E. coli increasingly is emerging as an organism expressing multidrug resistance (MDR) in dogs or cats. We have demonstrated that administration of enrofloxacin at routine doses results in the emergence of E. coli isolates exhibiting MDR, whereas amoxicillin at routine doses does not lead to MDR. The purpose of this study is to further characterize the phenotypic and genotypic expression of resistance in isolates in which non-MDR versus MDR has emerged following treatment with either amoxicillin at 10 mg/kg orally twice daily (n=8 dogs; no MDR detected) or enrofloxacin at 5 mg/kg orally once daily (n=8 dogs; only MDR detected; see sister abstract and poster). Ten E. coli isolates were randomly selected from the population of resistant isolates recovered from each dog receiving either drug. Isolate phenotype status was determined by a commercial antibiogram (Vitek®; Amoxicillin = A, Amoxicillin/clavulanic acid = X, Ceftiofur = C, Tetracycline = T, Enrofloxacin = E, Gentamicin = G, Sulfa/trimethoprim = S). Isolates collected from dogs treated with the same drug were subgrouped based on phenotype. For either drug, all resistant isolates cultured from the same dog were phenotypically similar. For amoxicillin treated dogs, 3 phenotypes were present among the 8 dogs studied: AC (G1), ACT (G2) and AT (G3). For enrofloxacin treated dogs, 2 MDR phenotypes were present among the 8 dogs: ATE (G4) and AXTEGS (G5) respectively.
Three representative isolates from each group were subjected to pulse-field gel electrophoresis (PFGE) with XbaI restriction for genotype analysis. For G1 and G2, genotypes were the same (based on 90% homology) despite differing phenotypes. For G3 (AT phenotype, representing 3 amoxicillin treated dogs), 3 distinct genotypes emerged, one in each dog. For enrofloxacin treated dogs, each phenotype was genetically distinct. These data support clonal expansion of resistant isolates (e.g., only one phenotype/genotype emerged in each dog) for both drugs. However, amoxicillin resistance tended to be genotypically similar among dogs despite phenotypic differences in dogs, whereas enrofloxacin resistance tended to be both genotypically and phenotypically different among dogs. These data suggest that routine antimicrobial use impacts on the emergence of antimicrobial resistance, with clonal expansion supportive of selection pressure as a mechanism of emergent resistance. Further, genotypes may not be related to phenotypes for some drugs (e.g., enrofloxacin). The differences in genotypes for enrofloxacin may contribute to the MDR.