Staphylococcus aureus

(genus Staphylococcus)

Morphology
-Gram positive, non sporing, non motile cocci.
- Arranged in grape- like clusters (due to sequential division of bacteria in 3perpendicular planes with daughter cells in close proximity)

Cultural characteristics.
- Grow in ordinary media.
- Aerobic (facultatively anaerobic)
- Temp=10-42c
- pH7.4-7.6
- eg; nutrient agar, blood agar, MacConkey’s agar.

Biochemical reactions.
- Ferment mannitol
- Beta hemolysis on blood agar
- Gelatin liquefaction
- Phosphatase production
- Tellurite reduction

Antigenic structure
- Capsule = inhibit phagocytosis
- Peptidoglycans= activates complement, evokes production of inflammatory cytokines.
- Teichoic acid= antigenic determinant
- Protein A= when coated with IgG antiserum, will agglutinate if mixed with its corresponding antigen

Pathogenicity
- Due to production of toxin and enzymes.
o Toxins
§ Hemolysis
· Alpha lysine= lytic for rabbit erythrocyte, less active against human, causes injury to circulatory system and muscle tissues
· Beta lysine= llytic for sheep, not for human.
· Gamma lysine = acts on human, rabbit, sheep.
· Delta lysine= acts on human, rabbit, sheep.
§ Leucocidin= damages polymorphomuclear leucocytes and macrophages.
§ Enterotoxin= food poisioning, resistant to guts enzyme.
o Enzymes
§ Coagulase= clot human plasma, converts fibrinogen to fibrin.
-
- Dies at 62c for 30mins @ 80c for 1hr.
- Resistant to drying
- Local effect= inhibiting phagocytosis, form wall of fibrin clot around the lesion, pus is formed.
o Cutaneous infections= abcesses, boils, carbuncles.
o Deep infections= tonsillitis, osteomyelitis, sinusitis, pharyngitis, pneumonitis
o Food poisioning
o Toxic shock syndrome

Antibiotic sensitivity
- Resistance to panicilin= has panicilinase, reduction affinity to penicillin binding proteins

Laboratory diagnosis
- Collection of specimen
- Direct microscopy= gram staining-positive
- Culture
o Blood agar= colonies are circular, raised, opaque, produce golden yellow pigment(not diffusible into medium, beta hemolysis seen around colonies )
o Peptone water.

Treatment
- Benzyl Penicillin
- Cloxacillins
- Vancomycin

Coagulase negative staphylococci
- Form part of normal flora of the skin
- Opportunistic pathogen= infection in debilitated @ immunocompramised patients.
- Staph. Epidermis;
o Skin commensal
o Opportunistic pathogen in prosthetic devices
- Staph. Saprophyticus
o Causes urinary tract infection in young women
o Septicaemia and endocarditis in patients with cardiac surgery.