Trimethoprim resistance in commensal bacteria isolated from farm animals.

Trimethoprim resistance was examined in faecal bacteria obtained from chickens, sheep, cattle and pigs. The incidence of trimethoprim resistance in porcine strains was 17% (157/922) and, whereas 15.8% (146/922) of these bacteria were highly resistant, only 4% (37/922) of the isolates possessed trimethoprim resistance plasmids. Highly resistant porcine strains were obtained from 44% of the pig farms (41/93) but transferable trimethoprim resistance was found in isolates from 11% (10/93) of the farms. There was an association between the carriage of trimethoprim resistance plasmids and certain farms. Most of the resistance plasmids were not identical with those found in human clinical bacteria but one porcine plasmid was the same as the most ubiquitous trimethoprim resistance plasmid in Edinburgh.