Staphylococcus – summary

One of the major cause of mortality & morbidity
Infectious range from relatively minor skin & soft tissue infections to life threatening systemic infections

Morphology & Taxonomy

Solid media – readily grown, 10°C-42°C ( optimum 37°C )
- Nutrient agar – circular, convex, smooth, shiny, opaque colonies
  - Golden yellow pigment by most strains
  - On nutrient agar slope – oil paint appearance
- Blood agar – hemolytic especially under 20-25% CO2
- MacConkey agar – smaller pink colonies ( due to lactose fermentation )
Liquid media – uniform turbidity
Selective media – for isolation of S. aureus
- Salt milk agar, salt broth – 8-10% NaCl
- Ludlam medium, Baird Parker agar – Lithium Chloride, tellurite
- Polymixin

S. aureus is commensal as well as opportunistic pathogen
Carriers may be present
Person to person transfer also happens
Most common cause of surgical wound infections, health care associated infections
MRSA – Methicillin resistant S. aureus

Primary response – PMNs
For infections – through damaged skin, mucus membranes
For intoxications – by Bacterial toxins produced invitro or in infectied host
Virulence factors :-
- Cell wall associated factors – peptidoglycan, Teichoic acid, capsular polysaccharide ; Protein A, bound coagulase
- Extracellular enzymes – coagulase, lipid hydrolases, hyaluronidase, DNAase
- Toxins – alpha hemolysin ( inactive at 70°C, active at 100°C ), ß hemolysin ( hot cold phenomena – hemolysis starts at 37°C but evident after cooling ), gamma hemolysin, Delta hemolysin
- PVL ( Panton Valentine leucocidin ) – 2 components ( S,F ), associated with CA-MRSA
- Enterotoxin – superantigen, symptoms after 2-6 hrs of consumption, relatively heat resistant, 8 antigenic types, action on CNS rather than GI mucosa
- TSST ( Toxic shock syndrome toxin ) – 1&2, superantigen
- Epidermolytic toxin – causes SSS- staphylococcal scalded skin syndrome
Evasion of host response by – antiphagocytic polysaccharide capsule, zwitterionic S. aureus capsule, CHIPS ( Chemotaxis inhibitory protein of Staphylococci, capacity of intracellular survival

Skin & soft tissues – folliculitis, abscess, furuncle, carbuncle, cellulitis, impetigo, Mastitis, Surgical wound infections
Musculoskeletal – septic arthritis, osteomyelitis ( hematogenous or direct spread ), pyomyositis, Psoas abscess
Respiratory – pneumonia, Septic pulmonary thrombi, empyema
Bacteremia – sepsis, septic shock, metastatic foci, infective endocarditis ( native valve, prosthetic valve )
Device related
Invasive – necrotizing fasciitis, Waterhouse friedrichsen syndrome ( adrenal failure due to adrenal hemorrhage ), necrotizing pneumonia, purpura fulminans
Urinary tract infections
Toxin mediated – Food poisoning, Toxic shock syndrome, SSS

Microscopy – gram +ve cocci
Culture
Biochemical tests
- Coagulase tests – tube coagulase ( based on free coagulase ), slide coagulase ( based on bound coagulase )
- Catalase test + ve
- MR, VP +ve
- Indole -ve
Antibiotic susceptibility tests
Serological tests
Molecular dignosis

For parenteral therapy for serious infections
- Sensitive to penicillin – Penicillin G
- Sensitive to Methicillin – Oxacillin/ Nafcillin
- Methicillin resistant – Vancomycin; Daptomycin for infective endocarditis, bacteremia, complications
- Methicillin resistant & intermediate resistance to Vancomycin – Daptomycin
- Resistance unknown/ Empirical – Vancomycin, Daptomycin
For skin & soft tissue infections –
- Dicloxacillin cephalexin/ cegadroxil ( for Methicillin sensitive )
- Clindamycin, cotrimaxazole, Mini/ Doxycycline, linezolid/ tedizolid ( for Methicillin resistant )

Check out other bacteria too :-

Pneumococcus- summary
Vibrio- summary
Haemophilus- summary
Bordetella- summary
Brucella- summary