Enrichment of periodontal pathogens from the biofilms of healthy adults.

Periodontitis is associated with shifts in the balance of the subgingival microbiome. Many species that predominate in disease have not been isolated from healthy sites, raising questions as to the origin of these putative pathogens. The study aim was to determine whether periodontal pathogens could be enriched from pooled saliva, plaque and tongue samples from dentally-healthy adult volunteers using growth media that simulate nutritional aspects of the inflamed subgingival environment. The microbiome was characterised before and after enrichment using established metagenomic approaches, and the data analysed bioinformatically to identify major functional changes. After three weeks, there was a shift from an inoculum in which Streptococcus, Haemophilus, Neisseria, Veillonella and Prevotella species predominated to biofilms comprising an increased abundance of taxa implicated in periodontitis, including Porphyromonas gingivalis, Fretibacterium fastidiosum, Filifactor alocis, Tannerella forsythia, and several Peptostreptococcus and Treponema spp., with concomitant decreases in health-associated species. Sixty-four species were present after enrichment that were undetectable in the inoculum, including Jonquetella anthropi, Desulfovibrio desulfuricans and Dialister invisus. These studies support the Ecological Plaque Hypothesis, providing evidence that putative periodontopathogens are present in health at low levels, but changes to the subgingival nutritional environment increase their competitiveness and drive deleterious changes to biofilm composition.

Root Surface Biofilms and Caries.

Following gingival recession, which increases with age, the root surface becomes exposed, creating new environments for microbial colonization and biofilm formation. The formation of root surface biofilms is influenced by the availability and composition of saliva and gingival crevicular fluid; they provide components for the conditioning film (acquired root surface pellicle) and also act as a source of nutrients. The early bacterial colonizers of the root surface are similar to those found on the enamel, and Gram-positive species such as Streptococcus sanguinis, S. oralis, S. mitis, and Actinomyces species predominate. The root surface has a lower mineral and higher organic content than enamel, and so is more vulnerable to demineralization. The characterization of the microbiota associated with root surface lesions is still ongoing. Traditional culture-based studies have implicated species such as mutans streptococci, lactobacilli, bifidobacteria, and Actinomyces species, while molecular-based studies have provided evidence for a more complex microbiota with many Gram-negative and anaerobic bacteria being detected in addition to the more conventional cariogenic organisms. Ecological concepts have been applied to explain the microbial etiology of root caries. The acidic environment generated from the fermentation of dietary sugars selects saccharolytic bacteria that can preferentially grow and metabolize under low pH conditions, and then proteolytic Gram-negative species are selected when the dentin is exposed and collagen and other proteins become accessible to be catabolized. These species act in concert to degrade the inorganic and organic components of the dental tissues.