USMLE® Step 1 style questions USMLE
A study is done to identify the normal bacterial flora of the mouth. One bacterial species that was found to colonize the mouth is a gram-positive coccus that grows in chains and is catalase-negative. When it is grown on blood agar, alpha-hemolysis is noted. Additionally, this bacterium was tested for optochin sensitivity and was optochin-resistant. Which of the following is an additional feature of this bacterium?
Content Reviewers:Viviana Popa, MD
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 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.
That’s where an optochin-containing paper disc is placed on a plate of blood agar that has been inoculated with the bacteria.
Another test that can be used to distinguish Strep viridans from Strep pneumoniae is the bile solubility test, which can be done in a test tube, or more commonly, by dripping drops of bile on bacterial colonies.
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.
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.