Insights into the virulence of oral biofilms: discoveries from proteomics

Abstract

This review covers developments in the study of polymicrobial communities, biofilms and selected areas of host response relevant to dental plaque and related areas of oral biology. The emphasis is on recent studies in which proteomic methods, particularly those using mass spectrometry as a readout, have played a major role in the investigation. The last 5-10 years have seen a transition of such methods from the periphery of oral biology to the mainstream, as in other areas of biomedical science. For reasons of focus and space, the authors do not discuss biomarker studies relevant to improved diagnostics for oral health, as this literature is rather substantial in its own right and deserves a separate treatment. Here, global gene regulation studies of plaque-component organisms, biofilm formation, multispecies interactions and host-microbe interactions are discussed. Several aspects of proteomics methodology that are relevant to the studies of multispecies systems are commented upon.

Figure 1. Toxin–antitoxin module stress-response regulator

(A) Toxin–antitoxin (TA) module genetic organization. TA systems are typically two-gene operons with the antitoxin encoded upstream of the toxin gene. Antitoxins bind their cognate toxins and inhibit toxin activity. Antitoxins and TA complexes repress TA module transcription. Proteomic methods have played an important role in elucidating TA module structure and function [86,87]. (B) The stress-response regulator hypothesis of chromosomal TA function. Environmental stresses such as amino acid starvation activate chromosomal TA modules through sequential events starting with (p)ppGpp biosynthesis. ppGpp inhibits exopolyphosphatase and thereby leads to polyP accumulation; polyP in turn activates Lon to degrade idle ribosomal proteins. Antitoxins are degraded either by Lon, ClpAP or ClpXP proteases, and activated toxins inhibit cell growth and/or lead to cell death. tmRNA: Transfer-messenger RNA.

Figure 2. Response of energy and cytotoxin metabolism in Porphyromonas gingivalis to internalization

Proteins catalyzing each step from the preferred metabolites l-glutamate, l-aspartate and l-asparagine to ATP and cytotoxin production are shown by their P. gingivalis PGN designation. Red up arrows indicate statistically significant increases in protein levels 18 h after internalization into human gingival epithelial cells. Green down arrows indicate decreased levels and pink squares no statistically significant change. Acetyl-CoA appears as a substrate and product at multiple points and is shown in purple. The metabolic precursors l-asparagine, l-aspartate, l-glutamate and 2-oxoglutarate as well as ATP and the cytotoxic end products acetate, butanoate and propionate are shown in bold. The map shows a shift toward l-asparagine/l-aspartate metabolism and acetate and propionate production and a shift away from production of the more toxic butanoate after internalization. Data taken from [72].