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, 50 (6), 1031-4

Biochemical Analysis of the Ceftazidime-Hydrolysing Extended-Spectrum Beta-Lactamase CTX-M-15 and of Its Structurally Related Beta-Lactamase CTX-M-3

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Biochemical Analysis of the Ceftazidime-Hydrolysing Extended-Spectrum Beta-Lactamase CTX-M-15 and of Its Structurally Related Beta-Lactamase CTX-M-3

Laurent Poirel et al. J Antimicrob Chemother.

Abstract

The extended-spectrum beta-lactamase CTX-M-15 confers resistance to ceftazidime, unlike the majority of CTX-M-type enzymes. Kinetic parameters were determined from purified CTX-M-15 and CTX-M-3, which differ by the single amino acid substitution Asp-240 to Gly, according to the Ambler numbering of class A beta-lactamases. Relative molecular masses of CTX-M-15 and CTX-M-3 were approximately 29 kDa and pI values were 8.9 and 8.4, respectively. CTX-M-15 had higher affinities for beta-lactams (lower K(m) values) than those of CTX-M-3 but catalytic efficiency (k(cat)/K(m) values) was variable depending on the beta-lactam substrate. Only CTX-M-15 showed a measurable catalytic efficiency for ceftazidime. Clavulanic acid and tazobactam were good inhibitors of both enzymes. MICs of beta-lactams for Escherichia coli reference strains expressing cloned beta-lactamase genes in the same genetic background were similar except for ceftazidime. This work underlines the fact that some CTX-M enzymes may hydrolyse ceftazidime and thus confer resistance to this expanded-spectrum cephalosporin in Enterobacteriaceae.

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