Members of oral biofilms: to inhibit or to be inhibited?

Objectives: The objectives of this study were to identify beneficial species which can antagonize the growth of the main periodontopathogens, to determine the antimicrobial substances which are involved in these inhibitory activities and to evaluate the influence of environmental factors on the magnitude of these inhibitions.
Methods: The spotting technique, a competitive assay which consists of inoculating two spots of competing bacteria nearby, was used to detect and quantify inhibitory effects of 14 commensal bacteria on 3 periodontopathogens. The amount of inhibition was quantified by measuring the distance between the edge of the commensal spot and the edge of the spot of the pathogen. Different techniques were used to determine the relative contribution of bacteriocins, acids and H₂O₂ production. Sequence of inoculation and the effect of environmental conditions were analyzed as determining factors for the amount of inhibition.

Results: Of the 14 tested commensal species, only 6 streptococcus species could antagonize the growth of the periodontopathogens. For these 6 species, H₂O₂ production was the most important inhibitory mechanism. Inhibition was only observed when the commensal species were inoculated prior to the pathogens. The inhibitory effect was strictly regulated by environmental factors: oxygen availability, cell density and nutritional depletion were crucial for the competition and coexistence between the commensal and pathogenic species.

Conclusions: Due to their inhibitory effects, 6 commensal species could be classified as beneficial bacteria. Their primary mode of interaction was H₂O₂ production. Although H₂O₂ production is highly regulated by environmental factors, it might have a much more important role on biofilm development and composition than previously anticipated.