Patterns and mechanisms of resistance to β-lactams and β-lactamase inhibitors in uropathogenic Escherichia coli isolated from dogs in Portugal

Little is known about beta-lactam and beta-lactamase inhibitor susceptibilities of uropathogenic Escherichia coli isolates from animals. Seventy-two isolates collected from canine urinary tract infections were studied by disc diffusion and microdilution methods. The mechanisms responsible for amoxicillin resistance were associated with the production of beta-lactamases in 26 (36%) isolates. These beta-lactamases were further characterized by isoelectric focusing (IEF) and PCR, and bla(TEM), bla(OXA-1), bla(SHV) and ampC genes were detected. The isolates were highly resistant to amoxicillin and ticarcillin, with MIC(90)s of 2048 mg/L. The MIC90 of cefalothin was 128 mg/L, but the MIC(90)s of ceftazidime, ceftriaxone, cefotaxime and aztreonam were lower (0.5, 0.06, 0.25 and 0.5 mg/L, respectively). Fourteen isolates were not inhibited by clavulanate. The mechanisms of resistance to beta-lactams and beta-lactamase inhibitors involved the presence of TEM-1 beta-lactamase in 20 isolates, which had an isoelectric point (pI) of 5.4 and were positive for the presence of the bla(TEM) gene. Fourteen of these isolates produced TEM-1 beta-lactamase alone, and the other six showed an additional band at pI 9.0-9.2 on IEF and the ampC gene by PCR, indicating the simultaneous production of AmpC enzyme. IEF showed that one isolate produced AmpC alone and PCR detected the presence of the ampC gene. Three of the 26 beta-lactamases with a pI of 7.6 belonged to the SHV family, which was confirmed by the presence of the bla(SHV) gene. The remaining two beta-lactamases were OXA-1 focusing at 7.4, and were encoded by the bla(OXA-1) gene. Resistance to beta-lactamase inhibitors was mediated mainly by TEM-1 alone (six of 26) or together with AmpC (four of 26), AmpC alone (one of 26), SHV (one of 26) and OXA-1 (two of 26) enzymes. Clear resistance to extended-spectrum cephalosporins, ceftazidime and ceftriaxone (64 mg/L), was found in one isolate. Isolates producing either AmpC or OXA-1 enzymes or producing high levels of TEM-1 beta-lactamases had susceptibility patterns that were difficult to distinguish without IEF and/or amplification of the corresponding specific genes. This work supports the need for antimicrobial resistance surveillance in veterinary medicine.