The study shows that a gum‑disease bacterium, Porphyromonas gingivalis, uses special surface proteins (Pgm6/Pgm7) to hide from the human antimicrobial peptide LL‑37. When those proteins are removed, LL‑37 sticks to the bacteria and kills it more easily, and it even works better together with other natural peptides. This explains why simply boosting LL‑37 may not clear this bug on its own.

Subgingival bacteria are continually exposed to gingival crevicular fluids that are derived from serum, which contain various bactericidal agents. The periodontopathic bacterium <i>Porphyromonas gingivalis</i> has been demonstrated to possess a variety of abilities to resist bactericidal agents, due to which it is able to propagate in the subgingival environment. We previously demonstrated that the major surface glycoproteins of <i>P. gingivalis</i>-Pgm6 and Pgm7, also called outer membrane protein A-like proteins (OmpALPs)-mediate resistance to the bactericidal activity of human serum, but their precise role remains unknown. In this study, we investigated the sensitivity of the wild-type and Pgm6/Pgm7-deficient <i>P. gingivalis</i> strains toward major antimicrobial peptides in the oral cavity, human <i>&#x3b2;</i>-defensins (hBDs) 1-3, and human cathelicidin LL-37. hBDs showed a considerably weak bactericidal activity against both bacterial strains. LL-37 also showed a weak activity against the wild-type strain; however, it showed a significant activity against the Pgm6/Pgm7-deficient strain. In the Pgm6/Pgm7-deficient strain, LL-37 remarkably accumulated on the bacterial cell surface, which may result in the destruction of the outer membrane. Additionally, the bactericidal activity of hBDs against the Pgm6/Pgm7-deficient strain was found to be synergistically promoted in the presence of LL-37. Our results suggest that OmpALPs specifically protect <i>P. gingivalis</i> from the bactericidal activity of LL-37; thus, <i>P. gingivalis</i> may adeptly survive in LL-37-producing subgingival environments.