Bacteria SMG

Bacteria - General characteristics.

1. BACTERIA Dr. Saji Mariam George Associate Professor, Assumption College Autonomous Changanacherry

2. BACTERIA Unicellular, microscopic prokaryotes Highly adaptable Cosmopolitan in distribution Study of bacteria – Bacteriology Morphology – Size, shape, structure and arrangement.

3. Size of Bacteria Small, microscopic; 0.5 – 1 µm in diameter e.g. E.coli - 2 - 6 µm long and 1.1 – 1.5 µm wide. E.coli Image https://step1.medbullets.com/microbiology/104052/escherichia-coli

4. Epulopiscium fishelsoni 200 – 500 µm Image :https://alchetron.com/

5. Thiomargarita namibiensis 100 – 750 µm Image: https://www.sciencespacerobots.com/thiomargarita-namibiensis-61420191

6. Size Of Some Common Bacteria Bacteria Disease Length ( µm) Clostridium botulinum Food poisoning 3.8 C.tetani Tetanus 2-5 Mycobacterium tuberculosis Tuberculosis 0.5-4 Salmonella typhi Typhoid 0.5-4

7. Shape Of Bacteria Shape is governed by rigid cell wall. Bacteria vary in shape. i ) Coccus ( kokkos = berry pl. cocci ) – spherical Image : https://www.atsu.edu/

8. ii) Bacillus ( baculus = rod pl. ) – Straight rods Image : https://www.atsu.edu/

9. iii) Vibrio – Comma shaped, curved rods,vibratory motility Image : https://en.wikipedia.org/

10. iv) Spirillum (pl. Spirilla ) – Rigid rods that are helically curved. Images :https://www.gettyimages.in/

11. v) Spirochetes ( speira = coil, chaite = hair) flexuous spiral forms Imagehttps://microbiologyinfo.com/

12. vi) Acinomycetes or filamentous bacteria ( actis = ray, mykes = fungus) – branching filamentous

13. vii) Pleomorphic – variable in shape e.g. Arthrobacter Image :https://microbewiki.kenyon.edu/ Arthrobacter

14. viii) Mycoplasma – cell wall deficient , no stable morphology- round or oval or interlacing filaments. Image :https://www.akronbiotech.com/

15. Unusual shapes of Bacteria ix) Flat, square –to- rectangular boxes e.g. Walsby’s square bacterium x) Lobed spheres e.g. Sulfolobus xi) Disks arranged like stacks of coins e.g. Caryophanon xii) Rods with helically sculptured surfaces e.g. Seliberia Image https://en.wikipedia.org/ Haloquadratum

16. Arrangement Of Bacterial Cells Unicellular Bacteria may be found in characteristic cellular arrangements or groupings – characteristic of the particular species. Depends on the plane through which binary fission takes place the tendency of the daughter cells to remain attached even after division.

17. Arrangement Of Coccus Monococcus (Pl. monococci )- single cells Diplococcus – divide in one plane – in pairs- e.g. Neisseria Streptococcus – divide in one plane – linear chain. e.g. Enterococcus Tetracoccus –divide in 2 planes – square groups of 4 cells – tetrads. e.g. Micrococcus Sarcinae - divide in 3 planes - cubical packets of 8 cells.

18. Image: https://en.wikipedia.org/

19. Arrangement Of Bacillus Monobacillus - Most cells occur as single cells Diplobacillus - in pairs Streptobacillus - in chains like straws. e.g. Bacillus subtilis Trichomes – like chains but large area of contact between adjacent cells e.g. Saprospira Palisade arrangement – cells arranged side by side and at right angles to one another e.g. Corynebacterium diphtheriae .

20. Image: https://en.wikipedia.org/

21. Ultrastructure Of a Bacterial Cell The bacterial cell is prokaryotic. Electron microscopic studies revealed the ultrastructure of bacterial cells. A bacterial cell has a rigid cell wall, plasma membrane, cytoplasm, nucleoid etc. Image :https://www.brainkart.com/

22. Cell Wall Found outside the plasma membrane,10 - 20 nm thick, 10 – 40% of the dry weight of the bacterial cell. Made of peptidoglycan or mucopeptide or murein - A very large polymer composed of two sugar derivatives, N- acetyl glucosamine and N- acetyl muramic acid and a tetrapeptide . Vary in the chemical composition and structure from one species to another. Peptidoglycan give strength and rigidity to the cell wall.

