MICROBIOLOGY

• Coagulase-negative staphylococci (CoNS) are aerobic, Gram-positive coccus, occurring in clusters.
  • Predominantly found on skin and mucous membranes.
  • Heterogeneous group
  • Catalase positive but coagulase negative (S. aureus is coagulase positive).
• Major pathogens:
  • S. epidermidis: colonies typically small, white-beige (about 1-2 mm in diameter).
  • S. haemolyticus:
  • S. lugdunensis:
  • Over forty recognized species of CoNS capable of causing human disease.
• Many strains with propensity to produce biofilm, allowing for adherence to medical devices.
• Susceptibility profile for CoNS:
  
  • Vancomycin (CLSI): MIC cutoffs
    • Sensitive: ≤ 4 mg/L
    • Intermediate: 8-26 mg/L
    • Resistant: ≥ 32 mg/L
    • Note: EUCAST states resistance is MIC > 4 mg/L
    • Vancomycin resistance usually due to cell-wall thickening mechanisms.
  • Oxacillin:
    • Sensitive: ≤ 0.25 mg/L
    • Resistant: > 0.5 mg/L
    • Note: for S. lugdunesis sensitive if ≤ 2 mg/L and resistant for 4 mg/L.
  • Usually resistant of penicillin and typically (80%) to methicillinor oxacillin.
    • mecA gene encodes for low-affinity penicillin-binding protein.
    • Resistance can be heterotypic, so usually like to see multiple isolates determined as susceptible to beta-lactams, as only a minority of isolates typically express resistance phenotypes.
  • Linezolid resistance described, but remains rare.