Transfer of Pheromone-Inducible Plasmids between Enterococcus faecalis in the Syrian Hamster Gastrointestinal Tract

Pheromone-responsive plasmids are common to Enterococcus faecalis, transfer at high frequency in vitro, and carry cytolysin and other gene products implicated in the pathogenesis of enterococcal infection. A Syrian hamster model of enterococcal intestinal overgrowth was used to test for transfer of three isogenic plasmids differing in conjugative and cytolytic phenotypes. Transconjugants were found in 8 (44%) of 18 and 6 (35%) of 17 hamsters given donor strains containing cytolytic (pAM714) and noncytolytic (pAM771) pheromone-responsive plasmids. Of the 14 hamsters from which transconjugants were isolated from stool, 9 (64%) had transconjugants 1 day after donor strain inoculation. The frequency of transfer (mean +/- SD) for pAM714 and pAM771 was 1.4 +/- 2.2 x 10(-1) and 2.9 +/- 4.2 x 10(-2) transconjugants/donor, respectively (P > .20). Transconjugants were not recovered from hamsters receiving a cytolytic, nonconjugative plasmid (pAM930; transfer frequency < 2 x 10(-5) transconjugants/donor). Pheromone-responsive plasmid transfer between E. faecalis strains occurs at high frequency in the gastrointestinal tract of hamsters and may be one means by which enterococcal resistance and virulence factors disseminate.