Medical bacteriology: Pyogenic Cocci: Staphylococci. Streptococci Neisseriae species.
Staphylococci : Morphology: It is a Gram positive Cocci, one micrometer in diameter, arranged in clusters, non spore formers, and non-motile. Staph. aureus has a microcapsule and other virulence factors.
Cultural Characteristics: Staphylococcus species are facultative anaerobic bacteria. They can grow on nutrient agar and blood agar, forms large yellow to creamy colonies. S. aureus is often hemolytic on blood agar. Mannitol salt agar is a selective media from Stool; food poisoning. They can grow at a temperature range of 15-45 C, and NaCl concentration as high as 15%.
Cultural Characteristics: Cultural characteristics of Staph. aureus and Staph saprophyticus:
Mannitol fermentation test: Staphylococcus aureus ferments mannitol. Staphylococcusepidermidis is non- fermenter.
Biochemical activities: Staphylococcus are catalase positive bacteria. Staph.aureusproduces DNase, ferments mannitol, and glucose which converted to lactic acid. Staph.aureusliquefy gelatin and produce coagulase enzyme that coagulates plasma.
Staphylococcus aureus virulence factors. S.aureus has different virulence factors That play a role in pathogenesis process: 1- Enzymes that promote bacteria spread in tissues, such as leukocidin, hemolysins, kinases, and leukotoxins. 2- Surface proteins that inhibit phagocytosis: protein A, and Capsule. 3 - Biochemical proteins that enhance their survival in phagocytes, such as carotenoids,and catalase.
a 4- Coagulase enzyme that clotting plasma and protect bacteria from immune system. 5- Toxins: A- Exfoliative toxin (dermatotoxin): scaled skin syndrome. B- Toxic shock toxin-1: toxic shock syndrome. C- Enterotoxins: food poisoning.
The Streptococcus species: Gram positive cocci 0.5-1.0µm in diameter, occur in pairs and in short chains. They are catalase negative, non-motile, non- sporing, some strains are capsulated. They include strict anaerobes and facultative anaerobes. Facultative anaerobes are classified according to the type of hemolysis on blood agar (alpha, beta and none). It can be classified according to a carbohydrate antigen present in the cell wall (Lancefield, A-H and K-W).
Streptococcus species can be differentiated by hemolysis activity on blood agar. a
Types of hemolysis: Beta hemolysis: Streptococcus pyogenes and Streptococcus agalactiae
Alpha hemolysis Streptococci: Streptococcus pneumoniae, Streptococcus viridans