Streptococcus viridans

Definitions & Key takeaways

Streptococcus viridans is a group of gram-positive, alpha-hemolytic streptococci that is commonly found in the oropharynx, epithelial surfaces of the oral cavity, teeth, skin, the gastrointestinal and genitourinary tract of humans. The term streptococcus viridans �, refers to five groups of Streptococci that include Streptococcus anginosus group, the mitis group, the sanguinis group, the salivarius group, and the mutans group.

S. viridans is part of the normal human microbiota and usually does not cause disease, however, it has been implicated in causing several infections, especially in immunocompromised individuals. S. viridans dental caries, periodontal disease, brain and liver abscesses, and bacterial endocarditis.

Some strains of S. viridans are capable of forming biofilms on medical implants such as catheters and prosthetic heart valves, which can offer them some antibiotic resistance. Treatment of S. viridans infections typically involves antibiotics such as penicillin, vancomycin, or erythromycin.

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The term “streptococcus viridans”, refers to five groups of Streptococci that include Streptococcus anginosus group, mitis group, sanguinis group, salivarius group, and finally the mutans group.
And each of these groups has approximately 30 species that can cause disease in humans. So the more correct term would be “viridans streptococci”, to reflect that there’s more than one of them.
Generally speaking, viridans streptococci can’t be assigned to a Lancefield group based on their cell wall antigens. However, in the Streptococcus anginosus group, some species can be assigned to Lancefield groups A, C, F or G.
Now, viridans streptococci have a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so they’re gram-positive bacteria.
They’re also catalase negative - which means they don’t produce an enzyme called catalase. Ok, now, when cultivated on a medium called blood agar, viridans streptococci colonies cause alpha hemolysis, also called green hemolysis, because they produce hydrogen peroxide, which partially oxidizes initially red hemoglobin in the blood agar to green methemoglobin.
One way to remember this is to think that 'viridans’ means ‘green’ in Latin. Some species in the Streptococcus anginosus group, however, can also be be beta-hemolytic or nonhemolytic.
These guys are special little snowflakes. Now, other Streptococcus species, like Streptococcus pneumoniae, are also alpha hemolytic.
So, an optochin test is done to distinguish them, which is when a few drops of optochin are added to the culture. Streptococcus pneumoniae are optochin sensitive, meaning the bacteria dies after adding optochin, whereas viridans streptococci are usually optochin resistant - meaning they survive.
Another difference between the two is that Viridans streptococci are usually bile insoluble, which means that when bile salt is added to a test tube with the bacteria, they don’t break down.
Alternatively, if bile salt is added to a test tube with Streptococcus pneumoniae, they break down - which means that it’s bile soluble.
Finally, viridans streptococci also lack the polysaccharide-based capsule typical of Streptococcus pneumoniae. Viridans streptococci are commensal organisms that reside in the oropharynx, epithelial surfaces of the oral cavity, teeth, skin, the gastrointestinal and genitourinary tract.
So, they live alongside their host without causing harm unless there’s a change in their environment. Inside the mouth, viridans streptococci like Streptococcus mutans, and Streptococcus mitis adhere to tooth enamel with the help of a dextran polysaccharide, and aggregate, forming dental plaques which can lead to dental caries, dental caries, and gum disease.
In contrast, Streptococcus sanguinis promotes dental health by inhibiting the growth of Streptococcus mutans. However, when dental work is done, like a tooth extraction, viridans streptococci can enter the bloodstream and cause bacteremia.
From there, they can make their way to the heart, where they may stick to a thrombus - an aggregate of platelets, red blood cells, and a mesh of cross-linked fibrin protein - on a damaged or prosthetic heart valve and cause subacute valve endocarditis.
This usually happens on the mitral valve. Here, the microorganisms form small vegetations composed of bacteria and a protective sheath of fibrin-platelet aggregates - which can detach and travel to different parts of the body as a septic emboli.
In fact, viridans streptococci, especially Streptococcus sanguinis, are the most common cause of subacute bacterial endocarditis in both adults and children.
Children with central venous catheters or immunodeficient states, like neutropenia in the context of chemotherapy, are especially at risk.
A particular complication in children is viridans group streptococcal shock syndrome, a toxic shock-like syndrome, characterized by widespread systemic vasodilation, which leads to low blood pressure and poor perfusion of vital organs.
Another viridans species, namely Streptococcus intermedius, of the Streptococcus anginosus group, doesn’t cause bacterial endocarditis.
Instead, from the bloodstream, it lodges in other organs, like the brain and the liver, forming abscesses. In fact, Streptococcus intermedius is the species of viridans streptococci responsible for most cases of brain abscesses.
Now, symptoms vary depending on the infection. If the infection is in the mouth, symptoms may include bleeding or receding gums, bad breath, toothache or tooth loss.
With bacterial endocarditis, symptoms may include fever, and a heart murmur upon auscultation. With toxic shock, there may be a fever, low blood pressure and tachycardia.
Brain abscesses associate neurological symptoms, like aphasia, or the inability to use or understand words. With liver abscesses, there may be fever, chills, anorexia, malaise and pain in the upper right quadrant, anorexia.
Diagnosis relies on cultures from biological samples, like abscesses, and blood cultures, to identify viridans streptococci in the bloodstream.
Alternatively, rapid antigen tests or antibody titers can be done. Additionally, an echocardiography can visualize the heart valve vegetations.
When a brain or liver abscess, imaging like a computed tomography of the head or abdomen may show a mass. Treatment consists of the use of antibiotics like penicillin G.
Other antibiotics with good activity include ampicillin, cephalosporins, vancomycin, and linezolid. Alright, as a quick recap, Streptococcus viridans is also known as viridans streptococci since it encompasses multiple Streptococcal species that usually do not possess Lancefield antigens.
Now, viridans streptococci are Gram-positive, catalase negative, alpha-hemolytic, optochin resistant, and bile insoluble, these are significant characteristics for diagnosis.
The viridans streptococci reside in the oropharynx, and the gastrointestinal and genitourinary tract. They can cause a wide-ranging type of conditions like dental plaques, dental caries, periodontal disease, brain and liver abscesses, and bacterial endocarditis.
The mainstay treatment consists of the use of antibiotics.