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.