Streptococcus agalactiae (Group B Strep)

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.