Microbial interactions in dental plaque - News & Features
11 August 2014
The September 2014 edition of Microbiologist will be themed around dental microbiology. Here, Executive Committee member and Features Editor, Nick Jakubovics, discusses microbial interactions in dental plaque - a typical bacterial biofilm.
Despite decades of research effort, it is estimated that we can still only culture around 50% of the 200 or so dental plaque microorganism species in the laboratory. Yet, with very little effort we all culture the entire microbial consortium in our mouths every day! The key to this is the dependence that microorganisms have for one another to survive, and a better understanding of microbial interactions within dental plaque may potentially lead to new targets for disease control.
Figure 1. Dental plaque after careful cultivation for 2 days, stained with a disclosing agent:
Throughout the last century a great deal of work was targeted towards finding a single pathogen associated with dental caries, but both dental caries and periodontitis are almost certainly better explained by an ‘ecological plaque hypothesis’. This says that disease is attributed to shifts in the microbial population and the overgrowth of selected species (but not just a single organism). A simplified version of the hypothesis is presented in Figure 2.
Figure 2. A simplified version of the ecological plaque hypothesis. Health-associated dental plaque is a microbial homeostasis that may shift to a disease-associated community in response to changes in the host or environment:
According to this theory, dental plaque reaches a ‘microbial homeostasis’ after around 24 h if left undisturbed. From this point, the microbial population in dental plaque is stable until it is perturbed by changes in host, bacterial or environmental factors such as eating sugary snacks or smoking.
The formation of dental plaque on a clean tooth surface begins with the attachment of primary colonizing bacteria, such as Streptococcus, Neisseria, Haemophilus, Actinomyces or Veillonella species, to the salivary pellicle that coats tooth surfaces. These species promote the colonization of other organisms through cell-cell adhesion, cell-cell signalling or by modifying the local environment, for example, by reducing the oxygen tension.
The adhesion of microorganisms to pre-bound species is known as ‘coadhesion’, whereas microbial cell-cell binding in the fluid phase is termed ‘coaggregation’. Coadhesion and coaggregation interactions only occur between compatible partners, and involve the specific recognition of protein or carbohydrate receptors on one organism by protein adhesins on the other.
As dental plaque accumulates, an extracellular macromolecular matrix is produced that enhances the adhesion of microbial cells to one another and to the tooth. Even when physically dislodged by scraping with a toothpick, dental plaque microorganisms remain bound to one another and can be visualized as large aggregates under a microscope (Figure 3).
Figure 3. Dental plaque removed from the mouth of a volunteer using a toothpick. Individual microbial cells are small compared with the epithelial cell (coloured blue with yellow nucleus), but together the microbial aggregate is large:
Microbial cell-cell binding helps to keep cells in close proximity to one another and consequently may promote a wide range of distance-critical interactions including the exchange of metabolites, signals or genetic information (Figure 4).
Figure 4. An illustration of the varied interactions in dental plaque. The primary colonizing bacteria adhere to salivary receptors and recruit later colonizers through specific interbacterial interactions. Metabolic exchange occurs in two directions between Porphyromonas gingivalis and Treponema denticola. Lactic acid produced by streptococci such as Strep. gordonii promotes the growth of Aggregatibacter actinomycetemcomitans and Veillonella sp. At the same time, streptococcal H2O2 triggers the production of the matrix-degrading enzyme dispersin B:
For example, Veillonella spp. require lactate for growth, and this nutrient is readily available from Streptococcus spp. which secrete lactate as a waste product from carbohydrate metabolism. Studies have shown that coaggregating strains of Strep. mutans and Veillonella alcalescens colonize the teeth of a rat more effectively than non-coaggregating strains.
The major cell-cell signals in dental plaque are thought to be autoinducer-2, a product of the luxS gene that is present in many oral bacteria, and peptides that are produced by Gram-positive species and are often linked to the development of a genetically competent state. The physical proximity of different species to one another is likely to enhance the exchange of signals, and will increase the efficiency of DNA transfer.
The exchange of transposon constructs has been shown to occur in model oral biofilms. There is also extensive evidence in the genome sequences of oral bacteria that indicate genetic exchange has taken place. Somewhat worryingly, the development of penicillin resistance in Strep. pneumoniae has been attributed to the transfer of a penicillin-binding protein gene (pbp2x) from oral streptococci.
Not all interactions between microorganisms are beneficial to the species involved, and cell-cell binding may also promote competition. Many oral bacteria produce small peptide bacteriocins that kill selected competitors, and under laboratory conditions, oral streptococci produce hydrogen peroxide (H2O2) at concentrations that inhibit the growth of different species. It appears that this inhibition is likely to be part of a network of controls that dictate the spatial organization of organisms in the biofilm and maintain a fine balance of competition.
It will be a long time before all the different intermicrobial interactions in dental plaque are resolved, but even without a complete knowledge of these processes it may be possible to develop strategies that support good oral health. For example, pre-biotic approaches that enhance the production of H2O2 by oral streptococci may help to control the growth of other, less desirable, microorganisms.
There is also evidence that our ability to culture microorganisms can be improved by taking intermicrobial interactions into account. For example, the isolation of Fretibacterium fastidiosum, a previously uncultured member of the phylum Synergistetes, from dental plaque was reported recently using an enrichment strategy that selectively depleted different members of the community. In the end, Fretibacterium fastidiosum was shown to grow in the laboratory when cross-streaked with other bacteria or when incubated with bacterial extracts.
Despite the apparent paradox, the isolation of new strains is critical in order to understand why these strains are so difficult to isolate in the first place. Studies using mixed species of bacteria were once shunned in the laboratory as being too complex to interpret. With the development of massively parallel technologies and sophisticated bioinformatics, it is now realistic to start deciphering these interactions even in complex communities such as dental plaque.
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