Streptococcus mutans biofilm

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Biofilms and Disease

Bacteria make biofilms, which contain a heterogeneous mixture of microorganisms attached to a surface [12]. Biofilms offer bacteria protection from hostile environments [3], promote efficient food storage and nutrient acquisition mechanisms, and encourage synergistic relationships for community benefit. Biofilms are involved in a variety of infectious diseases, causing persistent problems such as cystic fibrosis pneumonia, otitis media, urinary tract infections, and dental plaque formation [12].

Dental Plaque and Disease

Dental plaque is a biofilm of diverse bacteria and extracellular polymeric substances. It builds up over time and leads to dental caries and periodontal diseases such as gingivitis, a gum inflammation. As many as 400 species of bacteria have been associated with dental plaques, the most common being Streptococcus and Lactobacillus, which are associated with acid formation leading to tooth decay [12]. Multiple related Streptococcus strains such as S. gordonni, S. mitis, and S. mutans have been associated with dental caries [12]. with S. mutans being the major contributor [18] and model organism.

Description

As a part of the Streptococcaceae family and Streptococcus genus, S. mutans is a Gram-positive cocci concentrated in fissures and crevices of the oral cavity [12] and it is the main cause of cariogenesis. The completely sequenced genome of S. mutans UA159 offers many insights into the role of gene expression in virulence and disease pathogenesis in dental plaques [17].


Dental Plaque Formation

Dental plaque formation follows three steps: salivary glycoprotein layer formation on the tooth enamel, cell-to-surface adherence of primary colonizers, and cell-to-cell adherence of secondary colonizers 19.

Attachment and Colonization

Attachment to teeth is the initial step in plaque formation [7]. S. mutans, a primary colonizer, can use both sucrose-independent and sucrose-dependent attachment mechanisms. The sucrose independent pathway involves the expression of an adhesion molecule called antigen I/II, also known as SpaP [14]. This virulence factor is a fibrillar protein with an adhesion domain which binds to salivary agglutinin [5], a glyprotein coating the tooth surface. The antigen I/II family is also present in S. gordonii and S. oralis with the same function, which offers insight into the similarity of dental biofilm colonizers [15].

The more efficient sucrose-dependent pathway requires glucans, which are polysaccharides of glucose. Glucans mediate S. mutans attachment to tooth surfaces, and are a part of the extracellular polysaccharides involved in cell-to-cell adhesion [9]. S. mutans make 3 types of glucosyltranferases (GTF) – GtfB, GtfC, and GtfD, which synthesize both insoluble and soluble glucan chains from sucrose 9. The optimum cellular ratio of 20:1:4 for GtfB:GtfC:GtfD contributes to glucan products with the highest adhesive ability [16]. In addition, other biofilm colonizers such as Streptococcus sanguinis and S. gordonni have comparable forms GTFs performing similar functions [15].

Glucan-binding proteins (GBP) at the bacterial surface mediate cell-to-cell attachment via glucans (Diagram1). GbpC is involved in glucan-dependent aggregation of cells, while GbpA is essential for biofilm structure integrity and GbpD for biofilm rigidity [2].