Interference in Pheromone-Responsive Conjugation of a High-Level Bacitracin Resistant Enterococcus faecalis Plasmid of Poultry Origin
Bile-resistant bacteria, particularly gram-positive Enterococcus faecalis and Enterococcus faecium, play an important role in biliary stent occlusion, because their sessile mode of growth protects them against host defenses and antimicrobial agents. Twelve E. faecalis and seven E. faecium strains isolated from occluded biliary stents have been investigated for slime production, presence of aggregation substance genes, and ability to adhere to Caco-2 cells. Ten isolates were strong producers of slime, and seven isolates produced clumps when exposed to pheromones of E. faecalis JH2-2 and/or OG1RF. The small E. faecium clumps differed from the large clumps of E. faecalis and were similar to those of E. faecium LS10(pBRG1) carrying a pheromone response plasmid. After induction with pheromones, the adhesion to Caco-2 cells of clumping-positive strains was found to increase from two- to fourfold. Amplicons of the expected size were detected in three clumping-positive and three clumping-negative E. faecalis isolates by using primers (agg) internal to a highly conserved region of the E. faecalis pheromone response plasmids pAD1, pPD1, and pCF10 and primers internal to prgB of the E. faecalis plasmid pCF10. The agg/prgB-positive E. faecalis strains were also positive in Southern hybridization experiments with a prgB-specific probe. No PCR products were obtained with the same primers from four clumping-positive isolates (one E. faecalis and three E. faecium strains), which were also Southern hybridization negative. Our results demonstrate that slime production and pheromone response are both present in isolated enterococci, suggesting that clinical strains with these features might have a selective advantage in colonizing biliary stents.