Streptococcus pyogenes (Group A Strep)

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Streptococcus pyogenes (Group A Strep)



Streptococcus pyogenes (Group A Strep)


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USMLE® Step 1 questions

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Streptococcus pyogenes (Group A Strep)

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USMLE® Step 1 style questions USMLE

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A 5-year-old boy is brought to the pediatrician by his parents due to a sore throat and fever. The patient's symptoms started three days ago. Examination of the patient’s throat shows wide exudates on the tonsils, pharyngeal erythema, and cervical adenopathy.  A pharyngeal swab is performed and sent to the laboratory for Gram stain and culture, and bloodwork is drawn. The patient is started immediately on antibiotics.  Anti-streptolysin O antibody titers return several days later and are elevated. Which of the following characteristics of the most likely pathogen causing this patient’s condition?  

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Cellulitis p. 491

Streptococcus pyogenes p. , 134

Erysipelas p. 491

Streptococcus pyogenes p. , 134

Exotoxin A

Streptococcus pyogenes p. , 131


Streptococcus pyogenes p. , 131, 134

Impetigo p. 487

Streptococcus pyogenes p. , 134


Streptococcus pyogenes p. , 134

Rheumatic fever p. 321

Streptococcus pyogenes p. , 134

Scarlet fever

Streptococcus pyogenes p. , 134

Streptococcus pyogenes (Group A strep) p. 134

bacitracin response p. 720

β -hemolysis p. 133

clindamycin p. 189

exotoxin production p. 131

Gram-positive testing p. 132

M protein and p. 127

penicillin G/V for p. 184

rash p. 178

skin infections p. 491

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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 StrepGAS - 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.


Streptococcus pyogenes, often called group A streptococcus (GAS), is a gram-positive, beta-hemolytic, and bacitracin-sensitive bacteria. Group A streptococcus normally colonize the pharynx, vagina, or the skin, where they're part of the normal flora, but in some cases, they may take advantage of a weakened immune system and causes infections like strep pharyngitis, scarlet fever, impetigo, necrotizing fasciitis. Infections due to certain strains of this bacteria can involve certain bacterial toxins, leading to scarlet fever or toxic shock syndrome. Group A streptococcus is also associated with post-infectious sequelae, like acute rheumatic fever and post-streptococcal glomerulonephritis. Treatment involves antibiotics like penicillin G, cephalosporins such as ceftriaxone, and macrolides such as azithromycin.


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