Enhancing Phenotypic Detection of ESBL in AmpC co-producers by using Cefepime and Tazobactam

INTRODUCTION: Routine phenotypic methods employing clavulanate and third generation cephalosporins to detect ESBL are not promising for isolates that co-produce an inhibitor-resistant beta lactamase like AmpC. AIM: Enhancing phenotypic detection of ESBL in AmpC co-producers by using cefepime and tazobactam. MATERIALS AND METHODS: A total of 245 isolates of Escherichia coli (123), Klebsiella spp. (87), Proteus spp.(20), Enterobacter spp. (9) and Citrobacter spp.(6) obtained over a period of 2 years from January 2013 to December 2014 from urine samples of hospitalized patients were studied. The isolates were simultaneously screened for ESBL and AmpC production. AmpC production was confirmed by modified three -dimensional test (MTDT). ESBL production was confirmed by original double disc synergy test, phenotypic disc confirmatory test (PDCT) and modified double disc synergy test (MDDST) and the results compared. RESULTS: AmpC production was confirmed in 113 (46.1%) isolates by modified three dimensional test out of 143 screened positive for AmpC. Of the 192 isolates screened positive for ESBL, ESBL production was confirmed in 162 (66.1%). DDST detected ESBLs in only134 (54.7%) while additional 28 (11.4%) ESBL positive isolates were detected by MDDST. PDCT detected total 145(59.2%) ESBL positive isolates, with cefotaxime and cefotaxime + clavulanate detecting 139 (56.7%) and ceftazidime and ceftazidime + clavulanate detecting additional 6 isolates. All the 28 (11.4%) isolates which were additionally detected ESBL producers by MDDST showed positive three dimensional test i.e. AmpC co producers. DDST detected ESBL in none of AmpC positive isolates while PDCT detected ESBL in 11 isolates showing AmpC co-production. In MDDST cefepime was the best cephalosporin in detecting ESBL in presence of AmpC production. It showed synergism with amoxicillin-clavulanate in 11(39.3%) isolates and in 24(85.7%) isolates with piperacillin-tazobactam. Third generation cephalosporins -cefotaxime, ceftazidime and cefpodoxime were not able to detect ESBL in AmpC-co producers. CONCLUSION: Modification of double disc synergy tests that combine piperacillin-tazobactum with cefepime enhances the possibility of ESBL detection.