The effect of chlortetracycline feed additive on the antibiotic resistance of fecal coliforms of weaned pigs subjected to experimental salmonella infection

The effect of chlortetracycline fed at concentrations of 10 or 20 grams per ton on the antibiotic resistance of fecal coliforms from weaned pigs from four different farms was studied. At admission to experimental quarters, coliforms resistant to at least one antibiotic predominated in all groups of animals. In pigs from three farms, chlortetracycline caused a reduction in the proportion of sensitive coliforms excreted, the extent of reduction depending on feeding practices on the farm of origin. In another experiment using pigs from a fourth farm, almost all coliforms isolated were resistant. Of all resistant strains isolated, those carrying resistance to a multiplicity of antibiotics predominated, and resistance to tracycline unaccompanied by that to other agents was relatively infrequent. Special studies of hemolytic coliforms from pigs from one farm identified two particular antibiotic sensitive types. No further isolations of these types were made once chlortetracycline administration began although 21 additional isolates were made from littermates receiving unsupplemented ration. These findings suggest that the elimination of sensitive strains resulting from chlortetracycline administration was due to a replacement of these strains by different resistant types rather than by acquisition of resistance by previously sensitive strains.