Streptococcus viridans

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.

With Strep pneumoniae, the growth of the bacteria is inhibited in the zone that surrounds the optochin disc, meaning that Strep pneumoniae is optochin sensitive.

On the flip side, viridans streptococci are optochin resistant - meaning there’s no inhibition of the growth around the optochin disc.

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.

Strep pneumoniae is bile soluble, meaning that the bile salts will promote the lysis of the bacteria; whereas viridans streptococci are bile insoluble, or bile resistant.

Finally, viridans streptococci also lack the polysaccharide-based capsule typical of Streptococcus pneumoniae.