Effect of flavophospholipol on conjugation frequency between Escherichia coli donor and recipient pairs in vitro and in the chicken gastrointestinal tract.

OBJECTIVES To examine the ability of flavophospholipol to inhibit bacterial conjugation between Escherichia coli donor and recipient pairs in vitro and in day-of-hatch chickens. METHODS In vitro donor cultures were incubated in the presence of 0, 2, 4, 8, 16, 32 and 64 mg/L flavophospholipol during primary overnight mono-cultures only, secondary conjugation cultures only, or throughout primary and secondary cultures. Transconjugants were selected using oxytetracycline and nalidixic acid. Treatment groups A-G (n=20) of day-of-hatch broiler chickens received 0, 2, 4, 8, 16, 32 and 64 g/ton flavophospholipol, respectively, in their feed throughout the experiment. On day 4, all treatment groups were given 0.25 mL of donor and recipient E. coli at 7.0 and 9.0 log10 cfu/mL, respectively. On day 10, the birds were euthanized and the caecal contents were cultured on selective medium (oxytetracycline and nalidixic acid). RESULTS A dose-dependent reduction in transconjugant populations was observed in vitro when flavophospholipol was present in the secondary conjugation culture. The susceptibility profiles of transconjugants obtained from in vitro studies were identical to the predicted profile of the donor and recipient combination. There was no significant difference (P>or=0.05) in the number of transconjugants isolated from chickens among any of the flavophospholipol treatment groups when compared with the controls. The susceptibility profiles of chicken transconjugants suggested acquisition of naturally occurring plasmids. CONCLUSIONS Flavophospholipol strongly inhibited conjugation in vitro, but did not prevent recipient E. coli from acquiring resistance determinants in vivo.