial
activityisantagonizedbycitrate.Ovotransferrinappearstobetheinhibitorysubstancein
raweggwhitethatinhibitsSalmonellaenteritidis.Cellvacuolesofcruciferousplants
(cabbage,Brusselssprouts,broccoli,turnips,etc.)containglucosinolates,whichupon
injuryormechanicaldisruption,yieldisothiocyanates.
Antimicrobialstructures:-Thenaturalcoveringofsomefoodsprovidesexcellent
protectionagainsttheentryandsubsequentdamagebyspoilageorganisms.Inthis
categoryaresuchstructuresasthetestaofseeds,theoutercoveringoffruits,theshellof
nuts,thehideofanimals,andtheshellsofeggs.Inthecaseofnutssuchaspecansand
walnuts,theshellorcoveringissufficienttopreventtheentryofallorganisms.Once
cracked,ofcourse,nutmeatsaresubjecttospoilagebymolds.Theoutershelland
membranesofeggs,ifintact,preventtheentryofnearlyallmicroorganismswhenstored
undertheproperconditionsofhumidityandtemperature.Fruitsandvegetableswith
damagedcoveringundergospoilagemuchfasterthanthosenotdamaged.Theskin
coveringoffishandmeatssuchasbeefandporkpreventsthecontaminationandspoilage
5.
Dr. Sujeet KumarMrityunjay
6.

28. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 27 of28
ofthesefoods,partlybecauseittendstodryoutfasterthanfreshlycutsurfaces.Thesesix
intrinsicparameters(pH,Moisturecontent,Oxidation-reductionpotential(Eh),
Nutrientcontent,Antimicrobialconstituents,Biologicalstructures)representnature's
wayofpreservingplantandanimaltissuesfrommicroorganisms.
ImplicitfactorsThatInfluenceMicrobialGrowth(Physiologicalpropertiesof
microorganisms):-Theimplicitfactorsarethefactorsrelatedtothemicroorganismsthemselves,
includinginteractionsbetweenthemicroorganismscontaminatingthefoodandbetweenthese
microorganismsandthefood.
1.Specificgrowthrate:-
2.Mutualism:-Mutualismisdefinedasaninteractionbetweenindividualsofdifferentspecies
thatresultsinpositive(beneficial)effectsonpercapitareproductionand/orsurvivalofthe
interactingpopulations.Or,Mutualismisaninteractionbetweentwopopulationsthat
positivelyaffectsthefitnessofbothpopulations,wherethefitnessofapopulationisdefined
asthesuccessofapopulationinpropagatingitsgeneticmaterial.Digestivebacteriaand
humans-Humanbeingshavewhatareoftencalled"good"bacteriaintheirdigestive
systems.This"good"bacteriaexistsinordertohelpthehumantodigestfood.
3.Antagonism:-Antagonism,inecology,anassociationbetweenorganismsinwhichone
benefitsattheexpenseoftheother.
4.Commensalism:-Commensalismbenefitsthesymbiontwithoutsignificantlyaffectingthe
host.Thisisarelativelyraretypeofinteractionbecausefewhostscanbeconsideredtobe
completelyunaffectedbytheirsymbionts.Incommensalism,oneorganismbenefitswhilethe
otherisunaffected.Forexample,oneorganismcanprovideanessentialgrowthfactor,such
asavitamin,foranotherorganism.Thistypeofcross-feedingiscommoninsoilorganisms.
Theoppositeofcommensalismisamensalism,whereoneorganismisharmedwhiletheother
isunaffected.Agoodexampleofthisinteractioniswhenoneorganismproducesan
antibioticagainstanotherorganism.Suchaninteractionisoftenthebasisofbiological
control.Forexample,someisolatesofthebacteriumPseudomonasfluorescenscansuppress
thefungalpathogenGaeumannomycesgraminis,responsiblefor‘take-all’inwheat
(Triticumaestivum).
MicrobialAntagonisms:-Somemicroorganismscaninhibitothermicroorganismsorreducetheir
growthinmediumthankstotheirmetabolites,indirect(bychangingpH,osmoticpressureand
surfacetension)ordirect(byproducingtoxiccomponent,antimicrobialcomponent,bacteriocin,

29. Dr. Sujeet KumarMrityunjay
FoodMicrobiology Page 28 of28
antibioticetc.),thissituationiscalledasantagonisticrelation.Forexample,organicacidsproduced
bymicroorganismshaveantimicrobialeffectonsensitivemicroorganismstoacids.Alsosome
bacteriapreventgrowthofsomeotherbacteriainmediabyproducingmetabolitecalledas
bacteriocin.Themostintensivestudiesandcommercialpracticesaboutmicrobialantagonismshave
focusedonlacticacidbacteria.Inadditiontolacticacidbacteria,itwasreportedthatprobiotics,
yeasts,fungusandbacteriophageshaveantagonisticcharacteristics.