The effects of oxytetracycline on the intestinalEscherichia coliflora of newly weaned pigs

Abstract

SUMMARYFour recently weaned pigs were dosed orally with oxytetracycline. This caused a rapid increase in the incidence of tetracycline resistance (TcR) amongEscherichia coliisolates from the faecal flora. The isolates were differentiated further on the basis of O-serogroup, biotype and resistance pattern. There was no evidence that the administration of the antibiotic selected for a few TcR clones, but rather a relatively large number of TcR strains were identified during the dosing period. Using selective isolation media a proportion of these strains were demonstrated in the minority faecalEsch. coliflora before dosing, while the remainder were recognized for the first time after dosing commenced.The incidence of TcR amongEsch. coliisolates also increased after weaning in other pigs which were not dosed with oxytetracycline or any other antibacterial agent. In a proportion of these animals this increase was associated with the dominance of a TcR enteropathogenic serotype (0149:K 91, K 88a, c) in the faecalEsch. coliflora which was probably ingested in small numbers before weaning. The source of other TcR strains was probably the environment in which each pig was placed after weaning.