23. Peptidoglycan Network Structure Image :https://www.creative-proteomics.com/

24. Cell wall of Gram + ve bacteria Thick - Several layers of peptidoglycans (60 to 90 %) Less amount of lipids (1 – 4 %) Teichoic acids present Basal body of flagellum consists of 2 rings Cell wall of Gram – ve bacteria Thin - A single layer of peptidoglycan (10 %) High amount of lipids (11 -22%) Teichoic acids absent Basal body of flagellum consists of 4 rings.

25. Functions Of Cell Wall Gives definite shape to bacteria Gives protection to all internal structures Cell wall is elastic and porous and freely permeable to salts and low molecular weight substances Protect the cells from toxic substances Cell wall gives protection from osmotic lysis (prevent the cell from expanding and eventually bursting because of uptake of water since most bacteria live in hypotonic environments).

26. Cell wall is essential for bacterial growth and division (by forming an in growth from the cell wall). Cell wall is the site of action of several antibiotics.

27. Gram Staining Cell wall is responsible for Gram staining reaction. Developed by Hans Christian Gram (1884) Gram staining – a differential staining procedure – differentiate bacteria into 2 groups. Gram positive - appear as purple blue e.g. Staphylococcus aureus ii) Gram negative - appear pink or red e.g. E. coli .

28. Procedure Of Gram Staining Prepare the bacterial smear, air dry and heat fix. Add the primary stain crystal violet , keep it for 1 minute, wash. Add mordant Gram’s Iodine, keep it for 1 minute, wash. Add decolouriser , Absolute alcohol for a few seconds (20 -40 sec.) , wash. Add counter stain, Safranin , keep it for 1 minute, wash. Blot dry and observe under the microscope.

29. The decolourising with Absolute alcohol generates the differential aspect . The property of being coloured purple blue (Gram + ve ) or pink or red (Gram – ve ) is associated with the structure and composition of the cell wall.

30. The cell walls of Gram + ve bacteria are thick (several layers of Peptidoglycan ). The peptidoglycan itself is not stained; but it acts as a permeability barrier preventing the loss of crystal violet. The Gram + ve cell walls have lower lipid content . During alcohol treatment, the alcohol extracts the lipid and increases the porosity . During alcohol treatment , the cell walls also get dehydrated and the pore size decreases and the permeability is reduced. Thus the crystal violet – Iodine complex is retained during the decolourisation step and the bacteria retain the purple blue colour .

31. The cell walls of Gram – ve bacteria are thin, have a single layer of peptidoglycan . The cell walls contain higher quantity of lipids. The alcohol treatment extracts the lipid, which results in increased porosity or permeability of the cell wall. Thus the crystal violet - Iodine complex is extracted and the Gram – ve bacteria are decolourised . These colour less cells subsequently absorb the counter stain Safranin and appear pink or red in colour .

32. Staphylococcus aureus - appear as purple blue - Gram positive E. coli - appear pink - Gram negative Image :https://en.wikipedia.org/

33. Capsule Jelly like or viscous outer covering seen outside the cell wall in some bacteria . e.g. Streptococcus pneumoniae , Klebsiella pneumoniae . Ability to form capsule is genetically determined. Secreted from the inner side of the cell which get firmly attached to the surface of the cell. Image : https://microbeonline.com/

34. Sticky in nature. Well organized and not easily washed off. 0.2 µm in width , contains 98% water and 2% solids (Complex polysaccharides - Klebsiella or polypeptides – Bacillus anthracis ) Capsulated bacteria are smooth where as non-capsulated are rough in appearance. Protect the bacteria from desiccation. Prevent the attachment of bacteriophages .

35. Flagella Bacteria may be motile or non- motile( Coccus forms). Motile bacteria have flagella – swimming motility (Bacillus and Spirilla forms) Long (up to 20 µm ), hair- like, helical appendage that protrude through the cell wall. Thinner (.01 – 0.02 µm) than the flagella of eukaryotes.

