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Antimicrobial peptides are among the repertoire of host innate immune defenses. In mucosal immunity, the health-disease balance can be greatly modulated by the interplay between host immune factors and colonized microflora. Microbial ecology within dental plaque is constantly shaped by environmental factors present within the oral cavity. Several antimicrobial peptides are detected in saliva and their bactericidal activities against oral bacteria, including Streptococcus mutans, the primary etiologic agent of dental caries, have been clearly demonstrated. However, the role of these antimicrobial peptides in S .mutans ecology and host caries experience is not well-defined. We hypothesized that various strains of S. mutans possess different inherent susceptibility/resistance profiles to host salivary antimicrobial peptides and that host-specific quantities of these peptides may influence plaque colonization by particular S. mutans strains.
S. mutans strains from subjects with variable caries experience were tested for susceptibility to a panel of antimicrobial peptides, including HNP-1-3, HBD-2-3 and LL-37, revealing that the susceptibilities of S. mutans to these peptides were strain-specific. S. mutans strains from high caries subjects showed greater resistance to these peptides at varying concentrations than those from caries-free subjects. In addition, when combinations of these peptides were tested, they showed either additive or synergistic interaction against S. mutans.
Determinations of the salivary levels of these peptides showed that their concentrations were highly variable among subjects with no correlation to host caries experience. However, positive relationships between the salivary concentrations of HNP-1-3 and MS in dental plaque were found. Additionally, the levels of a number of these peptides in saliva appeared to be positively correlated within an individual. An analysis of the salivary peptide concentrations and the susceptibility profiles of S. mutans strains showed that S. mutans strains obtained from subjects with higher concentration of HNP-1-3 in saliva appeared to be more resistant to HNP-1.
Collectively, our findings showed that salivary antimicrobial peptides affect S. mutans ecology by restricting the overall growth of this bacterium within the oral cavity and that their activity may help select resistant strains of S. mutans to colonize within dental plaque. The relative ability of S. mutans to resist host salivary antimicrobial peptides may be considered a potential virulence factor for this species.