The effect of chlortetracycline feed additive on experimental salmonella infection of swine and antibiotic resistance transfer.

Four Salmonella strains were used. With two, S. canada and S. enteritidis, establishment of infection in pigs was poor. Although improved establishment was achieved with two strains of S. typhimurium, no infections were extensive. No significant difference was detected between excretion of infecting strains by pigs receiving chlortetracycline at levels of ten gm per ton of feed and that by control counterparts. Comparable establishment of S. typhimurium S192 was achieved in pigs receiving 20 or 40 gm of chlortetracycline per ton. Resistant variants were isolated from some animals in all experiments using S. typhimurium S192. Almost without exception such variants were isolated only after specimen enrichment. The numbers of specimens from test animals yielding resistant variants were more than twice those of control counterparts. Numbers of such variants in material from the former were much greater. In vitro transfer of resistance to S. typhimurium S192 was demonstrated both in waters from drinking troughs and during specimen enrichment. The former, indicating the risk of transfer in pigs' environments and packing houses, has disturbing implications for public health. The latter emphasizes the difficulties of determining antibiotic sensitivity of pathogens as they occur in the animal body.