Evaluation of three methods for detection of ampc β-lactamase in clinical isolates of escherichia coli and klebsiella pneumoniae at a tertiary care hospital

Abstract

Aims: Detection of AmpC β-lactamases is a great concern as phenotypic methods are misleading and results in treatment failure. There are no recommended guidelines for detection of this resistance mechanism and it is important to address this issue as much as the detection of extended spectrum beta lactamases (ESBLs) since both may co-exist and mask detection of later. Though resistance to cefoxitin is used as a screening test, it does not reliably indicate Amp C production and several phenotypic and genotypic methods were studied. We have undertaken this study to evaluate three different phenotypic methods for detection of AmpC β-lactamase in Escherichia coli (E.coli) and Klebsiella pneumonia (K.pneumoniae). Mthods: A total number of 190 consecutive, non-repetitive, imipenem sensitive clinical isolates of E.coli (n=118) and K.pneumoniae (n=72) were obtained over a period of six months, were screened for AmpC β-lactamase by using cefoxitin disk and confirmed by boronic acid (BA) inhibitor based test, modified three dimensional test (M3DT) and novel fashion method. Results: Out of 190 isolates, 84 (44.21%) were cefoxitin resistant, 76 (40%) were AmpC β-lactamase positive by M3DT and BA inhibitor based test, while 64 (33.68%) were positive by novel fashion method. Conclusion: Inhibitor based method using boronic acid is a practical and efficient method for detection of plasmid-mediated AmpC β-lactamases and clinical microbiology laboratories should consider testing the presence of AmpC β-lactamase using this method.