All susceptibility testing for enterococci should follow the standards defined by CLSI. Selection of drugs for testing will follow the same criteria defined previously.
Because the enterococci possess many intrinsic resistance factors, there are many antibiotics that should not be tested (or if tested, suppressed from the final report). CLSI document M100 defines all applicable criteria.
Both Disk Diffusion (Kirby Bauer) and broth dilution (MIC) methods are commonly employed, utilizing the following testing conditions:
- Medium: MHA for disk diffusion; CAMHB for broth dilution(supplemented to 50 µg/mL for daptomycin)
- Inoculum: Growth method or direct colony suspension, equivalent to a 0.5 McFarland standard
- Incubation: 35 + 2°C; ambient air. Disk diffusion; 16 to 18 hours; Dilution methods; 16 to 20 hours. All methods: 24 hours for vancomycin.