MedCompanion

Coagulase-Negative Staphylococci

CoNS

Morphology: Gram-positive cocci in clusters, coagulase-negative (distinguishes from S. aureus). Most clinically relevant species: **S. epidermidis** (most common, prosthetic device infections), **S. lugdunensis** (aggressive — behaves like S. aureus, can cause native-valve IE), **S. saprophyticus** (uncomplicated cystitis in young women — see organisms list for cystitis), **S. hominis**, **S. haemolyticus**.

MRSAGramPosOther

Typical drugs

  1. #1Vancomycin— Empiric — ~80% of clinical CoNS isolates are methicillin-resistant (mecA+).
  2. #2Nafcillin— ONLY if methicillin-susceptible on susceptibility report (rare). Cefazolin equivalent.
  3. #3Cefazolin— MSSE / methicillin-susceptible CoNS — preferable to nafcillin for tolerability.
  4. #4Daptomycin— Bacteremia / endocarditis if vanco intolerance or rising MIC.
  5. #5Linezolid— Salvage; not first-line for IE (bacteriostatic).

Empiric therapy when resistant

Empirically vancomycin for any clinically significant CoNS isolate. Narrow to cefazolin/nafcillin only if methicillin-susceptible. Contaminant vs pathogen: single positive culture from blood usually contamination; ≥2 sets with same speciation/susceptibility + clinical picture (fever, device, prosthesis) = pathogen.

Resistance mechanisms

  • altered-target

    mecA-encoded PBP2a → methicillin resistance

    Example: 70–90% of clinical CoNS isolates are methicillin-resistant. Empirically treat with vancomycin until susceptibility back.

  • biofilm

    Biofilm formation on indwelling devices (catheters, prosthetic valves, joints)

    Example: Device-related infections often require source removal — antibiotics alone fail.

Resistance notes

Empirically methicillin-resistant until susceptibility returns. Biofilm makes device removal usually mandatory for cure.

Common syndromes

Pearls

Most common CLABSI / prosthetic IE pathogen. Distinguish contaminant (1/4 bottles, no device, no symptoms) from pathogen (≥2 sets, device or prosthesis, clinical syndrome). S. lugdunensis is the dangerous exception — treat like S. aureus, can destroy native valves rapidly. Empirically MR — vancomycin is the default. Source control is the rule for prosthetic device infections — antibiotics alone rarely cure biofilm.

References