Table 3: Recommendations given by EUCAST and CLSI for the six antibiotics examined in the present study.

Guideline EUCAST MIC breakpoint (mg/L) EUCAST Zone diameter breakpoint (mm) * CLSI interpretive criteria MIC (mg/L) **
Antibiotic S ≤ R > S ≥ R < S ≤ I R ≥
Benzylpenicillin1 / penicillin2 0.125 0.125 21 21 0.12 0.25-2 4
Cefotaxime NR NR NR NR 1 2 4
Meropenem 0.25 0.25 31 31 0.5 NR NR
Vancomycin 1 1 16 16 1 NR NR
Linezolid NR NR NR NR 2 NR NR
Rifampicin 0.125 0.125 25 25 NR NR NR

S, Susceptible.
I, Intermediate.
R, Resistant.
NR, No recommendation.
1 Benzylpenicillin for EUCAST testing.
2 Penicillin for CLSI testing.
Quality control strain Streptococcus pneumoniae ATCC 49619 for both EUCAST and CLSI.
* Susceptibility recommendation method: Medium: Mueller-Hinton (MH) plates with 5% defibrinated horse blood and 20 mg/L β-NAD (MH-F). Inoculum: McFarland 0.5. Incubation: 5% CO2, 35±1 °C, 18±2h. Isolates with insufficient growth after 16-20 h incubation were re-incubated immediately and inhibition zones and MICs read after a total of 40-44 h incubation. Reading: Read zone edges as the point showing no growth viewed from the front of the plate with the lid removed and with reflected light.
** Susceptibility recommendation method: Medium: Cation-adjusted Mueller-Hinton broth (CAMHB) supplemented with 2.5% lysed horse blood (CAMHB-LHB). Inoculum: Direct colony suspension, equivalent to a 0.5 McFarland standard. Incubation: 35 °C, 5% CO2, 20-24 h.