Table 2.
Piperacillin-tazobactam MICs with variable testing parameters.
| Organism | Isolate # | Genotype | Piperacillin-tazobactam MIC (µg/mL) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Standard | 5 × 104 CFU/mL | 5 × 106 CFU/mL | pH 5 | pH 6 | pH 8 | CAMHB + 2.5% LHB | HTM | Trace calcium | 50 mg/L calcium | CO2 | |||
| E. coli | ATCC 25922 | WT | 2 | 2 | 4 | 8 | 4 | 2 | 2 | 2 | 2 | 2 | 4 |
| E. coli | CML2255 | CTX-M-15, TEM-1, OXA-1 | 32 | 16 | 512 | 64 | 128 | 16 | 16 | 64 | 32 | 32 | 64 |
| E. coli | CML1468 | KPC | >512 | 512 | >512 | 256 | >512 | 512 | 512 | >512 | >512 | >512 | >512 |
| K. pneumoniae | ATCC 700603 | SHV-18 | 16 | 8 | 16 | 64 | 64 | 8 | 16 | 16 | 16 | 16 | 8 |
| K. pneumoniae | CML138 | CTX-M-15 | >512 | >512 | >512 | >512 | >512 | 16 | >512 | >512 | >512 | >512 | >512 |
| K. pneumoniae | CML128 | KPC-2, SHV-27, TEM-1 | >512 | >512 | >512 | >512 | >512 | >512 | >512 | >512 | >512 | >512 | >512 |
| K. pneumoniae | CML148 | ΔKPC-2, SHV-27, TEM-1 | 256 | 64 | >512 | 256 | 256 | 16 | 64 | 256 | 128 | 128 | 256 |
| K. pneumoniae | CML2280 | CTX-M-15, TEM-1, OXA-1 | >512 | 256 | >512 | >512 | >512 | 32 | >512 | >512 | >512 | >512 | >512 |
| K. oxytoca | CML2251 | TEM-129 | >512 | >512 | >512 | 512 | >512 | 256 | >512 | >512 | >512 | >512 | >512 |
| P. aeruginosa | ATCC 27853 | WT | 4 | 4 | 8 | 8 | 4 | 4 | 2 | 2 | 2 | 4 | 4 |
| P. aeruginosa | CML1039 | AmpC | 512 | 512 | >512 | 128 | 512 | >512 | 512 | 512 | 512 | 512 | 512 |
| P. aeruginosa | CML1040 | AmpC, TEM-24 | 64 | 64 | 256 | 256 | 256 | 32 | 64 | 64 | 64 | 128 | 256 |
| P. aeruginosa | CML1047 | AmpC | >512 | 512 | >512 | 128 | 512 | >512 | 512 | 512 | >512 | >512 | 256 |
MICs obtained under standard CLSI conditions are indicated by the “standard” column header. Parameters that were varied include inoculum density (5 × 104 CFU/mL and 5 × 106 CFU/mL), pH of medium (pH 5, 6, and 8), medium supplements (2.5% laked horse blood (LHB) and HTM), cation content (trace amounts of calcium and 50 mg/L calcium), and incubation with 5% CO2. Bolded values indicate a change in MIC of ≥4-fold.