Streptococcus mutans, Candida albicans, and the Human Mouth: A Sticky Situation

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The human mouth with its diverse niches and ample supply of nutrients is undoubtedly conducive for the unrestricted formation of natural microbial biofilms. The oral microbial communities are some of the most complex microbial floras in the human body, consisting of more than 700 different bacterial species [1], [2]. Occurrence of disease results from disturbance of the equilibrium of this complex ecosystem, where population shifts lead to overrepresentation of pathogenic species which contribute to the onset and progression of the most common oral diseases, caries and periodontal disease [3]. Culture-independent molecular methods such as proteomics and 16S rRNA sequencing aiming to determine the bacterial diversity in the human oral cavity have demonstrated that in the supragingival plaque, S. mutans was the dominant species, with elevated levels of other streptococci including S. sanguinis, S. mitis, and S. salivarius in addition to lactobacilli and Veillonella. In contrast, the subgingival plaque was made up primarily of Gram-negative anaerobic bacteria such as Fusobacterium nucleatum, Porphyromonas gingivalis, and Prevotella intermedia which are known to be periodontal pathogens [3]–[5]. The dental tissues—enamel, dentin, and cementum—constitute the oral solid surfaces coated by a pellicle to which the microbial cells attach. The primary colonizers and secondary organisms stick to each other on the surface of teeth and generate a matrix of exopolysaccharide within which cells grow, forming a community with a collective physiology [6]. The resulting biofilm formed, known as dental plaque, subjects the teeth and gingival tissues to high concentrations of microbial metabolites which result in dental disease [2], [7]. The interactions between the various species in these mixed biofilms can be synergistic in that the presence of one microorganism generates a niche for other pathogenic microorganisms which can serve to facilitate the retention of organisms, an oral phenomenon known as coaggregation [3], [8]. The bacteria in the biofilm are always metabolically active which causes fluctuations in pH and loss of minerals from the tooth, ultimately resulting in dissolution of the dental hard tissues and formation of lesions known as dental caries [6], [9]. Interestingly, metabolic communications among oral bacteria may occur where the excretion of a metabolite by one organism is used as a nutrient by other organisms and breakdown of a substrate by enzymatic activity of one organism creates available substrates for different organisms [10], [11].