Interactions of Pathogenic Oral Bacteria

The human oral microbiome comprises over 700 bacterial species, the majority of which occur in polymicrobial biofilms accreted to the non-shedding hard surfaces of the teeth. These species form oral bacterial communities whose composition and metabolism changes over time. Chronic periodontitis is associated with the development of polymicrobial subgingival plaques that contain high levels of Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia. Objectives: To define the interactions of oral bacteria and emergent properties of pathogenic communities that lead to disease progression. Methods: Transcriptomic, proteomic and metabolomic analyses have been coupled with novel polymicrobial planktonic and biofilm culture systems. Visualisation of polymicrobial biofilms was achieved using fluorescent in situ hybridisation and confocal scanning laser microscopy or electron microscopy. Results: When grown in coculture P. gingivalis and T. denticola specifically associate and communicate with each other using soluble signals. They alter the expression of their metabolic pathways to avoid direct competition and display synergistic growth. When grown as part of a polymicrobial biofilms P. gingivalis and T. denticola act synergistically to form large structured biofilms. There is differential expression of a number of both P. gingivalis and T. denticola cell surface proteins, including the P. gingivalis gingipains, during polymicrobial biofilm growth. P. gingivalis produces large amounts of vesicles in these biofilms that enable the bacterium to extend its sphere of influence outside the biofilm. Production of these vesicles that are decorated with a wide variety of adhesins and proteases, including the gingipains, will increase damage to host tissue and subvert the host immune response. Conclusion: We are gaining a better idea of how these opportunistic bacteria interact to cause disease by using innovative culture systems and new visualisation techniques.