Staphylococci and streptococci
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7Chapter No # 4 Staphylococci And StreptococciClassification. 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.
Virulence factorsStaphylococci 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.
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. b-hemolysis A,B,C,G and some D strains a hemolysis S. pneumoniae and others collectively called viridans
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. Beta-hemolysis (-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 (-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. pyogenesProduces 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. pyogenesExtracellular 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 factorVirulence Factors of b-hemolytic S. pyogenesExtracellular 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 untreatedImpetigo (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 systemicThroat infectionsStreptococcal pharyngitis strep throatRheumatic fever Associated carditis, arthritis, chorea and feverAcute 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.TreatmentsThis bacterium remains acutely sensitive topenicillin. Failure of treatment with penicillin is generally attributed to other local commensal organisms producing-lactamase, or failure to achieve adequate tissue levels in the pharynx. Certain strains have developed resistance tomacrolides,tetracyclines, andclindamycin. Prepared By Amjad Khan Afridi Date: 18th March, 2017