Phenotypic and genotypic characteristics of antibiotic resistance of commensal Escherichia coli isolates from healthy pigs

The objective of the study was to examine the characteristics of the resistance profiles of Escherichia coli isolated from healthy pigs from three farms in Western Poland. The sensitivity to 13 antimicrobial agents was tested by a disk diffusion method, and the presence of 13 resistance genes was determined by PCR. The majority of the isolates were multi-resistant. The most common multi-resistance patterns were streptomycin, trimethoprim, sulfisoxazole, ampicillin, tetracycline. Although some resistance genes, such as strA/strB, bla_TEM, sul1, sul2, and tetA, were equally represented in isolates from each farm, differences in the distribution of tetB and tetC, hfrV, dhfrXII, and sul1 resistance genes were observed among the isolates from different farms. Approximately one-third (35.9%) of the isolates possessed a class 1 integron. The four major different variable regions of the class 1 integron contained streptomycin (aadA1, aadA2, and aadA5) and/or trimethoprim (dhfrI, dhfrV and dhfrXVII), and/or sulphonamides (sul1) resistance genes. The results of this study emphasise that uncontrolled use of antibiotics causes the development of resistance and provides the evidence of frequent occurrence of more than one gene encoding the resistance to the same antimicrobial agent in the multi-resistant strains.