36. Image : https://www.easybiologyclass.com/

37. Flagella has 3 parts – basal body, hook and filament(Shaft) Location of flagella varies depending on the species – polar ( at one or both ends) or lateral(along the side) . Number varies depending on species (10-20/cell)

38. Types Of Bacteria Based On Number & Position Of Flagella Presence/absence ; number & arrangement of flagella is characteristic of different genera- useful for identification & classification. Atrichous – No flagella . (e.g. Coccus ) Monotrichous ( trichous = hair) – single polar flagellum (e.g. Vibrio cholerae ) Lophotrichous ( Lopho =tuft)- a cluster of polar flagella (e.g. Pseudomonas fluorescens )

39. iv) Amphitrichous ( Amphi =both ends)-single or cluster of flagella on both ends ( e.g. Aquaspirillum serpens ) v) Peritrichous ( Peri = around) - several lateral flagella (e.g. Escherichia coli )

40. Image :https://www.dummies.com/

41. Fimbriae or Pili Present mostly in Gram negative bacteria (e.g. Proteus , Salmonella ) Hollow, straight, hair- like structures on the surface of bacterial cells , extruding from the plasma membrane Occur in flagellate(motile) & non-flagellate (non-motile) bacteria – not involved in motility The number of fimbriae may be around 1000. Composed of protein sub units called fimbrilin . Help the bacteria to attach to their natural substrate

42. Pili Genetically controlled by plasmids Specialised fimbriae Number – 3 to 5 Made of pilin protein Help to make contact between donor bacterium and recipient bacterium during conjugation. Possess a hollow core and act like a conjugation tube through which genetic material is transferred from the donor bacterium to the recipient bacterium

43. Plasma Membrane Found just beneath the cell wall 7.5 cm thick Composed of phospholipids (20 – 30%) and proteins(60 – 70%) Does not contain sterols such as cholesterol (difference from eukaryotic Plasma membrane) – but many bacterial Plasma membranes contain pentacylic sterol - like molecules called hopanoids – stabilize the membrane.

44. Mesosomes – localised invaginations of plasma membrane in the form of lamellae, vesicles, tubules – more prominent in Gram + ve bacteria – principal sites of respiratory enzymes. (Respiratory enzymes are found throught out the plasma membrane). Form a link between plasma membrane and nuclear material. May be involved in chromosome replication and distribution of DNA to daughter bacterial cells.

45. Mesosomes may be involved in septum formation during binary fission. Plasma membrane contains special receptor molecules – help to detect and respond to chemicals in their surroundings. Image :https://link.springer.com/

46. Cytoplasm Portion of protoplast in between plasma membrane and nucleoid . 70 % is water. Colloidal system of a variety of organic and inorganic solutes in a viscous watery solution. No cyclosis ( cytoplasmic streaming) unlike cytoplasm of eukaryotes. Endoplasmic reticulum, mitochondria, golgibodies etc. are absent.

47. Ribosomes 70 S type. Reserve food materials (storage granules or inclusion bodies) – characteristic for different species and depend on the age and conditions of culture. Include Volutin granules – inorganic – polyphosphate-reserve source of phosphate Polyß – hydroxybutyrate (ß – hydroxybutyrate molecules joined by ester bonds) – reserve source for carbon and energy Glycogen – polymer of glucose – source of carbon.

48. Nucleoid Bacterial cell is prokaryotic – no true nucleus – DNA is not enclosed in a nuclear envelope Nucleolus is absent Single, circular dsDNA - bacterial chromosome – in the centre – haploid Replicates by simple fission Image :https://courses.lumenlearning.com/

49. Plasmids Extrachromosomal DNA , small, circular, in the cytoplasm of some bacteria – tools in genetic engineering - vectors Self replicating Contain only a few genes – less than 30- useful , not essential to the survival Image :https://www.whatisbiotechnology.org/

50. Most Bacteria have only one chromosome under normal circumstances , but may contain – 1 or 2 (Single copy plasmid or low copy number or stringent plasmid) 100 or more copies ( Multicopy plasmid or high copy number plasmid or relaxed plasmid ) of a given plasmid. Types of plasmids based on mode of existence and spread : Episome - A plasmid that can exist either with or without being integrated into the bacterial chromosome. Conjugative plasmids (Transmissible ) – They have tra genes for pili and can transfer copies of themselves to other bacteria during conjugation. Non- conjugative plasmids – These are plasmids which can not transfer themselves but can be transduced by co -resident tra plasmids.

