Effect of D240G substitution in a novel ESBL CTX-M-27

Escherichia coli clinical strain Gre-1 collected in 2000 from a French hospital harboured a novel CTX-M-encoding gene, designated bla(CTX-M-27). CTX-M-27 differed from CTX-M-14 only by the substitution D240G and was the third CTX-M enzyme harbouring this mutation after CTX-M-15 and CTX-M-16. The Gly-240-harbouring enzyme CTX-M-27 conferred to E. coli higher MICs of ceftazidime (MIC, 8 versus 1 mg/L) than did the Asp-240-harbouring CTX-M-14 enzyme. Comparison of CTX-M-14 and CTX-M-27 showed that residue Gly-240 decreased K-m for ceftazidime (205 versus 940 muM), but decreased hydrolytic activity against good substrates, such as cefotaxime (k(cat), 113 versus 415 s(-1)), probably owing to the alteration of beta3 strand positioning during the catalytic process.