In vivo transfer of an Escherichia coli enterotoxin plasmid possessing genes for drug resistance.

: Experiments were conducted to study transfer of an enterotoxin (Ent) plasmid from a porcine enteropathogenic Escherichia coli to an E coli K12 strain in the intestine of newly weaned pigs. The Ent plasmid carried genes for resistance to tetracycline, streptomycin, and sulfonamides, thereby permitting a selection for tetracycline-resistant exconjugants in the feces of the pigs. In vivo transfer of the Ent plasmid was demonstrated to occur when the pigs were given large oral inocula of donor and recipient cultures, 1 hour apart. Differences in extent of transfer were not detected in pigs given antibiotic-free feed compared with littermates on feed containing oxytetracycline at 50 g/ton. In one experiment, tetracycline-resistant Ent- exconjugants were found which appeared to have received an R plasmid from an enteropathogenic type of E coli resident in the intestine.