51. Types Of Plasmids i ) Fertility (F) factor Plays a role in conjugation There may be 1 to 3 F factors in a cell. F factor bears genes for cell attachment and plasmid transfer between specific bacterial strains during conjugation. Most of the information required for plasmid transfer is located in the tra operon which contains about 21 genes – many of these direct the formation of sex pili that attach the F + cell to an F - cell – Other gene products aid in DNA transfer. F factor also has several segments called insertion sequences that assist in plasmid integration into the host cell chromosome. Thus the F factor is an episome that can exist outside the bacterial chromosome or be integrated into it. When the integration is relatively stable, the cell in which it has occurred, gives rise to a clone of cells which are known as Hfr (High frequency recombination) strains.

52. ii)Resistant (R) factors These plasmids confer antibiotic resistance to the bacterial cell They have genes that code for enzymes capable of destroying or modifying antibiotics. These plasmids are not usually integrated into the host chromosome. Many R- factors are conjugative so that they can spread through out a population.

53. . iii) Col plasmids (Col factors) These plasmids are bacteriocinogenic factors that determine the formation of bacteriocins which are proteins that kill the same or other closely related species of bacteria. Often kill cells by forming channels in the plasma membrane, thus increasing permeability. They also may degrade DNA and RNA or attack peptidoglycan and weaken the cell wall. The bacteriocins of E.coli are called colicins . Bacterial strains producing bacteriocins are resistant to their own bacteriocin which helps interspecies typing. Some col plasmids are conjugative.

54. iv) Virulence plasmids These plasmids make their hosts more pathogenic because the bacterium is better able to resist the host defense or to produce toxins. e.g. Enterotoxigenic strains of E.coli cause traveler’s diarrhoea because of a plasmid that codes for an enterotoxin .

55. vi) Metabolic plasmids They carry genes for enzymes that degrade substances such as aromatic compounds (Toluene), pesticides (2,4, D - 2,4, dichlorophenoxy acetic acid) and sugars. Some plasmids help bacteria to make use of unusual food sources such as camphor or petroleum. Metabolic plasmids even carry the genes required for some strains of Rhizobium to induce legume nodulation and carry out nitrogen fixation.

56. Nutrition In Bacteria Most of the bacteria are heterotrophic in nutrition – they depend on other sources for their food. Parasitic bacteria These bacteria derive nutrition from other living organisms (hosts) such as plants and animals. b) Saprophytic bacteria These bacteria obtain food from dead organic matter. They secrete some enzymes , with the help of which decompose complex organic substances into simple substances and utilize a part of the released energy to run their metabolism.

57. c) Symbiotic bacteria Some bacteria live in close association with other organisms and both partners derive mutual benefits. e.g. Rhizobium seen in the root nodules of leguminous plants fix free Nitrogen of the atmosphere into nitrates in the soil for plants and in return the plants provide them protection and carbohydrates.

58. Some bacteria are autotrophic in nutrition. They are capable of synthesizing their own food from inorganic substances. On the basis of energy source, autotrophic bacteria can be divided into phototrophs and chemotrophs . Phototrophs (Photosynthetic bacteria) These bacteria are capable of doing photosynthesis . They use light as the source of energy and Carbon dioxide as the source of carbon. e.g. Green sulphur bacteria – Chlorobium limicola Purple bacteria - Rhodopseudomonas viridis .

59. But unlike plants, photosynthetic bacteria can not use water as an electron donor. Hence , Oxygen is not evolved as a byproduct in bacterial photosynthesis ( Anoxygenic photosynthesis). The pigments in photosynthetic bacteria are bacteriochlorophylls ( Bacteriochlorophylls a, b, c, d, e ).

