Streptococcus pyogenes (Group A Strep)

Streptococcus pyogenes, sometimes called Strep pyogenes, can be broken down into “strepto” which means chain, “coccus”, which refers to round shape, “pyo” which means pus, and “genes” which refers to forming.

So, Strep pyogenes are round bacteria that grow in chains, responsible for a number of infections that often present with pus. Strep pyogenes are also called Group A Strep – GAS - in Lancefield classification developed by American microbiologist Rebecca Lancefield. 

Ok now, Strep pyogenes has a thick peptidoglycan cell wall, which takes in purple dye when Gram-stained - so this is a gram-positive bacteria.

It’s non-motile and doesn’t form spores, and it’s also a facultative anaerobe, meaning it can survive in both aerobic and anaerobic environments.

Finally, Strep pyogenes is catalase negative, meaning it doesn’t make an enzyme called catalase. 

However, unlike other common cocci like Enterococci, Strep pyogenes is pyrrolidonyl arylamidase positive, because it makes an enzyme called L-pyrrolidonyl arylamidase.

To test for this, a small sample is taken from a suspected bacterial colony, and then inoculated to a disk pad that’s embedded with pyrrolidonyl beta naphthylamide - another joy of a word.

With Strep pyogenes, pyrrolidonyl arylamidase hydrolyzes pyrrolidonyl beta—naphthylamide to produce beta-naphthylamide.

Try saying that 3 times fast! Finally, another reagent called N-methylamino-cinnamaldehyde is added to the disk, and it reacts with beta—naphthylamide, resulting in a bright red color that confirms Strep pyogenes is pyrrolidonyl arylamidase positive.

When cultivated on a medium called blood agar, Strep pyogenes colonies cause  beta-hemolysis, also called complete hemolysis. That’s because Strep pyogenes makes toxins known as streptolysins, which hydrolyze the hemoglobin in red blood cells to transparent yellow color byproducts. 

But some other Streptococcus species, like Strep agalactiae, are also beta-hemolytic. So a bacitracin test is done to distinguish Strep pyogenes.

That’s when a disk of bacitracin is added to the blood agar. Strep pyogenes is bacitracin sensitive, so the colonies die off, whereas with Strep agalactiae, the colonies remain intact.  

Now, Strep pyogenes has a number of virulence factors, that are like assault weaponry that help it attack and destroy the host cells, and evade the immune system.

So first, Strep pyogenes is encapsulated, meaning it’s covered by a polysaccharide layer called a capsule.