Method to determine effect of antibiotics at residue levels on R-factor transfer

An analytical system was developed which can assess the ability of antibiotic/antimicrobial residues (0.01-1.00 ppm) to affect the conjugal transfer of resistance among the Enterobacteriaceae. The donor strain, Escherichia coli RP-4 (Amr Tcr Nmr Kmr Lac+), and recipient strain, E. coli Sc-8632 (Smr Lac-), were incubated together in a 1:9 donor:recipient ratio for 18 h with gentle shaking (50 rpm) in brain heart infusion broth in the presence of residue levels of antibiotics. The mating cultures were serially diluted and spread-plated onto MacConkey agar containing 25 micrograms streptomycin/mL to select the total recipient population of sensitive E. coli Sc-8632 and transconjugants. After an 18 h incubation at 37 degrees C, the plates were replicated onto MacConkey agar containing 25 micrograms ampicillin/mL to select the ampicillin-resistant transconjugant population. Repeatability was good; the average transfer was 51.8%, with a coefficient of variation of 9.3%. Residue levels of tylosin (0.10 and 1.00 ppm) increased the transfer of the ampicillin marker beyond the 95% confidence limits. Oxytetracycline, bacitracin, streptomycin, penicillin, and virginiamycin did not increase the percent transfer. Oxytetracycline at 0.01 ppm decreased the percent transfer. In general, residue levels of antibiotics (0.01-1.00 ppm) did not affect the conjugal transfer of antibiotic resistance.