The persistence of drug resistant Escherichia coli strains in the majority faecal flora of calves.

Two groups of calves, one of three and the other of two animals, were purchased in markets and reared initially on a commercial veal unit for 1 month and 4 months respectively. They were then moved to the Veterinary School, Langford, and kept for a further 6 and 4 months respectively. The animals were sampled weekly and a continual turnover in the strains forming the majority Escherichia coli faecal flora was demonstrated for all calves. Antibacterial-drug resistance, as measured by an Antibiotic Resistance Index (ARI), increased after arrival on the veal unit and persisted at high levels during the whole of their stay. After moving to Langford the ARI fell. Initially there was a reduction in the average number of resistance determinants per resistant strain and then, after a delay of up to 8 weeks, by an increase in the proportion of isolates that were fully sensitive. The source of the sensitive strains was not ascertained, although their appearance was not associated specifically with either weaning or turning out to pasture.