The virulence of Streptococcus mutans and the ability to form biofilms

Abstract

In some diseases, a very important role is played by the ability of bacteria to form multi-dimensional complex structure known as biofilm. The most common disease of the oral cavity, known as dental caries, is a top leader. Streptococcus mutans, one of the many etiological factors of dental caries, is a microorganism which is able to acquire new properties allowing for the expression of pathogenicity determinants determining its virulence in specific environmental conditions. Through the mechanism of adhesion to a solid surface, S. mutans is capable of colonizing the oral cavity and also of forming bacterial biofilm. Additional properties enabling S. mutans to colonize the oral cavity include the ability to survive in an acidic environment and specific interaction with other microorganisms colonizing this ecosystem. This review is an attempt to establish which characteristics associated with biofilm formation--virulence determinants of S. mutans--are responsible for the development of dental caries. In order to extend the knowledge of the nature of Streptococcus infections, an attempt to face the following problems will be made: Biofilm formation as a complex process of protein-bacterium interaction. To what extent do microorganisms of the cariogenic flora exemplified by S. mutans differ in virulence determinants "expression" from microorganisms of physiological flora? How does the environment of the oral cavity and its microorganisms affect the biofilm formation of dominant species? How do selected inhibitors affect the biofilm formation of cariogenic microorganisms?

Fig. 1

The percentage composition of polysaccharide matrix (the figure was prepared based on the data published by Bowen and Koo [6])

Fig. 2

The function of Streptococcus mutans in the process of the formation of biofilms on the surface of teeth. Glycosyltransferases (GTF) are an indispensable element for the proper functioning of S. mutans. In the early phase of biofilm development, S. mutans are bound to the teeth surface. It is thought that this binding is the first step in the formation of plaque. Surface adhesins of S. mutans (so-called antigen I/II) interact with α-galactosides from saliva, forming the structure of pellicula. Other groups of compounds belonging to S. mutans and located on the surface of teeth which participate in the formation of pellicula are glucan-binding proteins (GBPs) or GTF. Salivary proteins with which can interact the surface adhesins of S. mutans can be divided into four groups depending on the time of pellicula formation. Proline-rich proteins (PRP) participate in the first stage of the formation of pellicula. Two hours later, enamel is formed by cystatins, peptides with high affinity to hydroxyapatite. Then, peptides with low molecular weight and with bacteriological properties against S. mutans start playing a role in this model. In the last stage, mucin-rich proteins, lactoferrin, lysozyme, amylase, albumin, or IgM and IgG antibodies are involved in the process. S. mutans are aggregated on the surface of teeth in the presence of saccharose. Additionally, GTF synthetizes extracellular glucans, which is another key step in the development of plaque. GBP is a S. mutans receptor that differs from GTF and specifically binds glucans. GTF contain a glucan-binding domain and, therefore, they act as receptors for glucans. Therefore, S. mutans binds initially developed glucan through GBP and GTF, which gives a basis for the aggregation of S. mutans

Fig. 3

Production of lactic acid by Streptococcus mutans. Metabolism of various carbohydrates (including glucose and fructose) by bacterial biofilm. Production and secretion of a significant amount of lactic acid, which can cause demineralization of teeth structure that can finally result in the development of decay