Porphyromonas gingivalis is a Gram-negative oral anaerobe that is regarded as a keystone pathogen in chronic periodontitis. Outer membrane vesicles (OMVs) produced by P. gingivalis are proposed to play a major role in secretion of virulence factors, nutrient acquisition and biofilm formation. The precise mechanisms involving OMV biogenesis remain unknown although selective sorting of proteins and lipopolysaccharides to OMVs is known to occur. Proteomic analysis has revealed that a specific, regulated and selective mechanism in P. gingivalis allows the preferential packaging of virulence factors, particularly C-terminal domain (CTD) containing proteins, including the gingipains onto the OMVs. These CTD proteins modified with anionic-lipopolysaccharide thus form an electron dense surface layer on OMVs surfaces.
PG1881, a hypothetical protein, preferentially localized to the OMV membrane was only found in the OMV fractions. Preliminary results from Northern blot suggest that pg1881 is expressed within an operon pg1878-pg1881. PG1878 is identified as a cysteinyl-tRNA synthetase, whereas PG1879 and PG1880 are predicted to be a patatin-like protein and a glycosyl transferase group 2 family protein, respectively. Co-transcription of these genes suggests their involvement in the same biochemical pathway or sharing of substrates and transporters. Partial characterisation of P. gingivalis W50 isogenic mutant lacking PG1881 protein (PG1881-) shows that the growth rate and cell morphology of PG1881- were similar to W50. Furthermore, PG1881- was shown to have a lowered OMV production with decreased Arginine-specific protease activity compared to W50. PG1881- also demonstrated an enhanced biofilm forming ability compared to W50. Interestingly, PG1881- homotypic biofilms were shown to contain more red-staining when stained with SYTO9 and propidium iodide, suggesting a possible release of extracellular DNA when compared to W50. These data suggest that PG1881 plays an important role in P. gingivalis OMV biogenesis and this study forms a good basis to investigate how OMVs may influence biofilm formation.