Molecular Characterization of Amoxicillin-Clavulanate Resistance in a Clinical Isolate of Escherichia coli

Abstract

The resistance phenotype of the clinical isolate of Escherichia coli 1941 was characterized by high-level resistance to penicillins and to combinations amoxicillin-ticarcillin/clavulanate and ampicillin/sulbactam. This resistance was carried by the conjugative plasmid pEC1941 that encoded a β-lactamase activity. The purified enzyme focused at pI 5.4 and was strongly inhibited in vitro by clavulanic acid (IC₅₀ = 0.09 μM). Nucleotide sequence analysis revealed identity between the plasmid borne bla_TEM gene of E. coli 1941 and the bla_TEM-1B gene, except for a single C-to-T substitution at position 32 in the promoter region leading to the overlapping promoters Pa and Pb. No alterations in the expression of outer membrane porins OmpC and OmpF have been detected. These findings show that the resistance of E. coli 1941 to the combinations of β-lactams with β-lactamase inhibitors is related to high-level production of TEM-1 enzyme expressed from the strong promoters Pa and Pb.