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Background Dysbiosis in subgingival microbial communities, resulting from increased inflammatory transudate from the gingival tissues, is an important factor in initiation and development of periodontitis. Dysbiotic communities are characterized by increased numbers of bacteria that exploit the serum-like transudate for nutrients, giving rise to a proteolytic community phenotype. Here we investigate the contribution of interactions between members of a sub-gingival community to survival and development of virulence in a serum environment—modelling that in the subgingival pocket. Methods Growth and proteolytic activity of three  Porphyromonas gingivalis  strains in nutrient broth or a serum environment were assessed using A 600  and a fluorescent protease substrate, respectively. Adherence of  P. gingivalis  strains to serum-coated surfaces was studied with confocal microscopy and 2D-gel electrophoresis of bacterial supernatants used to investigate extracellular proteins. A model multi-species sub-gingival community containing  Fusobacterium nucleatum, Streptococcus constellatus ,  Parvimonas micra  with wild type or isogenic mutants  of P. gingivalis  was then created and growth and proteolytic activity in serum assessed as above. Community composition over time was monitored using culture techniques and qPCR. Results The  P. gingivalis  strains showed different growth rates in nutrient broth related to the level of proteolytic activity (largely gingipains) in the cultures. Despite being able to adhere to serum-coated surfaces, none of the strains was able to grow alone in a serum environment. Together in the subgingival consortium however, all the included species were able to grow in the serum environment and the community adopted a proteolytic phenotype. Inclusion of  P. gingivalis  strains lacking gingipains in the consortium revealed that community growth was facilitated by Rgp gingipain from  P. gingivalis.  Conclusions In the multi-species consortium, growth was facilitated by the wild-type and Rgp-expressing strains of  P. gingivalis,  suggesting that Rgp is involved in delivery of nutrients to the whole community through degradation of complex protein substrates in serum. Whereas they are constitutively expressed by  P. gingivalis  in nutrient broth, gingipain expression in the model periodontal pocket environment (serum) appeared to be orchestrated through signaling to  P. gingivalis  from other members of the community, a phenomenon which then promoted growth of the whole community.

Polymicrobial synergy stimulates Porphyromonas gingivalis survival and gingipain expression in a multi-species subgingival community