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

Abstract

ABSTRACT 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 IS Ecp1 sequence and downstream by an orf477 sequence. The sequence of the 45-bp spacer region between the right inverted repeat (IRR) of IS Ecp1 and bla CTX-M-64 was exactly identical to that of IS Ecp1 - bla CTX-M-15-like . Moreover, the presence of a putative IRR of IS Ecp1 at the right end of truncated orf477 is indicative of an IS Ecp1 -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.