Project description:The lesions of enamel caries can be considered as the outcome of dysbiotic changes in the biofilm community of supragingival dental plaque. Demineralization occurs as the cumulative outcome of repeated shifts towards a less diverse microbiota that produces and tolerates a low pH environment in tooth sites that are sheltered from protective factors in host saliva. Although the etiology of caries is multifactorial, frequent consumption of foods rich in fermentable carbohydrates, notably sucrose, appears to be one of the major factors driving the microbiota in the direction of dysbiosis, particularly in the case of otherwise healthy children with normal salivary flow. Streptococcus mutans and closely related species (such as Streptococcus sobrinus) have long been considered to play a primary etiological role in dental caries. S. mutans responds to sucrose by producing large quantities of lactic acid. It is very tolerant of low pH, and produces an insoluble extracellular polysaccharide that may sequester acid at tooth surfaces. The mechanisms behind those putative virulence factors have been intensively studied in monoculture, and recently in simple multi-species consortia. Much less is known of other species that may also contribute to or protect against dysbiosis driven by dietary carbohydrates. Some strains of “non-mutans” streptococci produce and tolerate acid at levels comparable to S. mutans, while others show increased ariginolytic capabilities, which may act to raise pH within the biofilm matrix. S. mutans tends to be a minority species even in caries-active children, and carious lesions likewise can occur in children with no detectable S. mutans. 16S rDNA-based metagenomic comparisons of caries-active and caries-free subjects have detected associations between caries and a variety of oral species, including not only non-mutans streptococci, but also members of other genera, such as Scardovia and Bifidobacterium. Caries associations have not been consistent between studies. Moreover, different taxonomic clusters have been defined as subgroups within the same study. This raises an important point. Although caries-associated communities are typically less diverse than healthy supragingival plaque overall, those dysbiotic communities still display considerable taxonomic diversity between affected individuals. That in turn raises the question of whether it is desirable to define biomarkers of dysbiosis that are less dependent on taxonomy. The Human Microbiome project generated comprehensive metagenomic data for a wide variety of body sites in healthy subjects, including supragingival plaque. Although most of that data was based on 16S rDNA sequencing, shotgun metagenomics was also used to catalog the functional potential of all microbial genes within a smaller subset of subjects. One of the key findings was that healthy sites from different people were broadly similar with respect to their functional profiles, even though there was extensive individual variation in their taxonomic profiles. It is possible that the “conservation of function” concept may also extend to dysbiotic communities. This would explain why microbial communities associated with caries still show considerable taxonomic variation. In that case, differential patterns of community-wide gene and/or protein expression might provide a more accurate indicator of dysbiosis than can be achieved by counting caries-associated species. Metatranscriptomic or metaproteomic approaches can be used to provide information on function. A recent metatranscriptomic comparison of subgingival plaque from healthy and periodontally diseased sites in three subjects has provided data that support the “conservation of function” concept. They observed that taxonomically diverse diseased sites shared conserved gene expression profiles [20]. By the same token, a recent metaproteomic comparison of gut microbiotas from healthy controls to Crohn’s disease patients found that major shifts in protein expression by function did not always correlate with changes in taxon relative abundance [21]. In this metaproteomic study, we found that sucrose–induced changes in protein expression patterns for pathways involving glycolysis, lactate production, aciduricity and ammonia/glutamate metabolism were likewise conserved in taxonomically diverse dysbiotic oral microcosm biofilm communities.

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