Heterologous Inducible Expression of Enterococcus faecalis pCF10 Aggregation Substance Asc10 in Lactococcus lactis and Streptococcus gordonii Contributes to Cell Hydrophobicity and Adhesion to Fibrin

Aggregation substance proteins encoded by the sex pheromone plasmid family ofEnterococcus faecalis have been shown previously to contribute to the formation of a stable mating complex between donor and recipient cells and have been implicated in the virulence of this increasingly important nosocomial pathogen. In an effort to characterize the protein further,prgB , the gene encoding the aggregation substance Asc10 on pCF10, was cloned in a vector containing the nisin-induciblenisA promoter and its two-component regulatory system. Expression of aggregation substance after nisin addition to cultures ofE. faecalis and the heterologous bacteriaLactococcus lactis andStreptococcus gordonii was demonstrated. Electron microscopy revealed that Asc10 was presented on the cell surfaces ofE. faecalis andL. lactis but not on that ofS. gordonii . The protein was also found in the cell culture supernatants of all three species. Characterization of Asc10 on the cell surfaces ofE. faecalis andL. lactis revealed a significant increase in cell surface hydrophobicity upon expression of the protein. Heterologous expression of Asc10 onL. lactis also allowed the recognition of its binding ligand (EBS) on the enterococcal cell surface, as indicated by increased transfer of a conjugative transposon. We also found that adhesion of Asc10-expressing bacterial cells to fibrin was elevated, consistent with a role for the protein in the pathogenesis of enterococcal endocarditis. The data demonstrate that Asc10 expressed under the control of thenisA promoter in heterologous species will be an useful tool in the detailed characterization of this important enterococcal conjugation protein and virulence factor.