Competitive effect of commensal faecal bacteria from growing swine fed chlortetracycline-supplemented feed on β-haemolytic Escherichia coli strains with multiple antimicrobial resistance plasmids.

To determine the differences in competitive fitness among Escherichia coli strains with different plasmid profiles when grown in suspension with commensal faecal bacteria from growing swine fed chlortetracycline-supplemented or unsupplemented diets. Five multiple drug-resistant (MDR) E. coli strains that possessed 0, 2, 6 or 8 plasmids were inoculated into anoxic faecal cultures from swine fed an unsupplemented (control) or chlortetracycline (50 g ton(-1))-supplemented (experimental) diet. On days 21 of chlortetracycline supplementation, faecal growth competition studies were performed. MDR E. coli were enumerated at 0, 6 and 24 h. The plasmid-free strain was below culturable limits in both the control and experimental cultures by 24 h. For each plasmid-bearing strain, there was no statistically significant difference in population CFU ml(-1) (P < 0.05) between the control and experimental cultures. There was no significant effect on the faecal microflora, owing to the inclusion of chlortetracycline, in the swine diets, that affected the growth of E. coli in the competition studies employed. Furthermore, these results suggest that the cost of maintaining plasmids in these E. coli strains had little influence on survivability. Mutations that led to antimicrobial resistance may have a greater impact on survivability than multiple plasmid carriage.