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The potential of bambermycins (a growth-promoting antimicrobial approved for turkeys, broilers, and swine) to overcome or control plasmid-mediated antimicrobial resistance was determined in a series of in vitro experiments. Four possible modes of action of bambermycins were studied: synergistic effect with 12 other antimicrobials, elimination of resistance (R) plasmids from Escherichia coli, selective killing or inhibition of E coli carrying R plasmids, and inhibition of R plasmid transfer. Bambermycins had no synergistic activity with the other drugs tested and had little effect on eliminating plasmids from host bacteria. Dependent on plasmid type, bambermycins decreased or increased transfer frequency of R plasmids. Bambermycins also selectively inhibited growth of bacteria harboring certain R plasmids.

Twenty broiler chickens were fed bambermycins (Flavomycin; an antibiotic produced by Streptomyces) at the rate of 3 g/ton (US) for 63 days, and 20 control birds were fed nonmedicated feed. The birds were inoculated (dosed) on the 10th and 11th feeding day with Salmonella typhimurium. The study evaluated the effects of bambermycins on Salmonella incidence, shedding, and antimicrobial resistance. Bambermycins had no effect on body weights, duration of shedding of salmonellae, number of salmonellae shed on postdosing day 3, tissue recoverability of salmonellae, and total number of resistance patterns. Bambermycins resulted in the decrease of salmonellae to be more gradual; however, both treatments were comparable at the end of the study. The majority of S typhimurium from bambermycins-treated birds maintained the original antibiogram of streptomycin, sulfadiazine, and nalidixic acid. The salmonellae isolated from the control birds were more resistant to 2 drugs (varying antibiograms). Bambermycins as a feed additive in broiler diets given at the dose level of 3 g/ton had no detrimental effects based on salmonellae shedding and antimicrobial resistance.

Enteric bacteria isolated from feedlot heifers treated with tetracyclines (TET) were examined for resistance to TET and ampicillin. The effects of feeding (45 days) therapeutic and subtherapeutic quantities of chlortetracycline (CTC) and the injection of therapeutic doses of oxytetracycline (3 daily doses) were compared. Performance data from the 45-day trial did not identify an antibiotic effect on average daily gain, although cattle fed subtherapeutic quantities of CTC showed improved feed efficiency. Bacterial resistance to TET was increased in the heifers given the therapeutic and subtherapeutic quantities of CTC in the ration. However, an increase in resistance to ampicillin did not occur. Oxytetracycline injections had a short-term effect, increasing the number of resistant organisms only during the week of drug administration.