Inhibition of Vaginal Lactobacilli by a Bacteriocin-Like Inhibitor Produced by Enterococcus faecium 62-6: Potential Significance for Bacterial Vaginosis

OBJECTIVE Bacterial vaginosis (BV) is characterized by a shift in vaginal tract ecology, which includes a decrease in the concentration and/or prevalence of facultative lactobacilli. Currently, mechanisms which could account for the disappearance of lactobacilli are not well understood. The objective of this study was to determine whether vaginal streptococci/enterococci can produce bacteriocin-like inhibitors antagonistic to vaginal lactobacilli. METHODS Seventy strains of vaginal streptococci or enterococci were tested for antagonistic activities against vaginal lactobacilli using the deferred antagonism technique. RESULTS One strain, Enterococcus faecium 62-6, which strongly inhibited growth of lactobacilli was selected for further characterization. The spectrum of inhibitory activity of strain 62-6 included Gram-positive organisms from the vaginal environment, although native lactobacilli from the same host were resistant to inhibitor action. Following growth in MRS broth the strain 62-6 inhibitor was shown to be heat- (100 degrees C, 30 minutes), cold- (4 degrees C, less than 114 days) and pH- (4-7) stable. The sensitivity of inhibitor-containing supernatants to pepsin and alpha-chymotrypsin suggested an essential proteinaceous component. The inhibitor was sensitive to lipase but resistant to lysozyme. Dialysis of inhibitor-containing culture supernatants suggested a molecular mass greater than 12,000 Da. All physicochemical properties were consistent with its classification as a bacteriocin-like inhibitor. Kinetic assays demonstrated a sharp onset of inhibitor production coinciding with a concentration of 62-6 of 10(7) cfu/ml, suggesting that production may be regulated by quorum sensing. CONCLUSIONS These results may have clinical significance as a novel mechanism to account for the decline of vaginal Lactobacillus populations and contribute to both the establishment and recurrence of BV.