The viridans streptococci are important Gram-positive cocci comprising a large group of bacteria. These are found as mouth commensals, causing minor to life-threatening diseases. The word viridans has been derived from the Latin word Viridis as most species of this group gives green color due to alpha hemolysis (partial hemolysis) on blood agar.
Viridans streptococci show general characteristics similar to other streptococcal species. Like streptococcal species, these organisms are Gram-positive cocci arranged in chains and are facultative anaerobes and catalase-negative.
Like S. pneumoniae, the group is alpha-hemolytic streptococci. These bacteria can be differentiated from S. pneumoniae by optochin resistance, bile solubility, absence of capsule, and arrangement, i.e., cocci in a chain (whereas S. pneumoniae are diplococci).
Viridans streptococci include five groups based on phenotype and each group contains at least 30 species. These groups are;
- S. anginosus group,
- S. mitis group,
- S. sanguinis group,
- S. salivaris group, and
- S. mutants, group.
Viridans streptococci have no Lancefield antigen on the cell wall. Hence they are not classified serologically.
Habitat and Epidemiology
The organisms are generally present as harmless inhabitants of the mouth and oral cavity; gingiva, oropharynx, and gastrointestinal tract. They remain without causing harm unless there is a change in their environment like the host immune status is low or bad oral hygiene or the presence of an extra nutritional substrate. Hence infection caused by viridans streptococci is endogenous. The infection is low pathogenic in immunocompetent individuals. However, it can be severe and invasive, like endocarditis and septicemia, when low immune status.
Pathogenesis and Clinical Manifestations
Two virulence factors of viridans streptococci play vital roles in pathogenesis. The first is dextran which facilitates binding to fibrin platelet aggregates on damaged heart valves. Another is that these can form biofilms that assist in forming dental plaques.
Diseases caused by S. viridans are dental infections, sub-acute endocarditis, and others that may be life-threatening.
Viridans streptococci can bind to oral mucosa and tooth surfaces colonizing the oral cavity. Most common dental infections include dental caries, abscesses, and periodontal disease. If trapped in an oral cavity, a Sucrose-rich diet converts into dextrans, that creates a favorable environment for bacteria for demineralization and decay of teeth. Common etiological agents for dental infections are S. mutants and S. anginosus. At the same time, S. sanguinis and S. mitis are isolated mostly from patients having dental procedures, which may be invasive, leading to endocarditis. Hence patients with an existing heart problem need to use prophylactic antibiotics before surgery or dental procedure.
Sub- acute endocarditis
Viridans streptococci bind to fibrin platelet aggregates on damaged heart valves with the help of dextrans after entering the bloodstream. It leads to subacute endocarditis. A fibrin layer covers and protects the bacteria in the heart, which then causes bacterial endocarditis.
Viridans streptococci can cause several other infections. Some of those are brain abscesses, abscesses on abdominal viscera, septicemia, and rarely meningitis.
Sample of choice
Depending on the type of symptoms/infection, the specimen for lab diagnosis of viridans streptococci can range from pharyngeal swabs, supragingival dental plaques, to blood.
The following methods are used for isolation and identification of the organism:
Under the microscope, viridans streptococci are seen as Gram-positive cocci in a chain when observed after Gram staining. These bacteria resemble S. pyogenes morphologically but may be more ovoid and are usually in short-chain.
Most of the viridans streptococci grow in nutrient agar or blood agar but fail to grow in MacConkey agar. The growth of some strains is enhanced if incubated in 5-10 % carbon dioxide incubators. These organisms produce small, colorless colonies in blood agar. Most cause partial (alpha) hemolysis resulting in green discoloration of blood agar, whereas others do not cause any hemolysis on the blood agar (gamma hemolysis).
Viridans streptococci are optochin resistant. Optochin-containing paper disc is added on blood agar previously inoculated with S. viridans. On incubation, it shows no zone of inhibition around the disc.
Similarly, molecular techniques are also used to identify different species of viridans streptococci.
Viridans streptococci are harmless human normal flora. The organisms cause opportunistic and endogenous infections like dental caries when the host environment is changed, which ultimately may lead to endocarditis, septicemia, or brain abscesses. One should maintain oral hygiene properly by brushing teeth regularly and cleaning the mouth after having sweet foods to avoid such infections. Also, patients with existing heart problems need prophylactic treatment with antibiotics, like penicillin, before surgery or a dental procedure.
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