A total of 199 streptococci isolated from feces of healthy chickens, pigs, and beef cattle and 26 human clinical isolates were tested for resistance to kanamycin, streptomycin, tetracycline, erythromycin, and lincomycin. Of 66 isolates resistant to these antibiotics, 12 transferred one or more resistance traits by conjugation in broth. Erythromycin resistance (Emr) was transferred from 10 of the 12 successful donors. AvaI digests of plasmids isolated from Emr transconjugants derived from two human, two chicken, and one pig isolate contained three fragments similar in size to those produced from Tn3871, an Emr transposon. The three fragments from each of the five digests on Southern blots hybridized to radiolabeled Tn3871. Plasmid DNA from a transconjugant derived from a second pig isolate contained two of the three Tn3871-associated AvaI fragments. One of the AvaI fragments from each of the six plasmids hybridized with a radiolabeled probe containing a cloned AvaI fragment from Tn3871 that contained the Emr determinant. Transposition of the Emr trait was demonstrated for the plasmids derived from one human and one pig isolate. We concluded that extensive DNA homology existed between plasmids from streptococcal strains obtained from two human patients, two chickens, and two pigs and the Emr transposon Tn3871, which is very similar or identical to the well-characterized Emr transposon Tn917. The detection of Tn3871-like sequences in streptococcal isolates from Arkansas, Illinois, North Carolina, and Washington, D.C. indicates wide dissemination of Emr mediated by the same or closely related transposons.

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.

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