Horizontal transfer of blaCMY-bearing plasmids among clinical Escherichia coli and Klebsiella pneumoniae isolates and emergence of cefepime-hydrolyzing CMY-19

Nine Escherichia coli and 5 Klebsiella pneumoniae clinical isolates resistant to various cephalosporins and cephamycins were identified in a Japanese general hospital between 1995 and 1997. All nine E. coli isolates and one K. pneumoniae isolate carried bla(CMY-9), while the other four K pneumoniae isolates harbored a variant of bla(CMY-9), namely, bla(CMY-19). The pulsed-field gel electrophoresis patterns of the nine CMY-9-producing E. coli isolates were almost identical, suggesting their clonal relatedness, while those of the five K. pneumoniae isolates were divergent. Plasmid profiles, Southern hybridization, and conjugation assays revealed that the genes for the CMY-9 and the CMY-19 beta-lactamases were located on very similar conjugative plasmids in E. coli and K. pneumoniae. The genetic environment of bla(CMY-19) was identical to that of bla(CMY-9). A single amino acid substitution, I292S, adjacent to the H-10 helix region was observed between CMY-9 and CMY-19. This substitution was suggested to he responsible for the expansion of the hydrolyzing activity against several broad-spectrum cephalosporins, and this finding was consistent with the kinetic parameters determined with purified enzymes. These findings suggest that the bla(CMY-19) genes found in the four K. pneumoniae isolates might have originated from bla(CMY-9) gene following a point mutation and dispersed among genetically different K. pneumoniae isolates via a large transferable plasmid.