MICROBIOLOGY

• Coagulase-negative staphylococci (CoNS) are aerobic, Gram-positive coccus occurring in clusters [Fig].
  • Predominantly found on the 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: colonies typically small, golden yellow (about 1-2 mm in diameter).
  • S. lugdunensis: colonies are usually sticky, smooth, glossy, yellow-orange (2-4 mm).
    • Perhaps the most virulent of CoNS, it behaves similarly to S. aureus.
  • Over 40 recognized species of CoNS are capable of causing human disease.
    • Others seen on occasion: Staphylococcus auricularis, Staphylococcus capitis, Staphylococcus hominis, S. pettenkoferi, Staphylococcus saprophyticus, and Staphylococcus simulans.
• Many strains tend to produce biofilm, allowing for adherence to medical devices.
• Susceptibility profile for CoNS. Breakpoints vary by species.
  
  • Vancomycin (CLSI): MIC cutoffs
    • Sensitive: ≤ 4 mg/L
    • Intermediate: 8-16 mg/L
    • Resistant: ≥ 32 mg/L
    • Note: EUCAST states resistance is MIC > 4 mg/L
  • Oxacillin (CLSI)
    • S. epidermidis sensitive if ≤ 0.25 mg/L and Resistant if ≥ 0.5 mg/L.
    • S. lugdunensis sensitive if ≤ 2 mg/L and resistant if ≥ 4 mg/L.
  • Usually resistant to penicillin and typically (> 80%) to methicillin (oxacillin, nafcillin).
    • mecA gene encodes for low-affinity penicillin-binding protein.
    • Resistance can be heterotypic, so multiple isolates should be obtained to determine whether they are susceptible to beta-lactams, as only a minority of isolates typically express resistance phenotypes.
  • Linezolid resistance is described but remains rare.