Detection of AmpC β-lactamases in Escherichia coli, Klebsiella spp., Salmonella spp. and Proteus mirabilis in a regional clinical microbiology laboratory

There are currently no standardized diagnostic tests available for the reliable detection of AmpC β-lactamases in Klebsiella spp., Escherichia coli, Proteus mirabilis and Salmonella spp. A study was designed to evaluate a confirmation disk test using cefotetan (CTT) and cefoxitin (FOX) with phenylboronic acid (PBA). It also investigated the most suitable screening concentrations of FOX, ceftriaxone (CRO) and ceftazidime (CAZ) for the detection of AmpC β-lactamases. A total of 126 control (consisting of 11 laboratory and 115 well-characterized clinical strains) and 29 840 non-repeat clinical isolates were included. FOX with PBA used in a confirmation test and CRO and CAZ as screening agents were found to be unreliable. FOX at ≥ 32 mg/L was the best screening agent and CTT with PBA was the best confirmation test. Of the clinical isolates 635 (2%) were found to be resistant to cefoxitin (MIC ≥ 32 ug/mL) and 332 (52%) were AmpC positive. E. coli was the most common organism with AmpC β-lactamases and was mostly present in urines from community patients. It is recommended that laboratories use FOX at 32 mg/L as a screening agent and perform a disk test with CTT and PBA to confirm the presence of an AmpC cephalosporinase in isolates of Klebsiella spp., E. coli, Salmonella spp. and P. mirabilis. This approach is convenient, practical and easy to incorporate into the workflow of a clinical laboratory. False-positive AmpC detection may occur with KPC-producing bacteria when inhibitor-based methods are used.