# Characteristics of Clinical Isolates of _Streptococcus mutans_
## 1 Introduction  
In 1 mL of human saliva, there are approximately 20 to 400 million microorganisms belonging to more than 700 species, including a particularly important one discovered by [PERSON] in 1924, _Streptococcus mutans_[1; 2; 3]. In the oral cavity, certain species occupy specific places called ecological niches, which is due to the differences in aerobic and nutritional conditions prevailing in its particular parts. The major Gram-positive, non-haemolytic (y-haemolytic) species of oral streptococcal bacteria include _S. mutans_. It is a facultative anaerobe closely related to the etiopathogenesis of dental caries, and it has the ability to produce plaque (i.e., naturally occurring bacterial biofilm), which in turn is an important factor in its virulence. Importantly, microbiota of the oral cavity, along with the immune system, is an essential element of the body's defence against infections. Among the bacteria that make up the microbiota, there are opportunistic pathogens which are controlled by a well-functioning immune system. Oral cavity microbiota reaches a certain level of stability in adulthood; however, its state can be easily disrupted, for example, by insufficient oral hygiene and tooth loss [4; 5; 6]. It is noteworthy that during clinical research, _S. mutans_ strains were found in both healthy people (without carious defects) and patients with dental caries. Despite tremendous progress in civilization, dental caries continues to pose a challenge in dentistry. The availability of modern research methods provides an opportunity to understand complex processes occurring during caries development. From the point of view of clinical control of this disease, elimination of the etiological agent of caries, i.e., dental plaque, seems to be the most important factor [7; 8; 9]. It turns out that just a few minutes after brushing, the surface of tooth enamel is covered with pellicle containing glycoproteins from saliva, which plays an important role in protecting and remineralising the enamel. Importantly, the pellicle does not contain bacteria and the mechanism of its formation is based on physical and chemical interactions at the molecular level between the components of saliva and tooth surface Van der Waals forces, hydrophobic interaction and others. The next step in dental plaque formation is the adhesion of pioneer bacteria in saliva to the pellicle. At this stage, adhesion is still reversible. From the moment thebacteria begin to secrete extracellular polymeric substances (EPS), bacterial adhesion to the pellicle and to themselves becomes stronger. Dental plaque pioneer bacteria include: _Actinomyces_ spp., _Streptococcus sanguinis, S. oralis, S. mitis, Haemophilus_ spp. and _Neiseria_ spp. [10; 11]. The next step in the formation of bacterial plaque is its maturation. Once the pioneer bacteria attach to the pellicle, specific places of connection are created for subsequent species that have the ability to identify polysaccharides and receptor proteins on the surface of the pioneer bacteria. As a result of this process, they begin to form co-aggregates. Later colonising bacteria include: _Fusobacterium nucleatum_, _Treponema_ spp., _Tannerella forysthemis_, _Porphyomonas gingivalis_ and others. During plaque maturation, its composition changes as the number of bacteria belonging to _Streptococcus_ and _Neiseria_ types decreases and the number of bacteria of _Actinomyces_, _Fusobacterium_, _Veillonella_ or _Corynebacterium_ types increases [12; 13; 14]. A mature biofilm consists of many porous layers containing water channels, through which nutrients from saliva and waste products are delivered to the bacteria. In a mature biofilm, bacteria can detach individually and in whole plaque fragments as a result of the erosive force of saliva, and this phenomenon is called biofilm dispersion. Bacteria can also actively detach from the biofilm, seeking nutrient sources and new places of colonisation. A mature plaque has a complex bacterial structure with numerous interactions.