Haemophilus influenzae bla ROB-1 Mutations in Hypermutagenic Δ ampC Escherichia coli Conferring Resistance to Cefotaxime and β-Lactamase Inhibitors and Increased Susceptibility to Cefaclor

The clinical use of cefaclor has been shown to enrich Haemophilus influenzae populations harboring cefaclor-hydrolyzing ROB-1 beta-lactamase. Such a selective process may lead to the increased use of extended-spectrum cephalosporins or beta-lactams plus beta-lactamase inhibitors and, eventually, resistance to these agents, which has not previously been observed in H. influenzae. In order to establish which bla(ROB-1) mutations, if any, could confer resistance to extended-spectrum cephalosporins and/or to beta-lactamase inhibitors, a plasmid harboring bla(ROB-1) was transformed into hypermutagenic strain Escherichia coli GB20 (DeltaampC mutS::Tn10), and this construct was used in place of H. influenzae bla(ROB-1). Strain GB20 with the cloned gene was submitted to serial passages in tubes containing broth with increasing concentrations of selected beta-lactams (cefotaxime or amoxicillin-clavulanate). Different mutations in the bla(ROB-1) gene were obtained during the passages in the presence of the different concentrations of the selective agents. Mutants resistant to extended-spectrum cephalosporins harbored either the Leu169-->Ser169 or the Arg164-->Trp164 substitution or the double amino acid change Arg164-->Trp164 and Ala237-->Thr237. ROB-1 mutants that were resistant to beta-lactams plus beta-lactamase inhibitors and that harbored the Arg244-->Cys244 or the Ser130-->Gly130 replacement were also obtained. The cefaclor-hydrolyzing efficiencies of the ROB-1 variants were strongly decreased in all mutants, suggesting that if bla(ROB-1) mutants were selected by cefaclor, this drug would prevent the further evolution of this beta-lactamase toward molecular forms able to resist extended-spectrum cephalosporins or beta-lactamase inhibitors.