60. Chemotrophs (Chemosynthetic bacteria) They obtain energy from inorganic substances like ammonia, nitrite, nitrate, ferrous iron, hydrogen sulphide etc. for the synthesis of their food. Examples : Nitrifying bacteria Nitrosomonas – oxidise ammonia into nitrites Nitrobacter – oxidise nitrites into nitrates.

61. ii) Sulphur bacteria The sulphur bacterium Beggiatoa oxidise Hydrogen sulphide to elemental sulphur and it is deposited as granules in the cytoplasm. iii) Iron bacteria These bacteria (e.g. Ferrobacillus ) oxidize ferrous compounds into ferric forms and the energy released in this process is utilized for the synthesis of organic compounds.

62. Oxygen Requirement Depending on the Oxygen requirement, bacteria can be classified into different groups. Obligate aerobes e.g. Nitrobacter ii) Obligate anaerobes Strict or obligate anaerobes can grow only in the absence of oxygen. Oxygen is toxic for them and die in its presence (Stringent anaerobes) e.g. Bacteroides gingivalis Some bacteria can tolerate low levels of Oxygen (Non stringent or tolerant anaerobes).

63. iii) Facultative anaerobes These bacteria can grow in the presence or absence of Oxygen. They do not require Oxygen for growth, but may use it for energy production if it is available. iv) Microaerophiles These bacteria require low levels of Oxygen below the range of 2 to 10 % for growth They can not tolerate the level of Oxygen present in air.

64. Reproduction In Bacteria a) Binary Fission Common method of multiplication in bacteria A single bacterial cell divides into 2 daughter cells by a constriction in the middle First, cell elongates , DNA replicates, division of cytoplasm ( cytokinesis ) – 2 identical daughter cells A very rapid process. Under favourable conditions, binary fission may occur in every 20 -30 minutes.

65. b) Budding Some bacteria have semirigid extensions of the cell wall called prosthecae (Singular : Prostheca ) . Some prosthecate bacteria may form a new cell or bud at the end of a prostheca . The bud first appear as a small protrusion at a single point and enlarges to form a mature cell. The new cell is often smaller than its parent cell e.g. Hyphomicrobium .

66. Genetic Recombination In Bacteria In Bacteria, genetic recombination can takes place in three ways viz. Conjugation, Transduction and Transformation. Conjugation – (Lederberg & Tatum 1946)- direct transfer of DNA between two bacteria temporarily in physical contact.

67. Random collision of bacteria - sex pilus projects from the surface of the donor cell and the tip of the pilus attaches to the surface of the recipient cell. The pilus is retracted into the donor to draw the two cells together until direct contact is made. DNA passes into the recipient cell either through the sex pilus (hollow cored tube – conjugation tube) or through special conjugation bridges formed upon contact.

68. Conjugation

69. Types Of Conjugation F + x F - type The transfer of genetic material is from the donor strain (has sex factor or fertility factor or F factor, designated as F+ , 10 – 20 genes for pilus formation & plasmid transfer) to recipient strain(F-). F + donor replicates its sex factor (F – factor) and one copy is transferred to the recipient cell (F- strain). At the end of conjugation, F - strain is transformed into an F + strain. The transfer of F factor is independent of the transfer of chromosomal genes and the frequency of recombination is low (one recombinant in 10 4 to 10 5 cells) and the transfer of F factor is high (100 %).

70. Conjugation F + x F - type Image : http://bio3400.nicerweb.com/ ,

71. Hfr x F - Conjugation Most efficient natural mechanism of gene transfer between bacterial cells. In some bacteria, the F – factor may get integrated with the bacterial chromosome – episome - such a strain of bacteria takes part in recombinations at a higher frequency – Hfr (High frequency recombination ) strain. During conjugation, Hfr chromosome starts replicating at the point of insertion of the F factor – the initial break is within the plasmid.

72. Since the F factor can integrate in different positions of the Hfr bacterial chromosome, the first genes to enter a F - cell will vary with different Hfr strains. The Hfr chromosome is transferred to the F – cell in a linear fashion. Since only a part of the F factor is transferred at first , the F – recipient cell does not become F + unless the whole chromosome is transferred.

