Staphylococci and streptococci

Staphylococci and streptococci

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Chapter No # 4
Staphylococci And Streptococci
Classification.
Family Micrococcaceae,
Genus Staphylococcus.
According to the contemporary classification, staphylococci are subdivided into more then 30 species.
Among them: S. aureus, S. epidermidis, and S. saprophyticus, S. haemolyticus, S. capitis, S. hominis, S.
warneri, S. xylosus etc.
Morphology. Staphylococci are spherical in shape, 0.8-1 mcm in diameter, and form irregular clusters
resembling bunches of grapes. In smears from cultures and pus the organisms occur in short chains, in
pairs, or as single cocci. Large spherical (L-forms) or very small (G-forms) and even filterable forms may
be seen in cultures which have been subjected to various physical, chemical, and biological (antibiotics)
factors.

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Chapter No # 4
Virulence factors
Staphylococci express many cell surface-associated and extracellular proteins that are potential virulence
factors. For the majority of diseases caused by this organism, pathogenesis is multifactorial. Thus it is
difficult to determine precisely the role of any given factor. This also reflects the inadequacies of many
animal models for staphylococcal diseases.

Other Extracellular Proteins. Coagulase is an extracellular protein which binds to prothrombin in the
host to form a complex called staphylothrombin. The protease activity characteristic of thrombin is
activated in the complex, resulting in the conversion of fibrinogen to fibrin. This is the basis of the tube
coagulase test, in which a clot is formed in plasma after incubation with the S aureus broth-culture
supernatant. Coagulase is a traditional marker for identifying S aureus in the clinical microbiology
laboratory.
Enzymes. S aureus can express proteases, a lipase, a deoxyribonuclease (DNase) and a fatty acid
modifying enzyme (FAME). The first three probably provide nutrients for the bacteria, and it is unlikely
that they have anything but a minor role in pathogenesis. However, the FAME enzyme may be important
in abscesses, where it could modify anti-bacterial lipids and prolong bacterial survival. The thermostable
DNase is an important diagnostic test for identification of S aureus.

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Chapter No # 4

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Chapter No # 4
Laboratory diagnosis. Test material may be obtained from pus, mucous membrane discharge,
sputum, urine, blood, foodstuffs (cheese, curds, milk, pastry, cakes, cream, etc.), vomit, lavage fluids, and
faeces.
The material is examined for the presence of pathogenic staphylococci. Special rules are observed
when collecting the material since non-pathogenic strains are widespread in nature.
Treatment. Staphylococcal diseases are treated with antibiotics (penicillin,
phenoxymethylpenicillin, tetracycline, gramicidin, etc.), sulphonamides (norsulphazol, sulphazol, etc.),
and antistaphylococcal gamma-globulin.
General Characteristics of Streptococci
 Gram-positive spherical/ovoid cocci arranged in long chains; commonly in pairs
 Non-spore-forming, nonmotile
 Can form capsules and slime layers
 Facultative anaerobes
 Do not form catalase, but have a peroxidase system
 Most parasitic forms are fastidious and require enriched media.
 Small, nonpigmented colonies
 Sensitive to drying, heat and disinfectants
 25 species

Streptococci
 Lancefield classification system based on cell wall Ag – 17 groups (A,B,C,….)
 Another classification system is based on hemolysis reactions.
o b-hemolysis – A,B,C,G and some D strains
o a –hemolysis – S. pneumoniae and others collectively called viridans

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Chapter No # 4

Haemolysis of Streptococci and its types with examples
Hemolysis is the breakdown of red blood cells (RBC). A substance that causes hemolysis is a hemolysin.

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Chapter No # 4
 Beta-hemolysis (β-hemolysis) is associated with complete lysis of red cells surrounding the
colony. Beta hemolysis is caused by two hemolysins O and S; the former is inactive in the
presence of oxygen. Thus, stabbing of the plate increases the intensity of the hemolysis
reaction. S is an oxygen-stable cytotoxin.
 Alpha-hemolysis (α-hemolysis) is a partial or “green” hemolysis associated with reduction of red
cell hemoglobin. Alpha hemolysis is caused by hydrogen peroxide produced by the bacterium,
oxidizing hemoglobin to green methemoglobin.
 Gamma-hemolytic (Non-haemolytic) Streptococci Colonies show neither typical alpha nor beta
haemolysis. There may be, however, slight discoloration in the medium. The streptococci
included in this group are usually not pathogenic.
Human Streptococcal Pathogens
 S. pyogenes
 S. agalactiae
 Viridans streptococci
 S. pneumoniae
 Enterococcus faecalis
b-hemolytic S. pyogenes
 Most serious streptococcal pathogen
 Strict parasite
 Inhabits throat, nasopharynx, occasionally skin
Virulence Factors of b-hemolytic S. pyogenes
Produces surface antigens:
 C-carbohydrates – protect against lysozyme
 Fimbriae - adherence
 M-protein – contributes to resistance to phagocytosis
 Hyaluronic acid (nonsulfated glycosaminoglycan) capsule – provokes no immune response

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Chapter No # 4
Virulence Factors of b-hemolytic S. pyogenes
Extracellular toxins:
 streptolysins – hemolysins; streptolysin O (SLO) and streptolysin S (SLS) – both cause cell and
tissue injury
 pyogenic toxin (erythrogenic) – induces fever and typical red rash
 superantigens – strong monocyte and lymphocyte stimulants; cause the release of tissue necrotic
factor
Virulence Factors of b-hemolytic S. pyogenes
Extracellular enzymes
 streptokinase – digests fibrin clots
 hyaluronidase – breaks down connective tissue
 DNase – hydrolyzes DNA
Epidemiology and Pathogenesis
 Humans only reservoir
 Inapparent carriers
 Transmission – contact, droplets, food, fomites
 Portal of entry generally skin or pharynx
 Children predominant group affected for cutaneous and throat infections
 Systemic infections and progressive sequelae possible if untreated
Impetigo (pyoderma) – superficial lesions that break and form highly contagious crust; often occurs in
epidemics in school children.
Erysipelas – pathogen enters through a break in the skin and eventually spreads to the dermis and
subcutaneous tissues; can remain superficial or become systemic
Throat infections
Streptococcal pharyngitis – strep throat
Rheumatic fever – Associated carditis, arthritis, chorea and fever
Acute glomerulonephritis – nephritis, increased blood pressure, occasionally heart failure; can become
chronic leading to kidney failure
Chorea: a neurological disorder characterized by jerky involuntary movements affecting especially
the shoulders, hips, and face.
Treatments
This bacterium remains acutely sensitive to penicillin. Failure of treatment with penicillin is generally
attributed to other local commensal organisms producing β-lactamase, or failure to achieve adequate
tissue levels in the pharynx. Certain strains have developed resistance to macrolides, tetracyclines,
and clindamycin.
Prepared By Amjad Khan Afridi Date: 18
th
March, 2017