TABLE 2.
MIC range and resistance rates for ceftolozane-tazobactam and ceftazidime-avibactam according to results for expression of oprD, efflux pumps, and chromosomal ampC among non-carbapenemase-producing CRPA clinical isolates
| Resistance mechanism | No. of isolates | MIC range (median) (mg/liter) |
% resistance |
||
|---|---|---|---|---|---|
| Ceftolozane-tazobactam | Ceftazidime-avibactam | Ceftolozane-tazobactam | Ceftazidime-avibactam | ||
| Decreased oprD expression | |||||
| Positive (≤30% compared with PAO1)a | 39 | 1–32 (2) | 2–32 (8) | 5.1 | 25.6 |
| Negative (>30% compared with PAO1) | 3 | 2–4 (2) | 8–16 (16) | 0 | 66.7 |
| Overexpressed mexB | |||||
| Positive (≥3-fold compared with PAO1)a | 23 | 1–32 (2) | 2–32 (8) | 4.3 | 34.8 |
| Negative (<2-fold compared with PAO1) | 12 | 1–16 (2) | 2–16 (4) | 8.3 | 25.0 |
| Overexpressed mexD | |||||
| Positive (≥10-fold compared with PAO1) | 6 | 1–4 (2) | 2–8 (4) | 0 | 0 |
| Negative (<5-fold compared with PAO1) | 35 | 1–32 (2) | 2–32 (8) | 5.7 | 34.3 |
| Overexpressed mexF | |||||
| Positive (≥10-fold compared with PAO1) | 5 | 1–4 (2) | 2–16 (8) | 0 | 40 |
| Negative (<5-fold compared with PAO1) | 29 | 1–32 (2) | 2–32 (8) | 6.9 | 24.1 |
| Overexpressed ampC | |||||
| Positive (≥10-fold compared with PAO1) | 17 | 1–32 (4) | 2–16 (8) | 5.9 | 23.5 |
| Negative (<5-fold compared with PAO1) | 25 | 1–16 (2) | 2–32 (4) | 4.0 | 32.0 |
a
P < 0.05 in comparison of percent resistance to ceftolozane-tazobactam with that to ceftazidime-avibactam.