Abstract

The detection of extended spectrum -lactamases (ESBL(S)) in Gram-negative bacteria that produce AmpC -lactamases is problematic. In
the present study, the performance of modified double-disc synergy test (MDDST) that employs a combination of cefepime and piperacillin-tazobactam for the detection of ESBL(S) Klebsiella producing AmpC -lactamases was evaluated and compared with doubledisc synergy test (DDST). E-test phenotypic confirmatory and modified three-dimensional tests (MTDT) were adopted for more data confidence. A total of 100 clinical isolates of Klebsiella, which met the CLSI (2012) screening criteria as having broth microdilution (BMD) MIC > or =2 mg mL-1 for at least one extended-spectrum cephalosporin [ceftazidime (CAZ), cefotaxime (CTX) and cefpodoxime], were accurately-selected for the study. MDDST detected ESBLs in 62 out of the100 studied isolates with 100 % sensitivity and specificity, whereas DDST detected ESBLs in only 52 isolates with 92.9 % sensitivity and 100 % specificity. E-test could detect ESBLs in 62 isolates, while as many as 34/62 ESBL positive isolates were confirmed to be AmpC beta-lactamase positive by the MTDT. MDDST and E-Test could detect ESBLs in all the 34 AmpC positive isolates, whereas DDST detects ESBLs in only 26 isolates. The study recommended MDDST as superior to DDST for the detection of ESBLs in AmpC -lactamase-producing Klebsiella spp. and this was confirmed by MTDT and E-Tests.

Key words: AmpC β-lactamase, extended-spectrum β-lactamases (ESBLs), Klebsiella spp., modified double-disc synergy test (MDDST), double-disc synergy test (DDST), phenotypic confirmatory test (E-test), modified three-dimensional test (MTDT)