The term Streptococcus viridans is used to describe several groups of streptococcal bacteria. They can cause pathological processes such as tooth decay and inflammation.
What is Streptococcus viridans?
The name Streptococcus viridans is actually considered misleading. For example, it is not a single species, but different streptococcal species, which in turn consist of several subgroups. The term viridans streptococci is therefore considered more appropriate. This refers to spherical gram-positive bacteria that originate from the streptococcus genus (Streptococcus). The name viridans streptococci is due to historical reasons and is used in microbiology. Streptococcus viridans are greening streptococci. Thus, the Latin term ‘viridans’ means ‘greening’ or ‘vergrünen’. In addition, because the bacteria settle in the mouth and throat region, they are called oral streptococci. Although some species of Streptococcus viridans can cause disease, however, most subspecies are not considered pathogenic.
Occurrence, distribution, and characteristics
Although the viridans streptococci comprise several species, they share some common characteristics. For example, they form coccoid cells that arrange into chains in numerous representatives of this species. Endospores are not formed by them. In the Gram stain there is a positive progression. On blood-containing culture medium, greening or alpha-hemolysis of the grown streptococcal colonies occurs. Other typical characteristics of Streptococcus viridans include the arrest of their growth at temperatures above 10 degrees Celsius. However, at temperatures of 45 degrees, most streptococci are still quite capable of multiplying. For medical purposes, it is important to be able to distinguish Streptococcus viridans from other streptococcal species, such as Streptococcus pneumoniae. This species also belongs to the alpha-hemolytic streptococci. Identification is possible with the aid of an optochin test. In addition, members of Streptococcus pneumoniae can be identified as diplococci. Streptococcus viridans also lacks polysaccharide capsules and the antigens of Lancefield groups A, B, C, and D.
Viridans streptococci are usually found in the oral cavity as well as in the ear, nose, and throat region. They are also present in the gastrointestinal tract and vagina. At the beginning of the 20th century, scientists still assumed that Streptococcus viridans formed a single concrete species that could be distinguished from Streptococcus haemolyticus. Streptococci in which alpha-hemolysis occurred were given the name ‘greening streptococci’. However, numerous differences between viridans streptococci became known over the years, and further subdivisions were made beginning in 1937. In this way, the group Streptococcus viridans received more and more new species, among which there were also y-hemolytic streptococci, from which no hemolysis was performed. For this reason, medicine eventually divided viridans streptococci into several groups. The classification of Streptococcus viridans includes four groups. These are the Milleri group also called the Anginosus group, the Oralis group, the Mutans group, and the Salvarius group. Members of the Milleri group are Streptococcus intermedius, Streptococcus constellatus and Streptococcus anginosus. The Oralis group includes Streptococcus mitis, Streptococcus sanguinis, and Streptococcus mitior. The mutans group consists of Streptococcus mutans, Streptococcus cricetus, and Streptococcus subrinis, while the salvarius group is composed of Streptococcus salvarius, Streptococcus bovins, and Streptococcus thermophilus. The Bovis group does not belong to the oral streptococci. Although alpha-hemolysis is also possible in members of this group, Lancefield group D antigens are also present in them.
Diseases and medical conditions
Some species of Streptococcus viridans are capable of causing discomfort and disease. For this reason, physicians refer to them as opportunistic or pathogenic agents. For example, Streptococcus mutans and Streptococcus sobrinus, among others, cause dental caries by producing extracellular polysaccharides.Bacteria grow on the coating of polysaccharides, which in turn form substances from which the enamel of the human tooth is affected. Bacteremia is also possible in the case of oral injuries caused by chewing or in the course of dental treatment. This is the introduction of bacteria into the human bloodstream. Within the blood, the bacteria are usually eliminated immediately, but if this does not happen, there is a risk of life-threatening sepsis (blood poisoning). A particularly worrisome disease that can be caused by members of Streptococcus viridans is bacterial endocarditis. This inflammation is caused by viridans streptococci in 50 to 70 percent of cases, which applies to the subacute course endocarditis lenta of bacterial endocarditis. Sick individuals, who are primarily patients with heart valve damage, suffer from sweating, feelings of weakness, palpitations (tachycardia) and fever during endocarditis. In some cases, other organs may also be affected by the disease. Basically, endocarditis lenta takes a gradual course. There is a risk of the formation of clock glass nails, drumstick fingers and anemia. In order to diagnose endocarditis lenta, one of the procedures that can be performed is the creation of a bacterial culture, which is one of the microbiological procedures. For safety reasons, three samples are taken independently of each other. Echocardiography is another diagnostic option. Antibiotics are used to treat endocarditis. Streptococcus viridans, for example, is particularly sensitive to penicillin. Empirical therapy is usually given initially until the pathogens are specifically detected. Members of the viridans streptococcus group, such as Streptococcus milleri, sometimes cause purulent abscesses. Occasionally, they can also cause meningitis. Streptococcus viridans is also considered of concern for people taking immunosuppressants.
