Comparision of Three Laboratory Tests for Detection of AmpC β Lactamases in Klebsiella Species and E. Coli.

BACKGROUND AND OBJECTIVE: AmpC β lactamases are one of the important causes of drug resistance in gram negative bacteria. Failure to detect these enzymes in the laboratory has contributed to therapeutic failures but there are till date no standard guideline available. This study was therefore undertaken to evaluate three phenotypic laboratory tests and the inhibitors used in two of the tests to detect AmpC β lactamases produced by E. coli and Klebsiella species as they are most commonly isolated organisms.

METHODS: E. coli and Klebsiella isolates from different clinical samples were tested for ESBLs production as per CLSI guidelines and excluded from the study. The non-ESBLs isolates were then screened for AmpC β lactamases production, by cefoxitin and then confirmed by three different methods, i.e., Disc Potentiation Test (DPT) , Double Disc Synergy Test (DDST) and Modified Three Dimensional Test (M3DT) which in the absence of molecular methods, was taken as the gold standard. Boronic acid and cloxacillin were used as inhibitory agents in the Disc Potentiation and Double Disc synergy Tests.

RESULTS: A total of 2,933 isolates were tested out of which 165 isolates were detected as non ESBLs producers,135 (81.82%) when screened for AmpC β lactamases based on resistance to cefoxitin were labelled as positive. 30 (18.18%) cefoxitin sensitive isolates were labelled as probably non AmpC producers . M3DT, in addition to detecting all the 135 (100%) cefoxitin resistant isolates, also detected 5 (16.67%) cefoxitin sensitive isolates as AmpC producers. Other phenotypic tests, DPT and DDST with different inhibitors like boronic acid and cloxacillin in different potencies were all found to be less sensitive. The best results among these two methods were obtained with DDST using cloxacillin 500μg.

CONCLUSION: In the absence of recommended guidelines for AmpC detection, the study reports, among the tests performed, M3DT as the best phenotypic method for AmpC confirmation, as it is not only the most sensitive but also specific test for AmpC as it rules out the resistance due to other mechanisms like the porin channel.