How Are Streptococcal Species Classified?

Streptococcus is a group of Gram-positive cocci-shaped bacteria that occur in strips, or chains of cells. Understanding the classification of members of this genus can be tricky. There are many different traits used to distinguish one from another, and some of these traits are shared by several Streptococcal species.

One method of classifying Strep is based on whether or not they produce blood destroying enzymes called hemolysins. Another system for distinguishing different species of Strep uses the bacteria's antigens.

Classifying Streptococcus Based on Lancefield Antigens

Antigens are molecules that the human immune system recognizes as foreign, or non-self. When these uninvited particles enter the human body, the human immune system attempts to disable or eliminate the invaders. Antigens can be many different things, components of bacterial capsules, cell walls, even pollen and dust...anything that is recognized as not being part of the human body.

In the late 1930s, Rebecca Lancefield developed a classification scheme that divides members of the genus Streptococcus into twenty different serological groups based on the antigenic particles that they possess; Groups A - H and Groups K - V. Most members of this species that cause disease in humans are either Group A or B Streptococcus.

Classifying Streptococcus Based on Hemolysins

Certain bacteria produce enzymes (hemolysins) that act on red blood cells to lyse or break them down. Microbiologists can see which type of hemolysin a bacterium produces by plating the bacteria on a growth medium called Blood agar (BAP).

BAP is usually inoculated from a patient's throat swab, and is used to determine if Group A beta-hemolytic Strep (Streptococcus pyogenes) is present. This is the bacterium that causes strep throat, and that can also result in flesh-eating disease, toxic shock, scarlet and rheumatic fever.

There are three possible hemolysis patterns for bacteria grown on Blood agar.

Beta Hemolysis: This hemolysis pattern results when the bacteria's hemolytic enzymes completely break down the blood cells. β-hemolysis is revealed as clear halos around the bacterial colonies, areas where the agar has completely lost its blood-red color. Streptococcus pyogenes, S. agalactiae, S. equisimilis, and S. anginosis are all are all either strongly or weakly beta-hemolytic. For members of the genus Streptococcus, a beta reaction generally equates with "bad", i.e. bacteria that cause disease.

Alpha Hemolysis: An α-hemolysis pattern is displayed when bacterial enzymes only partially break down the blood cells. This results in the BAP showing a yellowish, greenish or brownish bruise-like discoloration around the colonies.

Gamma Hemolysis: This hemolysis pattern is essentially no hemolysis at all. The bacteria have no effect on the red blood cells and there is no change to the color of the medium.

Both Alpha and Gamma hemolysis indicate the presence of normal flora, bacteria that do not typically cause disease.

More Microbiology Information

To learn more about microbes, see Microbe Wiki or the Virtual Microbiology Classroom. Additional sources include:

Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.

Bauman, R. (2004) Microbiology. Pearson Benjamin Cummings.

Note: If you are reading this article because you suspect that you or a loved one are sick with a bacterial infection, please seek diagnosis from a medical professional, not a computer.