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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 ~29 kDa and pI values were 8.9 and 8.4, respectively. CTX-M-15 had higher affinities for [beta]-lactams (lower Km values) than those of CTX-M-3 but catalytic efficiency (kcat/Km 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.

(C) Copyright Oxford University Press 2002.