Streptococcus agalactiae (Group B Strep)





Introduction to bacteria

Streptococcus agalactiae (Group B Strep)


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Streptococcus agalactiae (Group B Strep)

Streptococcus agalactiae (Group B Strep)

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Streptococcus agalactiae (is/is not) an encapsulated organism.


USMLE® Step 1 style questions USMLE

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USMLE® Step 2 style questions USMLE

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A 5-day-old female newborn is brought to the emergency department by her parents with a fever and increased irritability for the last 12 hours. The mother states that the child was 3 days premature, and was otherwise healthy. However, she has been crying much more and not breast-feeding well. The patient's temperature is 39.2°C (102.6°F), pulse is 140/min, respirations are 38/min, and her blood pressure is 80/55 mmHg. Physical examination shows an ill-appearing child that is warm to touch, diaphoretic, and has a bulging fontanelle. A lumbar puncture analysis shows a leukocyte count of 1,100/mm3, glucose of 25 mg/dL, and protein of 300 mg/dL. Gram-stain and culture results are pending. Which of the following is the most common etiological cause of the patient's symptoms?

External References

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.

This results in greater areas of beta-hemolysis where these colonies cross each other on the blood agar plate.

Finally, the hippurate test is based on the fact that Strep agalactiae is the only hippurate positive Strep, because it produces an enzyme called hippuricase or hippurate hydrolase.

To test for this, a colony of bacteria is transferred from the culture to a test tube containing a special hippurate medium, and incubated for two hours. Then, a few drops of a substance called ninhydrin are added to the mix.

With Strep agalactiae, hippuricase converts hippurate to glycine and benzoic acid - and glycine reacts with ninhydrin, making the mix turn deep blue - or positive.

Ok, now, Strep agalactiae 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.

First, Strep agalactiae is encapsulated, meaning it’s covered by a polysaccharide layer called a capsule.

The capsule also has pilli, which are hair-like extensions that help the bacteria attach to a host cell.

Additionally, the capsule is rich in sialic acid, which is a substance that can also be found in human cells.

So inexperienced immune cells, like that of a newborn, may confuse Strep agalactiae with self cells, allowing it to survive inside the body.

And finally, Strep agalactiae makes beta-hemolysin, which destroys the host’s tissues and causes hemolysis, meaning it destroys red blood cells.