Periodontitis is a chronic, inflammatory disease characterised by the destruction of the gums and bone supporting the teeth. It is associated with a build-up of subgingival plaque bacteria, specifically, Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia that correlate strongly with the onset and progression of the disease. A fourth species, Fusobacterium nucleatum is considered a bridging species since it acts as an interface between early and late colonizers in dental plaque. The interaction between these bacteria has been documented, but the molecular mechanisms underlying the development of multi-species biofilms are poorly understood.
Our study aims to provide detailed descriptions of the architecture of biofilms containing these species at all stages of their development using high-resolution imaging modalities. We have grown all four bacteria inside the flow cell system for 90 hrs, resulting in about 300 µm thick microcolonies. The biogeography of the system will be explored by analysing it at several time points. Moreover, using cryo-electron tomography, we have recorded the molecular structures of individual bacteria in their native form. Interestingly, T. forsythia is decorated with a 2D crystalline surface (S-) layer, which is composed of two glycoproteins, TfsA and TfsB. We were able to get a high-resolution structure of the S-layer revealing a unique structure consisting of three layers, 13 nm thick, contributing to adhesion and invasion of cells, and they assist in bacterial co-aggregation. Also, T. denticola has been seen to have an appendage-like structure decorated by the outer membrane protruding out of the cell in the biofilm that helps in cell-cell connection. Further, thin sections of thicker biofilm produced by focussed ion beam (FIB) milling were recorded using cryoET where we could see molecular structures that mediate the co-bacterial interactions and thus hold the biofilm together.
This study will aid in the understanding of the structure and ecology of bacterial biofilms in general, and more specifically subgingival plaque. The knowledge gained could be used in the future for designing treatments to interfere with the development of the biofilms that cause periodontitis.