Staphylococci and streptococci

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1 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.

Transcript of Staphylococci and streptococci

Page 1: Staphylococci and streptococci

1Chapter 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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2Chapter No # 4Virulence 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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3Chapter No # 4

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4Chapter 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 strainso a –hemolysis – S. pneumoniae and others collectively called viridans

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5Chapter 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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6Chapter 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

Virulence Factors of b-hemolytic S. pyogenes

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7Chapter No # 4Extracellular 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: 18th March, 2017