TABLE 2.
Extended-spectrum β-lactamase assays for isogenic E. coli strains harboring KPC-3 or KPC-3 variants for cephalosporin versus cephalosporin-clavulanate
| KPC-3 or variant | ESBL testing results |
ESBL?a | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Disk diffusion (zone size in mm) |
Disk diffusion (zone size in mm) |
Etest MIC (μg/ml) |
||||||||
| Ceftazidime | Ceftazidime-clavulanate | Zone size difference | Cefotaxime | Cefotaxime-clavulanate | Zone size difference | Cefotaxime | Cefotaxime-clavulanate | No. 2-fold (actual fold) decrease in MICs | ||
| Wild-type | 10 | 15 | 5 | 14 | 18 | 4 | >16 | >1 | NAc | |
| D179Y | No zoneb | 23 | 17 | 23 | 31 | 8 | 4 | 0.06 | Six 2-fold (or 64-fold) | Yes |
| T243M | No zone | 20 | 14 | 19 | 24 | 5 | 4 | 0.06 | Six 2-fold (or 64-fold) | Yes |
| D179Y/T243M | No zone | 23 | 17 | 28 | 34 | 6 | 4 | 0.125 | Five 2-fold (or 32-fold) | Yes |
| EL165-166 | No zone | 23 | 17 | 13 | 32 | 19 | >16 | >1 | NA | Yes |
a
Isolates were identified as ESBL producers by CLSI criteria (9), i.e., a difference of ≥5 mm between zone diameters of a ceftazidime or cefotaxime disk and the respective clavulanate-containing disk or a ≥3 twofold (i.e., ≥8-fold) decrease in cefotaxime MIC in combination with clavulanate versus cefotaxime alone (as determined using an Etest strip).
b
When no zone was present, the zone size difference was calculated by subtracting 6 mm (the size of the disk) from the zone size around ceftazidime-clavulanate.
c
NA, not applicable.