Classification of Streptococci

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I am of the opinion (as is my idol, Mark Crislip) that microbiologists get together every few years, get drunk, and re-organize and re-classify microorganisms (its not like that, but Id like to believe this is the case). Streptococcus is one of those families of organisms that is difficult to organize in someone’s head, especially when it keeps changing so often. I will not belabor this post (as one of the ones coming up soon will deal with one of these, plus I’ve already talked about Strep pneumo and Enterococcus). 

I am sure that you know about hemolysis as a classification. Beta-hemolytic (read: red cell Busters) tend to cause a layer of hemolysis around the colony in agar plates, where as alpha-hemolytic (partial) cause a “green discoloration” and gamma-hemolytic (no hemolysis) do not:

This is the first classification that arose for streptococci. As time went on, Rebecca Lancefield found that the cell wall antigens can also be used to classify many of the beta-hemolytic strep (1). As it so happens, many of these did not apply non-beta hemolytic strep (with many exceptions. The classification below is a very simplistic way of looking at things. It takes into account both the hemolysis aspect of it and the lancefield classification:

Many issues to discuss here. First, lancefield D antigens are actually found in the enterococci (so while in the above image its its own separate class, they still have a group D antigen). Further, some members of the viridans strep actually display some beta-hemolysis (actually looking at the Strep milleri group, which can also have several Lancefield antigens). Indeed, Granulicatella and Abiotrophia were formally classified as streptococci (nutritionally deficient strep). 

This post is kind of useless (at least it will be once 16s ribosomal sequencing, MALDI-TOF, and the Karius next-gen sequencing become more mainstream), but at least for now, it allows me to think of the major groups of streptococci. 


  1. [edited by] John E. Bennett, Raphael Dolin, Martin J. Blaser. Mandell, Douglas, And Bennett’s Principles and Practice of Infectious Diseases. Philadelphia, PA :Elsevier/Saunders, 2015.

Is my post longer than the Mandell chapter on the topic? No? Is it easier to understand? Maybe? Oh well.

1 comments on “Classification of Streptococci”

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