Table 2.
MICs (μg/ml) of cefazolin by blaZ gene type and size of the bacterial inoculum
| blaZ gene type (n) and parameter | Cefazolin MIC (μg/ml) |
No. (%) of strains exhibiting inoculum effect |
||
|---|---|---|---|---|
| Standard inoculumb | High inoculumc | ≥4-fold increased | ≥4-fold increase to nonsusceptible MICe | |
| Type A (48) | ||||
| Mean (GM)a | 1.2 (1.1) | 7.6 (3.7) | 22 (46) | 8 (17) |
| Mode | 1 | 2 | ||
| Range | 0.5–2 | 1–32 | ||
| MIC50 | 1 | 4 | ||
| MIC90 | 2 | 16 | ||
| Type B (43) | ||||
| Mean (GM) | 1.2 (1.1) | 2.1 (1.9) | 3 (7) | 0 |
| Mode | 1 | 2 | ||
| Range | 0.5–2 | 1–4 | ||
| MIC50 | 1 | 2 | ||
| MIC90 | 2 | 4 | ||
| Type C (49) | ||||
| Mean (GM) | 1.1 (1.0) | 3.2 (2.8) | 22 (45) | 0 |
| Mode | 1 | 4 | ||
| Range | 0.5–2 | 1–8 | ||
| MIC50 | 1 | 4 | ||
| MIC90 | 2 | 8 | ||
| Type D (2) | ||||
| Mean (GM) | 1.0 (1.0) | 2.0 (2.0) | 0 | 0 |
| Mode | 1 | 2 | ||
| Range | 1–1 | 2–2 | ||
| MIC50 | 1 | 2 | ||
| MIC90 | 1 | 2 | ||
| blaZ negative (43) | ||||
| Mean (GM) | 0.8 (0.77) | 1.4 (1.2) | 3 (7) | 0 |
| Mode | 1 | 1 | ||
| Range | 0.25–2 | 0.25–2 | ||
| MIC50 | 1 | 1 | ||
| MIC90 | 1 | 2 | ||
a
GM, geometric mean.
b
Standard inoculum, 105 CFU/ml.
c
High inoculum, 107 CFU/ml.
d
Isolates exhibiting ≥4-fold increase in cefazolin MIC between standard and high inocula.
e
Isolates exhibiting ≥4-fold increase in cefazolin MIC between standard and high inocula that resulted in a nonsusceptible cefazolin MIC (≥16 μg/ml) at the high inoculum.