73. Transfer takes place in about 100 minutes in E. coli and the connection usually breaks before the process is completed. The F factor is usually not transferred fully so that the F - cell remains as F - at the end of conjugation. The frequency of recombination is very high The transfer of F factor is low.

74. Hfr x F - Conjugation

75. Sexduction (Jacob & Wollman ) - The bacterial genes are transmitted from donor to recipient as part of sex factor. The Hfr cells can revert to the F + state → F factor is released from the bacterial chromosome – resumes its autonomous replication.

76. Transduction Discovered by Lederberg and Zinder (1952) Process of genetic recombination between bacteria mediated by a temperate bacteriophage ( Lysogenic phage) [ e.g. Lambda phage, P1] Temperate phage do not lyse the host bacterial cell and their nucleic acid get integrated with the host bacterial chromosome in a non- infectious stage – prophage . Sometimes, the phage DNA may get excised from the host bacterial chromosome and enter into a lytic life cycle – prophage induction – start to replicate rapidly .

77. Occasionally error may occur during assembly of the phage components to form mature phages. As a result, phage particles may become filled with bacterial chromosomal DNA or a mixture of chromosomal and phage DNA. Such aberrant phages can attach to other bacteria and introduce bacterial DNA into them. Thus, in transduction, a bacteriophage is serving as a vector, transferring a portion of bacterial DNA (a few genes) from one bacterium to another.

78. Transduction Image https://www.facebook.com/drshivika.micro/photos/specialized-transduction/1062378443914269/

79. Types Of Transduction Generalized Transduction All fragments of bacterial DNA ( i.e , from any region of bacterial chromosome ) have a chance to enter a transducing phage Any chromosomal gene of a bacterium may be transduced at random by a phage. As the phage begins the lytic cycle, viral enzymes hydrolyse the bacterial chromosome into many small pieces of DNA.

80. During the assembly stage, when the viral chromosomes are packaged into protein capsids , any part of the bacterial chromosome may be incorporated into the phage capsids by mistake. Since the capsid can contain only a limited quantity of DNA , the viral DNA may be left behind. The quantity of bacterial DNA carried depends primarily on the size of the capsid . e.g. The P 22 phage of Salmonella typhimurium usually carries about 1% of the bacterial genome . The P1 phage of E.coli carries about 2 to 2.5% of the genome. The frequency of such abnormal phages are extremly low.

81. When such a defective transducing phage attaches to a new (recipient) bacterium, injects its DNA into the bacterial cell just as the normal phage infection, but does not initiate a lytic cycle. Instead, it may recombine with the homologous region of the bacterial chromosome. This phage is known as a generalized transducing phage and the process is generalized transduction.

82. Image :http://www.discoveryandinnovation.com/BIOL202/notes/lecture15.html

83. 2. Specialised Transduction (Restricted Transduction) The transducing phage carries only specific portions of the bacterial genome. It is made possible by an error in the lysogenic life cycle. Following prophage induction, excision of prophage genome occurs which is usually precise in producing normal phages. Some times, an imprecise excision occurs and the excised prophage carries along portions of the bacterial chromosome (about 5 to 10 % of the bacterial DNA) adjacent to the integration (insertion ) site while leaving behind some phage genes. Thus the genome of the transducing phage is usually defective and lacks some part of its attachment site.

84. It can infect another bacterium and transfer the defective gene set but the genes can not code for a lytic cycle. Instead, they integrate into the bacterial chromosome carrying the bacterial genes with them. Thus the recipient bacterium has acquired genes from the original bacterium and the recipient is transduced .

85. e.g. The phage Lambda of E.coli , inserts the genome into the host bacterial chromosome at specific locations known as attachment sites. The location of the lambda prophage in the bacterial chromosome is always between the bacterial genes for galactose metabolism, gal and bio. When ever the phage genome comes out of or is excised from the bacterial chromosome, it some times take with it gal or bio genes .

