Sub-lethal stress effects on virulence gene expression in Enterococcus faecalis

Enterococci are ubiquitous lactic acid bacteria commonly associated with the human digestive tract as commensal organisms. Additionally, these organisms have a long history of use in foods improving flavor as well as providing protective mechanisms as either a probiotic or antimicrobial additive. However, Enterococcus faecalis accounts for up to 10% of all nosocomial infections of the bloodstream, wounds, urinary tract and heart. Knowledge about the regulation of virulence factors is limited and the involvement of environmental signals contributing to E. faecalis pathogenicity is poorly documented. In this study, two clinical E. faecalis isolates, TMW 2.63 and OG1RF, as well as one food isolate, TMW 2.629, were subjected to six sub-lethal food- and host-related stresses including 6.8% NaCl, 200 ppm nitrite, 51 degrees C, 80 MPa, pH 4.1 and 0.08% bile salts (cholic acid:chenodeoxycholic acid 1:1), respectively, reducing their growth rate to 10%. Relative gene expression of 15 stress and virulence-associated genes including dnaK, groEL, ctsR, clpPBCEX, gls24, efaAfs, ace, fsrB, gelE, sprE and cylB, was quantified by using real time PCR and Lightcycler((R)) technology (reference conditions: BHI broth, 37 degrees C, pH = 7.4). Apart from strain-dependent differences, sub-lethal environmental stress was capable of provoking significant alterations in the expression of virulence-associated genes in E. faecalis from clinical as well as food origins of isolation. These results help to avoid preconditioning enterococci in food production processes and to understand the complex mechanisms in E. faecalis' switch to pathogenicity.