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DNA-DNA hybridization and sequencing were performed to determine the molecular basis of resistance to clavulanic acid in 107 inhibitor-resistant TEM (IRT) enzymes produced by Escherichia coli clinical isolates. These beta-lactamases derived from TEM-1 enzyme focused at pI 5.2 (n = 68) or 5.4 (n = 39) and were very poorly inhibited by clavulanic acid compared with TEM-1 enzyme. Results showed that the amino acid sequences of 84 of the 107 enzymes differ from TEM-1 by one or two substitutions previously described: Arg-244--&gt;Ser (IRT-2) in 22 strains, Met-69--&gt;Leu (TEM-33) in 17 strains, Met-69--&gt;Val (TEM-34) in 14 strains, Met-69--&gt;Ile (IRT-3) in 6 strains, Met-69--&gt;Leu associated with Asn-276--&gt;Asp (IRT-4) in 13 strains, and Met-69--&gt;Val associated with Asn-276--&gt;Asp (TEM-36) in 12 strains. A new combination, Met-69--&gt;Ile with Asn-276--&gt;Asp, was found in 20 strains and was called IRT-8. Two IRT enzymes not previously described were characterized. The substitution Met-69--&gt;Val associated with a novel substitution Arg-275--&gt;Leu occurred in one strain. The combination Met-69--&gt;Leu and Asn-276--&gt;Asp was associated with the novel substitution Trp-165--&gt;Arg in two strains. These two novel enzymes were called IRT-9 and IRT-10, respectively. The implication of these novel mutated positions, 165 and 275, in resistance to inactivation by clavulanate was supported by crystallographic data on the TEM-1 enzyme and results of site-directed mutagenesis. Molecular characterization of these mutants showed great diversity among the genes coding for inhibitor-resistant TEM enzymes produced by clinical E. coli isolates.

antimicrobial agents and chemotherapy

Molecular characterization of nine different types of mutants among 107 inhibitor-resistant TEM beta-lactamases from clinical isolates of Escherichia coli