Detection of Amp C beta lactamases production in Escherichia coli & Klebsiella by an inhibitor based method.

BACKGROUND & OBJECTIVE Detection of AmpC-mediated resistance in Gram-negative organisms poses a problem due to misleading results in phenotypic tests. There are no recommended guidelines for detection of this resistance mechanism and there is a need to address this issue as much as the detection of extended spectrum beta lactamases (ESBLs) since both may co-exist and mask each other. Though resistance to cefoxitin is used as a screening test, it does not reliably indicate Amp C production. This study was undertaken to detect Amp C beta lactamases in certain Gram-negative bacteria employing an inhibitor base test using boronic acid. METHODS A total number of 76 consecutive non repetitive clinical isolates of Escherichia coli (n=67) and 9 Klebsiella pneumoniae (n=9) obtained over a period of two months, were screened for amp C production by disc diffusion method using cefoxitin (30 microg) dics and confirmed by inhibitor based test using boronic acid as inhibitor. RESULTS A total of 36 of 76 isolates (47.3%) screened harboured amp C enzymes, of which a majority 31 (86.1%) co-produced ESBL enzymes. Pure ampC production was seen in 7 (9.2%) of isolates only. INTERPRETATION & CONCLUSION Most of the amp C producers also produced ESBL enzymes. The inhibitor based test was useful in identifying cefoxitin susceptible amp C producers and could also effectively differenciate ESBL from amp C producing isolates.