Effect of low concentrations of dihydrostreptomycin on drug resistance in enteric bacteria

Beagle dogs were fed a diet containing 0, 2, or 10 mug of dihydrostreptomycin (DSM) per g of feed. The 2-mug/g level was selected to represent a residue level of the antibiotic. In both treatment groups, medicated feed resulted in a shift from a predominantly streptomycin (SM)-susceptible coliform fecal population to an SM-resistant population. The proportion of resistant organisms was significant (P < 0.01) for both treatment groups. A definitive response did not occur with animals maintained on DSM-free diets. An increase in the prevalence of DSM-resistant organisms was observed after 15 days of DSM-supplemented feeding and persisted during the posttreatment phase of the study. The predominant pattern of resistance was SM-sulfamethoxypyridazine. Fifty-nine percent of SM-resistant strains transferred resistant determinants by conjugation to Escherichia coli K-12 recipients.