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 enterococci possess many intrinsic resistance factors, many antibiotics 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 testing conditions listed in Table 6.
Table 6. Testing Conditions for Susceptibility Testing of Enterococci.| Testing Conditions |
| Medium | - Mueller Hinton Agar (MHA) for disk diffusion
- cation-adjusted Mueller Hinton Broth (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 in ambient air - Disk diffusion 16 to 18 hours
- Dilution methods 16 to 20 hours
- All methods 24 hours for vancomycin
|
