Increasing trend in the prevalence of plasmid-mediated AmpC β-lactamases in Enterobacteriaceae lacking chromosomal ampC gene at a Korean university hospital from 2002 to 2004

The aim of the study is to investigate the prevalence of plasmid-mediated AmpC β-lactamases in Enterobacteriaceae naturally lacking chromosomal AmpC β-lactamases. A total of 1860 clinical isolates of Klebsiella spp., Salmonella spp., and Proteus mirabilis were collected from a Korean hospital between January 2002 and December 2004. For the isolates that are nonsusceptible to cefoxitin, polymerase chain reaction amplification of the bla_SHV, bla_TEM, and bla_AmpC genes and sequencing were performed. Plasmid-mediated AmpC β-lactamases were found in 2.9% (37 isolates of DHA-1, 1 isolate of CMY-1, 1 isolate of CMY-2, and 1 isolate of ACT-1) of Klebsiella pneumoniae, 2.5% (5 isolates of DHA-1) of Klebsiella oxytoca, 0.8% (1 isolate of DHA-1) of Salmonella spp., and none of P. mirabilis isolates. The DHA-1-producing K. pneumoniae was only 2 isolates (0.6%) in 2002, but the rate and the number significantly increased to 2.4% (13 of 538 isolates) in 2003 and to 4.3% (22 of 512) in 2004. In conclusion, DHA-1 is the most prevalent plasmid-mediated AmpC β-lactamase in Enterobacteriaceae lacking chromosomal ampC gene, and the DHA-1-producing K. pneumoniae isolates have rapidly increased since 2003 in a Korean hospital. In addition, this is the first report of the appearance of a K. pneumoniae isolate producing ACT-1 β-lactamase in Korea.