In vivo transfer of R factors between Escherichia coli strains inoculated into the rumen of sheep.

Substantial transfer of R factors occurred in vivo, under certain conditions, in the rumen of adult sheep in the absence of any antibiotic treatment. A starvation period of 24-48 hr. was required to produce the conditions necessary, when even quite low inocula (ca. 10(3) cells) of donor and recipient E. coli could grow within the rumen and reach a population density sufficient for transfer to take place. The results indicate that under the same conditions R factors may be transferred between organisms in the lower intestinal tract also. Without the starvation period, the inoculation of even massive numbers (10(10) cells) of the same organisms resulted in almost no detectable transfer. Some of the experimental animals on which a starvation period was imposed became carriers of either the inoculated recipient E. coli, or of R factor bearing coliforms, and these formed 1-10% of the total coliform population of the faeces for at least 6 weeks.