The Transfer Of Antibiotic Resistance Between Strains Of Enterobacteria In Chicken, Calves And Pigs

In mixed culture experiments, great variation was found between different strains of Escherichia coli and salmonellae in their ability to transmit or to receive antibiotic resistance (R factors).

Resistance transmission was demonstrated to occur in the alimentary tract of chicken given strains by mouth, but only between strains that were known to be good donors and recipients in vitro and able to colonise the alimentary tract reasonably well. Sometimes, the R factors appeared to infect the recipient organisms in the alimentary tract in epidemic form. Some strains of S. typhimurium, including one of phage-type 29, were outstandingly good recipients in vivo; organisms of these strains that had received R factors were not uncommonly found in the liver as well as in the faeces of experimental chicken. It was not necessary to give each chick both the prospective donor and the prospective recipient strain. For example, transfer occurred in chicken that were mixed together after some had been given a prospective donor strain of E. coli and others a prospective recipient strain of S. typhimurium.

During transfer studies in chicken, feeding with a diet containing an antibiotic that featured in the transmissible resistance pattern of the prospective donor strain occasionally had a suppressive effect on the numbers of prospective recipient organisms in the faeces. Usually, however, it led to a great increase in the proportion of R-factor-containing recipient organisms, both in the faeces and in the liver. Chloramphenicol did not appear to influence the situation. This was because the drug is absorbed in the anterior part of the alimentary tract and well before the caeca, the organs in which resistance transfer was found to occur.

Resistance transfer from E. coli to S. typhimurium was also demonstrated to occur in the alimentary tract of calves but not of pigs. Poor colonisation of the alimentary tract by S. typhimurium was the probable reason for the failure in pigs; transfer from E. coli to E. coli, however, was demonstrated in these animals.

The results are discussed with particular reference to the high incidence of antibiotic-resistant strains of S. typhimurium in intensively reared calves.