86. Such phage particles are called Lambda dgal because they carry galactose utilising genes. When phage carrying gal and bio genes infect a new host bacterium, recombination with the gal or bio genes of the host can occur. Specialized Transduction is very rare because genes do not easily break of from the bacterial chromosome.

87. Transformation Transformation is the direct uptake of cell free or ‘naked DNA’ molecule or a fragment by a bacterial cell from the medium and incorporation of it into the recipient chromosome in a heritable form. Transformation results in the expression of new genes.

88. When bacteria lyse , they release considerable amount of DNA into the surrounding environment. Some bacteria have unique ability to pick up additional DNA fragments from a medium. e.g. Bacillus subtilis , Haemophilus influenzae . After DNA entry into a cell, one strand is immediately degraded by deoxyribonucleases , while the other strand undergoes base pairing with a homologous portion of the recipient cell chromosome and becomes integrated into the recipient DNA.

89. Since complementary base pairing takes place between one strand of the donor DNA fragment and a specific region of the recipient chromosome, only closely related strains of bacteria can be transformed. Gene transfer by Transformation is an important means of genetic exchange in bacteria.

90. Transformation Transformation in bacteria was discovered by Griffith (1928) in the Pneumonia causing bacterium, Streptococcus pneumoniae ( Pneumococcus ). Image : https://en.wikipedia.org/wiki/Frederick_Griffith

91. Griffith’s Experiment There are two strains of Pneumococci , such as the capsulated, smooth, virulent ‘S’ strain which causes pneumonia and the non-capsulated , rough , avirulent ‘R’ strain which are harmless. Image: https://www.memorangapp.com /

92. The virulence is largely due to its polysaccharide capsule, which protects it from the host’s immune system. The composition of the polysaccharides defines the serotype, and about 91 pneumococcal serotypes have been found. A German bacteriologist, Fred Neufeld had discovered three pneumococcal types viz. Type I, II, and III- using immunological techniques. In the presence of type I antiserum, type I pneumococci would swell, likewise types II and III in the presence of their specific antisera . (Neufeld called this the Quellung reaction, after the German word for swelling.)

93. Griffith injected mice with live Type III S strain Pneumococci – mice died and pathogenic ‘S’ strain bacteria were isolated from the animal’s tissues. When live Type II ‘R’ strain Pneumococci were injected, the mice were healthy. When heat killed Type III ‘S’ strain were injected to the mice, they also were healthy. When a mixture of live, harmless Type II ‘R’ strain and heat killed pathogenic Type III ‘S’ strain was injected to the mice, they died and Griffith isolated live harmless Type II ‘R’ strain as well as live pathogenic Type III ‘S’ strain.

94. This experiment led to the discovery of Transformation in bacteria. In this experiment, when a mixture of live Type II ‘R’ strain and heat killed Type III ‘S’ strain were injected to mice, some of the harmless Type II ‘R’ strain bacteria had been transformed into pathogenic Type III ‘S’ strain. Griffith also showed that the ‘transforming principle’ could be passed from the transformed cells to their progeny and thus had the characteristics of a gene.

95. Endospores Endospores are asexual spores formed in certain species of Bacteria during unfavourable environmental conditions like starvation, desiccation , extreme temperature, chemical disinfectants etc. e.g. Bacillus subtilis , B. anthracis , Clostridium tetani , C. botulinum etc. They are thickwalled , highly resistant dormant stage of Bacteria. Heat resistance is due to the presence of large amount of dipicolinic acid in combination with Calcium, protoplast dehydration etc. Each species has its own characteristic size, shape and position of the spore and is of considerable value in identification.

96. Endospores may be spherical , ellipsoidal or cylindrical and may be located in central, subterminal or terminal position. During sporulation , each vegetative cell of bacteria forms only one endospore and during germination, each spore gives rise to only one vegetative cell. There fore , sporulation in bacteria is not a method of reproduction. Endospore formation is an ecological adaptation in some bacteria to tide over unfavourable conditions.

97. Bacillus bacterial cells stained with Schaeffer-Fulton endospore stain. Vegetative cells are stained pink, endospores are stained green © CNX OpenStax   CC BY 4.0

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