Difference between Group A and B Streptococcus

Serological Classification of S. pyogenes and S. agalactiae

Mar 26, 2009

Bacteria in the genus Streptococcus can be classified by species, hemolysis pattern and by their antigens, chemicals to which the human immune system reacts.

Streptococcus is a genus of Gram-positive cocci (round) shaped bacteria that exist in chains of cells. Classification of members of this genus can be challenging, as there are many different, overlapping classification systems used to distinguish one species of Streptococcus from another. One method of classifying Strep is based on the distinct types of antigenic particles that are part of the cell structure of different species.

What Is an Antigenic Particle?

Antigens are molecules that the human immune system recognizes as foreign, or non-self. When these unfamiliar particles enter the human body, the immune system launches an immune response to disable or eliminate the invaders. Antigenic particles can be many different things, components of bacterial cells walls, pollen, dust, viral proteins…anything that is nonself.

In 1938, Rebecca Lancefield originated a classification scheme dividing 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. However, many pathogenic Strep, those that cause disease in humans, are in Groups A and B.

Group A Strep consists of just one species, S. pyogenes (say pie-AAH-jen-eez), but this one species packs a wallop, being responsible for many serious infectious diseases. Many S. pyogenes bacteria have a capsule, as well as numerous other virulence factors that help them cause disease. The capsule is a form of glycocalyx external to the bacterial cell wall, which can protect the bacteria from destruction by the immune system.

Group A Strep usually only cause disease either when competing normal flora populations are reduced (such as after the use of antibiotics), or when a very large amount of Streptococcus grow in the body before the immune system can organize a counterattack. Streptococcal infection caused by Group A Strep include:

Group B Strep also consists of only one species, S. agalactiae (say a-ga-LACK-tuh-ee). With respect to serology, S. agalactiae is distinguished from Group A Streptococcus by cell wall antigens specific to the species.

The capsule that an individual Group B Strep forms doesn’t work well to protect the bacterium from destruction by the immune system, because humans have antibody molecules that can recognize components of its capsule. Group B Strep mainly pose a risk to newborns, because neonates do not have fully developed immune function at birth.

S. agalactiae can normally be found in the lower GI and lower urogenital tract. Babies can become inoculated with the bacteria when moving through the birth canal. If the mother has antibodies to the bacteria, so will the baby, and no disease will result. But if a newborn from an uninfected mother is exposed to the bacteria from another source, the resulting infection can be very dangerous. Although S. agalactiae can cause wound infection and infections in mothers following childbirth, the diseases caused by Group B Strep most often affect neonates and include:

More Microbiology Information

Bauman, R. (2007). Microbiology with Diseases by Taxonomy. 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 and treatment from a medical professional, not a computer.

The copyright of the article Difference between Group A and B Streptococcus in Microbiology is owned by Tami Port. Permission to republish Difference between Group A and B Streptococcus in print or online must be granted by the author in writing.