Oral Biofilms: Pathogens, Matrix, and Polymicrobial Interactions in Microenvironments

Abstract

Biofilms are microbial communities embedded within an extracellular matrix, forming a highly organized structure that causes many human infections. Dental caries (tooth decay) is a polymicrobial biofilm disease driven by the diet and microbiota-matrix interactions that occur on a solid surface. Sugars fuel the emergence of pathogens, the assembly of the matrix, and the acidification of the biofilm microenvironment, promoting ecological changes and concerted multispecies efforts that are conducive to acid damage of the mineralized tooth tissue. Here, we discuss recent advances in the role of the biofilm matrix and interactions between opportunistic pathogens and commensals in the pathogenesis of dental caries. In addition, we highlight the importance of matrix-producing organisms in fostering a pathogenic habitat where interspecies competition and synergies occur to drive the disease process, which could have implications to other infections associated with polymicrobial biofilms.

Copyright © 2017. Published by Elsevier Ltd.

Figures

Figure 1. Dental plaque architecture: the EPS matrix, spatial organization and polymicrobial composition

Plaque biofilm from caries-active subject (photo courtesy of Dr. Jaime A. Cury): microscopic image (inset) of plaque-biofilm showing a selected area containing bacterial cells (highlighted in orange) enmeshed in EPS (in dark blue); the image was pseudo-colored using ADOBE Photoshop software for visualization purposes (adapted from Hajishengallis et al., 2016 [19]). The middle panel shows bacterial clusters (green) surrounded by EPS matrix (red) detected in mature mixed-species oral biofilms formed in sucrose (adapted from Koo & Yamada, 2016 [5]). The right panel displays the spatial organization of human dental plaque showing multiple clusters of varying sizes containing different microbial species (adapted from Mark Welch et al., 2016 [31]).

Figure 2. The biofilm properties: assembling a complex microenvironment

In the oral cavity, a diet rich in sugar, particularly sucrose, provide substrate for the production of extracellular polysaccharides, which forms the core of the extracellular matrix in cariogenic biofilms. The matrix drastically changes the physical and biological properties of the biofilm. The exopolysaccharides enhance microbial adhesion-cohesion and accumulation on the tooth surface, while forming a polymeric matrix that embeds the cells. The matrix provides a multi-functional scaffold for structured organization and stability of the biofilm microbial community, where microorganisms co-exist and compete with each other. The diffusion-modifying properties of the matrix combined with the metabolic activities of embedded organisms help create a variety of chemical microenvironments, including localized gradients of oxygen and pH. Furthermore, the matrix can trap or sequester a diverse range of substances, including nutrients, metabolites and quorum sensing molecules. Similarly, enzymes can be retained and stabilized, transforming the matrix into a de facto external digestive system [1]. These properties provide the distinctive characteristics of the biofilm lifestyle, including mechanical stability, spatial and chemical heterogeneity and drug tolerance [1].

Figure 3. The biofilm battleground: antagonistic, synergistic and mutualistic interactions to build a pathogenic habitat

The social interactions of the oral microbial community start with early colonizers that can rapidly adhere to the tooth surface, and then co-adhere with other microorganisms. During this process the various species interact physically and metabolically to shape the initial biofilm community. The interactions can be both antagonistic and cooperative, which can dynamically change depending on the host. Certain interactions are beneficial as S. gordonii/S. salivarius and A. naeslundii interfere with caries pathogens such as S. mutans by secreting bacteriocins and hydrogen peroxide as chemical weapons or counter the deleterious effects of acidification by producing alkali. However, the balance between commensals and pathogens can be disrupted by frequent sugar consumption and poor oral care. When sucrose is available, EPS-producing exoenzymes such as Gtfs present in the pellicle and also bound to different microbes (including C. albicans) produce copious amounts of glucans. Furthermore, the Gtfs can also use starch hydrolysates (from α-amylase activity on pellicle and bacterial surfaces) to produce hybrid glucan polymers. The surface-formed glucans provide novel binding sites for adhesion and co-adhesion, which mediates new interspecies interactions and microbial clustering on the tooth surface, while assembling a polymeric matrix that provides protection and mechanical stability, making biofilm recalcitrant to antimicrobials and difficult to remove. Competitive and synergistic interactions continue to develop between the microbes embedded in the biofilm structure. If the biofilm remains on teeth and the consumption of carbohydrate-rich diets persist, the amount of EPS and extent of acidification of the matrix increases. The diffusion-modifying properties of the matrix combined with microbial metabolic activities help create a highly acidic and increasingly anaerobic (hypoxic) niche. Such conditions elicit biochemical, ecological, and structural changes that favor the survival and dominance of highly acid-stress tolerant organisms that can synergize with each other. The microbial diversity can further reduce in favor of an aciduric microbiota, help maintaining an acidified microenvironment. The low-pH condition at tooth-biofilm interface promotes demineralization of the enamel leading to the onset and progression of dental caries. This model may explain the rapid accumulation of cariogenic plaque in the presence of sucrose (and other fermentable sugars) in the diet, even if the initial population of pathogens such as S. mutans is numerically low.