Epidemiology of Conjugative Plasmid-Mediated AmpC β-Lactamases in the United States

Abstract

ABSTRACT A sample of 752 resistant Klebsiella pneumoniae , Klebsiella oxytoca , and Escherichia coli strains from 70 sites in 25 U.S. states and the District of Columbia was examined for transmissibility of resistance to ceftazidime and the nature of the plasmid-mediated β-lactamase involved. Fifty-nine percent of the K. pneumoniae , 24% of the K. oxytoca , and 44% of the E. coli isolates transferred resistance to ceftazidime. Plasmids encoding AmpC-type β-lactamase were found in 8.5% of the K. pneumoniae samples, 6.9% of the K. oxytoca samples, and 4% of the E. coli samples, at 20 of the 70 sites and in 10 of the 25 states. ACT-1 β-lactamase was found at eight sites, four of which were near New York City, where the ACT-1 enzyme was first discovered; ACT-1 β-lactamase was also found in Massachusetts, Pennsylvania, and Virginia. FOX-5 β-lactamase was also found at eight sites, mainly in southeastern states but also in New York. Two E. coli strains produced CMY-2, and one K. pneumoniae strain produced DHA-1 β-lactamase. Pulsed-field gel electrophoresis and plasmid analysis suggested that AmpC-mediated resistance spread both by strain and plasmid dissemination. All AmpC β-lactamase-containing isolates were resistant to cefoxitin, but so were 11% of strains containing transmissible SHV- and TEM-type extended-spectrum β-lactamases. A β-lactamase inhibitor test was helpful in distinguishing the two types of resistance but was not definitive since 24% of clinical isolates producing AmpC β-lactamase had a positive response to clavulanic acid. Coexistence of AmpC and extended-spectrum β-lactamases was the main reason for these discrepancies. Plasmid-mediated AmpC-type enzymes are thus responsible for an appreciable fraction of resistance in clinical isolates of Klebsiella spp. and E. coli , are disseminated around the United States, and are not so easily distinguished from other enzymes that mediate resistance to oxyimino-β-lactams.