Streptococcus mutans is a gram-positive, nonmotile, facultatively anaerobic pathogen of the genus Streptococcus. These are spherical bacteria that usually attach in chain form. S. mutans occurs as a commensal in the oral cavity and is largely responsible for the development of caries.
What is Streptococcus mutans?
Streptococcus mutans belong to the gram-positive spherical bacteria, which are physiologically found in the oral flora of almost all people. These grow under facultative anaerobic conditions, meaning they can multiply in the presence of oxygen but also in its absence. Streptococcus mutans, as well as other streptococci, are referred to as such because they grow in chains (gr. streptos = necklace). Streptococci are both catalase-negative and oxidase-negative. The special feature within the streptococci is their different hemolysis behavior on blood agar soils. Thus, they can be divided into three groups: S. mutans shows an α-hemolytic behavior, which means that they can only partially degrade hemoglobin in agar to a biliverdin-like product. The result is a greenish-looking border zone around the colony, which is why S. mutans and other streptococci (such as S. bovis or S. mitis) are called greening streptococci. In β-hemolysis behavior, complete degradation of hemoglobin to bilirubin occurs, whereas γ-hemolysis is a misleading term because no hemolysis occurs.
Occurrence, distribution, and properties
Streptococcus mutans is part of the normal flora of the human oral cavity and is present in almost everyone. In this context, parents are usually responsible for the first contact with S. mutans on the infant, for example, by putting the infant’s spoon or pacifier into the mouth, where the bacterium resides in masses. In addition, numerous microorganisms are transmitted through kissing. However, the presence of S. mutans is not synonymous with the development of caries. The bacterium must first be given the appropriate conditions by eating many sugary foods and drinks. Only then can the bacterium create the acidic environment that attacks the tooth enamel. The number of S. mutans correlates with the risk of a patient developing caries, i.e. the more bacteria there are in the saliva, the higher the probability of developing caries. S. mutans and other streptococci possess hemolysin. This enables them to destroy the erythrocytes on the blood agar, resulting in the typical α-hemolytic behavior, which is characterized by the colonies on a blood agar being surrounded by a greenish shimmering halo. In addition, S. mutans possesses other virulence factors associated with caries formation. For example, the pathogen can form glucan polymers from sucrose, a sugar found in food pulp, which are produced by glucosyltransferases. This enables the bacterium to attach itself to the tooth enamel and thus form a firm adherence. Furthermore, S. mutans can ferment carbohydrates found in food to lactate (lactic acid). The lactic acid forms the acidic environment that attacks the enamel, leading to demineralization of the substance. Since S. mutans is capable of excreting acids, it must be able to survive in this environment, so the pathogen has developed a certain acid resistance to protect itself from decomposition by the lactic acid. In addition to protection against the acid, S. mutans also forms a protection against the human immune defense. Antibodies of the immunoglobulin A (IgA) type are found primarily in saliva. S. mutans forms a so-called IgA protease, which can deactivate these antibodies and thus weaken the immune defense. Furthermore, S. mutans can compete with other bacteria for food. To win this, S. mutans produces bacteriocins that can be used to attack foreign bacteria.
Diseases and ailments
Streptococcus mutans is the main causative agent of dental caries, which is one of the most common diseases worldwide. Caries itself is not contagious, but the pathogen can be transmitted. Normally, the teeth are surrounded by a biofilm, which is full of bacteria, so also with S. mutans.However, if the tooth is exposed to some risk factors, the probability increases that the bacteria will ferment carbohydrates from the food and form lactic acid from them, which attacks the tooth and thus leads to caries. Such risk factors include the number of S. mutans, poor dental hygiene, or sugary foods. The dentist divides caries into different stages depending on how deeply the bacteria has penetrated the tooth. If the tooth has a filling, it can become loose and detach. In addition, patients usually complain of an unpleasant taste or bad breath. If the pathogen penetrates close to the pulp, toxins formed can be released and cause inflammation. The sensitivity of the nerve fibers is increased and the patient has an increased feeling of cold or heat. In addition, there is a strong feeling of pain, which can be permanent or recurrent. If the pathogen gets all the way into the pulp, an abscess may form and the tooth may die and must be removed.
