Molecular Screening of Enterococcus Virulence Determinants and Potential for Genetic Exchange between Food and Medical Isolates

Enterococci are used as starter and probiotic cultures in foods, and they occur as natural food contaminants. The genus Enterococcus is of increased significance as a cause of nosocomial infections, and this trend is exacerbated by the development of antibiotic resistance. In this study, we investigated the incidence of known virulence determinants in starter, food, and medical strains of Enterococcus faecalis, E. faecium, and E. durans. PCR and gene probe strategies were used to screen enterococcal isolates from both food and medical sources. Different and distinct patterns of incidence of virulence determinants were found for the E. faecalis and E. faecium strains. Medical E. faecalis strains had more virulence determinants than did food strains, which, in turn, had more than did starter strains. All of the E. faecalis strains tested possessed multiple determinants (between 6 and 11). E. faecium strains were generally free of virulence determinants, with notable exceptions. Significantly, esp and gelE determinants were identified in E. faecium medical strains. These virulence determinants have not previously been identified in E. faecium strains and may result from regional differences or the evolution of pathogenic E. faecium. Phenotypic testing revealed the existence of apparently silent gelE and cyl genes. In E. faecalis, the trend in these silent genes mirrors that of the expressed determinants. The potential for starter strains to acquire virulence determinants by natural conjugation mechanisms was investigated. Transconjugation in which starter strains acquired additional virulence determinants from medical strains was demonstrated. In addition, multiple pheromone-encoding genes were identified in both food and starter strains, indicating their potential to acquire other sex pheromone plasmids. These results suggest that the use of Enterococcus spp. in foods requires careful safety evaluation.