Mature beagles were fed a ground-meal diet containing 0, 2, or 10 μg of oxytetracycline per g for 44 days. The 10-μg/g diet resulted in a shift from a predominantly drug-susceptible population of enteric lactose-fermenting organisms to a multiply antibiotic-resistant population which peaked at 78% resistant organisms. Since a shift to drug-resistant organisms did not occur in the group fed 2 μg/g, the level of oxytetracycline that results in increased incidence of antibiotic resistance lies between 2 and 10 μg/g in this dog model. Rats and hamsters fed diets containing oxytetracycline (10 μg/g or greater) or dihydrostreptomycin (10 μg/g), and provided suspensions of drug-susceptible Escherichia coli , did not develop a population of antibiotic-resistant organisms.

Swine were fed either a diet containing 110 mg of chlortetracycline (CTC) per kg (100 g/ton) or a control diet and were inoculated orally with Salnonella typhimurium that was either susceptible or resistant to CTC. The quantity, duration, and prevalence of fecal elimination of S. typhimurium, as well as the effect of CTC on the transmission of S. typhimurium from infected to uninfected swine, were determined. When animals were infected with CTC-resistant S. typhimurium, CTC increased the quantity (P < 0.05), duration (P < 0.05), and prevalence (P < 0.01) of fecal shedding, the transmission from infected to uninfected swine, and the recovery of the infecting organism at necropsy. When animals were infected with CTC-susceptible S. typhimurium, CTC reduced the quantity (between 7 and 10 days postinfection) (P < 0.01), duration (P < 0.05), and prevalence (P < 0.05) of fecal shedding, the transmission from infected to uninfected swine, and the recovery of the infecting organism at necropsy. Resistance to tetracycline was transferred in vivo to 4 and 6% of the susceptible infecting S. typhimurium recovered from the untreated and treated groups, respectively.The increased reservoir of S. typhimurium and the transfer of resistance to susceptible S. typhimurium have implications for both animal and public health.

Salmonella cultures were obtained from outbreaks of animal disease from 37 states and 1 territory. They were screened for resistance to 11 antimicrobial drugs. Of the 1,251 strains studied, 935 were resistant to one or more of these agents. The three most common resistance patterns were ampicillin, dihydrostreptomycin, sulfamethoxypyridazine, tetracycline; ampicillin, dihydrostreptomycin, sulfamethoxypyridazine; dihydrostreptomycin, sulfamethoxypyridazine, tetracycline. Resistance transfer was demonstrated on 267 multiply resistant cultures, of which 181 were able to transfer all or part of their resistance pattern to a drug-sensitive recipient. The occurrence of transferable drug resistance of the kind mediated by plasmids, called R factors, has been extensively documented since it was first demonstrated in Japan in 1959. These reports from abroad (2, 3, 5, 6, 11, 12) and the United States (7-10, 15-17, 21) are primarily studies of the phenomenon in human beings and show that it is widespread. Less information is available about the incidence of this type of resistance in animal populations. Smith and Halls Per cent

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The incidence of lactose-fermenting enteric organisms resistant to chloramphenicol increased from less than 0.1% prior to treatment to more than 90% in two groups of chickens fed racephenicol-supplemented feed at 50 or 140 g/ton for 10 days, whereas controls remained unchanged. A concurrent increase in the incidence of organisms resistant to dihydrostreptomycin, oxytetracycline, ampicillin, and sulfonamides also occurred. Minimum inhibitory concentrations for chloramphenicol increased from <10 μg/ml to >300 μg/ml in most Escherichia coli isolates in response to treatment. The observed resistance was transferable to E. coli and Salmonella .