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.