Horizontal transfer of bla_CMY-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-9. 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 β-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 be responsible for the expansion of the liydrolyzing 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 diferent K. pneumoniae isolates via a large transferable plasmid.