DETECTION OF TETRACYCLINE RESISTANCE DETERMINANTS IN PIG ISOLATES FROM 3 HERDS WITH DIFFERENT HISTORIES OF ANTIMICROBIAL AGENT EXPOSURE

A total of 114 gram-negative fecal isolates from domestic pigs in herd s with different histories of antimicrobial agent exposure were screen ed for the presence of plasmid DNA and specific tetracycline resistanc e determinants. More than 84% of the isolates harbored plasmid DNA, wh ich ranged in size from 2.1 to 186 kb. A total of 78 isolates (68.4%) were resistant to tetracycline at concentrations greater than 4 mug/ml . Plasmid DNAs from about 56% of the tetracycline-resistant isolates h ybridized with DNA probes for class A, B, C, and D tetracycline resist ance determinants. The class B determinant was the most common determi nant (35% of the isolates), followed by the class C determinant (12%) and the class A determinant (1%). About 9% of the isolates contained t wo determinants on plasmids. None of the plasmids from isolates hybrid ized with the class D determinant probe. The class C determinant was t he most prevalent determinant on plasmids in isolates from pigs not ex posed to antimicrobial agents for more than 146 months, while the clas s B determinant was more prevalent on plasmids in isolates from pigs e xposed to either subtherapeutic or therapeutic levels of antimicrobial agents. Most tetracycline resistance determinants were localized on p lasmids which were more than 30 kb long. A great number of wild-type t etracycline-resistant Escherichia coli strains were found with the cla ss E determinant on their chromosomes. This study revealed a high prev alence of tetracycline resistance determinants in the fecal flora of p ig herds whether or not they were fed with antibiotics.