Background and purpose: Enterococcus faecium is a normal bowel commensal and lactic acid bacterium that is rarely found in the oral cavity. This study investigated whether a non-pathogenic and non-biofilm strain of E. faecium functioned as a probiotic strain toward biofilm formation by Streptococcus mutans, which is an etiological agent for dental caries.
Methods: The effects of E. faecium on streptococcal biofilm was evaluated by absorbance of safranin-stained biofilm at 492 nm in a 96-well microtiter plate.
Results: The E. faecium strain demonstrated cell-number-dependent inhibition of biofilm in dual cultures with 4 laboratory and 16 clinical S. mutans strains, as well as laboratory strains of Streptococcus sobrinus and Streptococcus sanguinis, in 96-well microtiter plates. The inhibiting effects of E. faecium were not dependent on the production of bacteriocin from streptococci and E. faecium, low pH after mix culture, or biofilm formation levels of S. mutans. A culture supernatant sample of more than 10 kDa from E. faecium showed direct inhibiting effects toward S. mutans biofilm formation. Treatment of heat, butanol, and phenol to a supernatant sample restored biofilm formation in culture of S. mutans with the sample. Moreover, the tendencies of inhibition levels by the supernatant sample were associated with those by bacterial cells of E. faecium to S. mutans strains.
Conclusions: The E. faecium non-biofilm strain produced an inhibiting protein to streptococci biofilm formation, showed various susceptibilities to inhibit streptococcal biofilm, and acted as a probiotic bacterial inhibitor of streptococcal biofilm formation.