AmpC Disk Test for Detection of Plasmid-Mediated AmpC β-Lactamases in Enterobacteriaceae Lacking Chromosomal AmpC β-Lactamases

Abstract

ABSTRACT Although plasmid-mediated AmpC β-lactamases were first reported in the late 1980s, many infectious disease personnel remain unaware of their clinical importance. These enzymes are typically produced by isolates of Escherichia coli , Klebsiella spp., Proteus mirabilis , and Salmonella spp. and are associated with multiple antibiotic resistance that leaves few therapeutic options. Plasmid-mediated AmpC β-lactamases have been associated with false in vitro susceptibility to cephalosporins. Many laboratories do not test for this resistance mechanism because current tests are inconvenient, subjective, lack sensitivity and/or specificity, or require reagents that are not readily available. In this study a new test, the AmpC disk test, based on filter paper disks impregnated with EDTA, was found to be a highly sensitive, specific, and convenient means of detection of plasmid-mediated AmpC β-lactamases in organisms lacking a chromosomally mediated AmpC β-lactamase. Using cefoxitin insusceptibility as a screen, the test accurately distinguished AmpC and extended-spectrum β-lactamase production and differentiated AmpCs from non-β-lactamase mechanisms of cefoxitin insusceptibility, such as reduced outer membrane permeability. The test is a potentially useful diagnostic tool. It can provide important infection control information and help to ensure that infected patients receive appropriate antibiotic therapy.