Streptococcus agalactiae (Group B Strep)


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Streptococcus agalactiae (Group B Strep)


Introduction to bacteria

Bacterial structure and functions

Gram positive bacteria

Staphylococcus epidermidis

Staphylococcus aureus

Staphylococcus saprophyticus

Streptococcus viridans

Streptococcus pneumoniae

Streptococcus pyogenes (Group A Strep)

Streptococcus agalactiae (Group B Strep)


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Streptococcus agalactiae (Group B Strep)


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Streptococcus agalactiae (Group B Strep)

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First Aid








Hippurate test for Streptococcus agalactiae p. , 135


Streptococcus agalactiae p. , 135


Streptococcus agalactiae in p. 135

Pneumonia p. 707

Streptococcus agalactiae p. , 135

Pregnancy p. 657

Streptococcus agalactiae in p. 135


Streptococcus agalactiae as cause p. 135

Streptococcus agalactiae (Group B strep) p. 135

β -hemolytic nature of p. 133

encapsulated bacteria p. 125

Gram-positive testing p. 132

immunodeficient patients p. 116

meningitis p. 177

in neonates p. 181

pneumonia p. 176

prophylaxis for p. 195

splenic dysfunction p. 96

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Content Reviewers


Evode Iradufasha, MD

Tanner Marshall, MS

With Streptococcus agalactiae sometimes called Strep agalactiae, strepto means a chain, coccus means round shape, and agalactiae literally means “no milk”.

So, Strep agalactiae refers to the round bacteria that grow in chains and that was previously known to infect cattle, resulting in reduced milk production.

Later on, Strep agalactiae was found to also be a human potential pathogen responsible for a number of infections that most commonly affect pregnant women and newborns.

Strep agalactiae are also called Group B Strep – GBS - in Lancefield classification developed by an American microbiologist Rebecca Lancefield.

Ok now, a little bit of microbe anatomy and physiology.

Strep agalactiae 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 also, it’s a facultative anaerobe, meaning that it can survive in both aerobic and anaerobic environments.

Now, a particular trait of Streptococcus species is that they are catalase negative, meaning they do not produce an enzyme called catalase.

This is unlike other common gram positive cocci, like Staphylococcus, which are catalase positive.

When cultivated on a medium called blood agar, Strep agalactiae colonies cause beta hemolysis, also called complete hemolysis.

That’s because Strep agalactiae makes a toxin called beta-hemolysin, that causes complete lysis of the hemoglobin in the red blood cells, making them blood agar change color from red to transparent yellow around the colonies.

However, other Streptococcus species, like Strep pyogenes, are also beta hemolytic.

So to identify Strep agalactiae specifically, the bacitracin test, the hippurate test, or the CAMP test can be done.

With the bacitracin test, a disk of bacitracin is added to the blood agar.

Strep agalactiae is bacitracin resistant, so the colonies remain intact, whereas Strep pyogenes is bacitracin sensitive, so the colonies die off.

With the CAMP test, Strep agalactiae is grown with Staphylococcus aureus on the same blood agar.

Both these bacteria are beta-hemolytic, but Strep agalactiae makes a substance called CAMP factor, which enhances the action of staphylococcal beta-hemolysin.


Streptococcus agalactiae, also known as group B streptococcus, is a gram-positive, beta-hemolytic, catalase-negative, and bacitracin-resistant bacterium, which can cause several infections in humans. Most frequently, Streptococcus agalactiae causes neonatal infections like pneumonia, sepsis, meningitis, and septic arthritis. It can also cause chorioamnionitis or cystitis in pregnant females. Treatment involves antibiotics like penicillin G or ampicillin, or Cefazolin and Vancomycin. To prevent Streptococcus agalactiae infections in neonates, intrapartum antibiotic prophylaxis can be administered to vaginally colonized pregnant females.


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