STREPTOCOCCI

In the upper respiratory tract, there is one genus of bacteria that cause 90% of infections (pharyngitis) Group A Streptococcus (aka S. pyogenes).

15-30% of all sore throats are caused by Group A, 40% by unknown causes, 30% by viruses. This means that only about 30% of sore throats require antibiotics, which you should remember when you’re a doctor and a kid comes to you with a sore throat.

There are three types of Strep based on what they look like on blood agar after 24 hours:

1. β Hemolytic  If you look at the blood agar, you will see a colony of bacteria surrounded by white space. This is because there is complete hemolysis of the RBCs. The hemolysins are mainly streptolysins S and O. Important members of this group includes Group A, and to a lesser extent B, C, and G. (90% vs. 5%).

2. α Hemolytic  These convert hemoglobin into methemoglobin, which is green, therefore on the plate you will see a colony surrounded by green stuff. This type includes S. viridans and S. pneumoniae.

3. γ Hemolysis  this is also called nonhemolytic Strep, because all you see is the colony with no hemolysis. This includes S. bovis and group D.

You might be wondering why all these bacteria are called Group A, B, C etc. It’s because a scientist in the 30’s, Rebecca Lancefield, decided to classify all the different kinds of strep. She extracted bacteria from throat swabs immunized rabbits with it  used the antisera to see if it reacted. If it didn’t, she did it again, and again, until she got tons of groups. But 90% of the bacteria reacted with the first group, which is called Group A or S. pyogenes. Both names are used in the literature, so remember both. The professor refers to it as GAS (Group A Streptococcus) and so will I from now on.

Ok, so we know that all of GAS is dangerous, why not make a vaccine?

Well there are problems. The groups were distinguished based on polysaccharides in their cell membranes, and antibodies can’t be made against polysaccharides. Antibodies can be made against proteins, and the protein on GAS is called M protein. There are at least 60 different epitopes of M protein in GAS, and all cause disease. This is a lot of antibodies to include in one vaccine!!

Category: Microbiology Notes