AmpC-type beta-lactamases in Clinical Isolated of Cefoxitin-resistant E. coli and K. pneumoniae

ABSTRACT

To elucidate the role of plasmid-mediated AmpC-type B-lactamases in clinical practice, cefoxitin-resistant isolates of E. coli (19 strains) and K. pneumoniae (7 strains) from three hospitals in Korea were studied. All of the 26 isolates produced at least one j3-lactamase and 16 (62%) isolates produced AmpC-type B-lactamases poorly inhibited by clavulanic acid. In 16 such isolates, 4 kinds of AmpC enzymes were detected; the pI 8.0 AmpC enzyme in 11 isolates, the pI 8.9 in 3 isolates of E. coli, the pI 8.5 in 1 isolate of E. coli, and the pI 7.8 in 1 isolate of K pneumoniae. The pI 8.0 and 7.8 AmpC enzymes had an apparent molecular mass of 38 kDa and the pI 8.5 and 8.9 AmpC enzymes had a molecular mass of 35 kDa. Cefoxitin resistance was transmissible in six E. coli and three K pneumoniae strains due to a common AmpC-type B-lactamase with a pl of 8.0. This enzyme was confirmed to be CMY-1 B-lactamase by Southern blotting and PCR analysis. Four E. coli isolates produced large amounts of AmpC-type j3-1actamase. They were chromosomal AmpC hyperproducers carrying some alterations in the promoter and attenuator regions of the ampC chromosomal gene. The pI 7.8 AmpC enzyme is currently under study. In conclusion, this study showed that the CMY-1 plasmid-mediated cephamycinase play an important role in cephamycin resistance of K. pneumoniae and E. coli clinical isolates in Korea.