Susceptibility of various oral bacteria to antimicrobial peptides and to phagocytosis by neutrophils

Background and Objective: The aim of this study was to compare the susceptibility of nonperiodontopathic and periodontopathic bacteria to major defense mechanisms for bacterial clearance in gingival sulcus. Material and Methods: Twenty strains of 13 oral bacterial species were studied for their susceptibility to phagocytosis by human neutrophils and to the antimicrobial peptides LL-37 and human beta defensin-3. The minimum inhibitory concentrations of LL-37 and human beta defensin-3 were determined by a liquid dilution assay, and susceptibility to phagocytosis was examined by a flow cytometric phagocytosis assay. Results: The minimum inhibitory concentrations of LL-37 and human beta defensin-3 varied greatly, depending on the strain and species. Although a significant difference between the non- and periodontopathic groups was not observed, the red-complex bacteria were more resistant to LL-37 than the others (p = 0.004). The susceptibility of oral bacteria to phagocytosis was quite variable, depending on the species but not on the strains. The periodontopathic bacteria, especially Actinobacillus actinomycetemcomitans and the red-complex triad, were more resistant to phagocytosis than were the nonperiodontopathic bacteria (p = 0.0003). In addition, bacteria resistant both to antimicrobial peptides and to phagocytosis were more common in the periodontopathic group. Conclusion: Our results indicate that immune evasion may contribute to the pathogenicity of some periodontopathic bacteria.