Staphylococci

Gram positive cocci that:

• Typically occur in bunches
• Culture well on enriched media
  e.g. Blood.
• All produce catalase
  Key distinguishing feature from Streptococci.
• Grouped by their ability to produce coagulase
  Enzyme which cleaves fibrinogen and leads to clot production. This clot also covers the bacteria in a film of clot, which protects them from immune detection and subsequent phagocytosis.
  • Coagulase positive
    Confined to S. Aureus.
  • Coagulase negative
    All the rest.

Staphylococcus Aureus

Common skin commensal that:

• Causes illness by local infection and toxin production
• Generally requires host compromise to cause infection:
  • Skin breaks
  • Foreign body
  • Immunosuppression
• May lead to infection in:
  • B
    • HAP
      Classically necrotising.
  • C
    • Infective endocarditis
      May be associated with local abscesses if related to injection.
    • Invasive vascular devices
      • CLABSI
      • Pacemaker
  • D
    • Osteomyelitis
    • Discitis
    • Epidural abscess
  • E
    • Skin
    • Soft tissue infections
    • Joint prostheses

Staphylococcal toxic shock is a complication of S. Aureus colonisation, and is covered under Toxic Shock Syndrome.

Methicillin-Resistant Staphylococcus Aureus

MRSA is a common mutation of S. Aureus that:

• Alters the Penicillin Binding Protein
• Significantly ↓ affinity of β-lactams for the cell wall so they become non-responsive
• Require treatment with a much more limited arsenal of antimicrobials

Coagulase Negative Staphylococcus

Coagulase-negative Staphylococcus:

• Include several strains, the most common which are:
  • S. Epidermidis
    IE, line infections.
  • S. Saprophyticus
    UTIs.
  • S. lugdunensis
    IE.
  • S. haemolyticus
    IE.
  • S. schleiferi
    IE.
  • S. warneri
  • S. hominis
  • S. capitis
• Usually exist as skin and nares commensals
  Common cause of false-positive blood cultures. True infection can be distinguished on:
  • Clinical suspicion
  • Number of positive cultures
  • Differential time to positivity
    Time between the bottle from an infected line (classically a CVC) flagging positive, and a peripheral culture taken at the same time; if the CVC culture becomes positive earlier it is more likely to be the source.
  • Positive in aerobic and anaerobic bottles
    85% of “true” infections, compared to ~35% of potentially contaminated specimens.
  • Culture positive in <16 hours
• Cause infections in:
  • Areas obscured from the immune system:
    • Septic arthritis
    • Endocarditis
    • Invasive devices
      • Lines
      • Pacemakers
      • Prosthetics
        • Valves
        • Joints
        • VP shunts
  • Immunocompromised host
  • Disrupted integumentary barriers
    • Burns
• Are generally multiresistant, and sensitive to glycopeptides and linezolid

Investigations

Bedside:

• TTE
  For vegetations. Does not exclude IE, but can rule-in.
• TOE
  Should be performed if TTE is negative.

Management

Specific therapy:

• Pharmacological
  • MSSA
    
    • β-lactams
      
      • Flucloxacillin
      • Cefazolin
    • Clindamycin
    • Moxifloxacin
    • Vancomycin
      Failure rates ~4× that of β-lactams.
  • MRSA
    • Glycopeptides
      • Vancomycin
      • Teicoplanin
    • Linezolid 600mg IV Q12H
      No dose adjustments required for renal or hepatic impairment.
    • 5^th generation cephalosporins:
      • Ceftaroline 600mg IV Q8H
      • Ceftobiprole 500mg Q8H
    • Daptomycin
    • Rifampicin/fusidic acid
    • Trimethoprim/sulfamethoxazole
  • Coagulase-negative
    • Vancomycin as empiric therapy
    • Flucloxacillin/Cephalosporin
      May be appropriate de-escalation once sensitivity testing completed.
• Procedural
  • Source Control

Desensitisation for β-lactam allergic patients is preferable - these are by far and away the best agents.

Prognosis

Mortality varies on site of origin, but can reach 50% with:

• Lung
• CLABSI
• Infective endocarditis

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References

1. Harvey RA, Cornelissen CN, Fisher BD. Lippincott Illustrated Reviews: Microbiology (Lippincott Illustrated Reviews Series). 3rd Ed. LWW.