Detection of Amp C Beta Lactamases in Clinical Isolates of Escherichia coli and Klebsiella

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. Several methods have been used to detect the presence of AmpC β-lactamase production in some isolates but most of these methods are not reliable. There is a need for a reliable method of evaluating the presence of AmpC β-lactamases in clinical isolates. A total of 81 consecutive non repetitive clinical isolates of Escherichia coli (n=40) and Klebsiella spp. (n=41) were screened for AmpC production by disc diffusion method using cefoxitin (30 Zg) disc and confirmed by inhibitor based test using boronic acid as inhibitor. A total of 16 E.coli isolates (40%) and 16 Klebsiella isolates (39.02%) screened harbored AmpC enzymes, of which 43.75% of E.coli and 56.25% of Klebsiella isolates coproduced ESBL enzymes. Pure AmpC production was observed in 56.25% of E.coli and 43.75% of Klebsiella isolates. The inhibitor based test was useful in identifying cefoxitin susceptible AmpC producers and could also effectively differentiate ESBL from AmpC producing isolates.