Evaluation of Two Phenotypic Methods for the Detection of Plasmid-Mediated AmpC β-Lactamases among Enterobacteriaceae Isolates

Abstract

Objectives AmpC β-lactamases are cephalosporinases that confer resistance to cephalothin, cefazolin, cefoxitin, penicillin, and β-lactamase inhibitor-β-lactam combinations. Even though the AmpC resistance is reported, but the accurate occurrence of AmpC β-lactamases in Enterobacteriaceae members is still unknown. Techniques to identify AmpC producers are still evolving but not yet optimized for the clinical laboratory. Here we aimed to compare the test performance of two different phenotypic methods, that is inhibitor-based assay using boronic acid and disk approximation test for AmpC detection in Enterobacteriaceae isolates from a tertiary hospital microbiology laboratory. Materials and Methods The study includes 137 nonrepeat Enterobacteriaceae strains. Bacterial isolates, that yielded a zone diameter of less than 18 mm for cefoxitin by disk diffusion method were considered potential AmpC producers and further confirmed by phenotype methods—inhibitor-based assay using boronic acid and disk approximation test. A multiplex polymerase chain reaction was used to detect the most common plasmid-mediated AmpC genes: ACC, FOX, MOX, DHA, CIT, and EBC. Results Of the 137 clinical isolates, 58 (42.33%) were cefoxitin resistant, while 53.4 and 18.9% of the cefoxitin-resistant isolates were positive by inhibitor-based assay and disk approximation test. Multiplex PCR detected 42 (30.6%) isolates with AmpC genes. Of the 42 isolates, the inhibitor-based assay detected 25 (59.5%) isolates, while the disk approximation test detected nine (21.4%) isolates. Conclusion Our findings suggest that inhibitor-based assay using boronic acid can be used for the detection of the isolates that harbor AmpC β-lactamases. This method is cost-effective, simple to perform, and easy to interpret. Thus AmpC detection as a routine in clinical laboratories can help in appropriate therapeutic intervention and improved infection control.