Novel chimeric β-lactamase CTX-M-64, a hybrid of CTX-M-15-like and CTX-M-14 β-lactamases, found in a Shigella sonnei strain resistant to various oxyimino-cephalosporins, including ceftazidime

The plasmid-mediated novel β-lactamase CTX-M-64 was first identified in Shigella sonnei strain UIH-1, which exhibited resistance to cefotaxime (MIC, 1,024 μg/ml) and ceftazidime (MIC, 32 μg/ml). The amino acid sequence of CTX-M-64 showed a chimeric structure of a CTX-M-15-like β-lactamase (N- and C-terminal moieties) and a CTX-M-14-like β-lactamase (central portion, amino acids 63 to 226), suggesting that it originated by homologous recombination between the corresponding genes. The introduction of a recombinant plasmid carrying bla_CTX-M-64 conferred resistance to cefotaxime in Escherichia coli, and the activities of cefotaxime and ceftazidime were restored in the presence of clavulanic acid. Of note, CTX-M-64 production could also confer consistent resistance to ceftazidime, which differs from the majority of CTX-M-type enzymes, which poorly hydrolyze ceftazidime. These results were consistent with the kinetic parameters determined with the purified CTX-M-64 enzyme. The bla_CTX-M-64 gene was flanked upstream by an ISEcp1 sequence and downstream by an orf477 sequence. The sequence of the 45-bp spacer region between the right inverted repeat (IRR) of ISEcp1 and bla _CTX-M-64 was exactly identical to that of ISEcp1-bla _{CTX-M-15-like}. Moreover, the presence of a putative IRR of ISEcp1 at the right end of truncated orf477 is indicative of an ISEcp1-mediated transposition event in the bla_CTX-M-64 gene. The emergence of CTX-M-64 by probable homologous recombination would suggest the natural potential of an alternative mechanism for the diversification of CTX-M-type β-lactamases.