Names of Streptococcus Species

So you’ve just received your lab report from a culture test to determine what kind of infection you have. The report simply says: “Alpha hemolytic streptococcus”. What does that mean, and what is the actual species name of the bacteria? Is “Alpha hemolytic streptococcus” the same thing as “Group A streptococcus” (GAS)?


There are two major methods of classifying Streptococcal bacteria.

The first method is based on how well a bacterial sample can break down or rupture red blood cells (RBCs) when it is cultured. This activity is called “hemolysis”. The bacteria is designated as belonging to one of three groups based on its capability to cause partial, complete, or zero hemolysis (alpha, beta, and gamma hemolysis, respectively). Other types of bacteria are also often tested for their hemolytic characteristics.

When Streptococcal bacteria are cultured on blood agar plates containing RBCs, some of those species produce hydrogen peroxide, while others do not. The peroxide causes oxidation of iron contained in the hemoglobin of RBCs, which subsequently results in a greenish color being produced on the culture plate at the location of the streak. This hemolytic mechanism is called “alpha (α) hemolysis”.

Some species of Streptococcus can additionally produce toxins that cause breakdown of the outer membrane of RBCs. It is a more thorough and stronger hemolytic mechanism than the alpha type, and is called “beta (β) hemolysis”. This phenomenon can also affect other types of cells in addition to RBCs.

In summary, hemolysis can be caused by more than one mechanism.

  • “Alpha (α) hemolysis” produces only partial/incomplete hemolysis on the culture plate.

  • “Beta (β) hemolysis” produces complete hemolysis of the RBCs around the streak.

  • “Gamma (γ) hemolysis” is a less commonly used term, denoting zero hemolysis.


Hemolyses of Streptococcus spp.: (left) α-hemolysis (S. mitis); (middle) β-hemolysis (S. pyogenes); (right) γ-hemolysis (= non-hemolytic, S. salivarius).

Image credit: https://en.wikipedia.org/wiki/Hemolysis_(microbiology) and https://en.wikipedia.org/wiki/File:Streptococcal_hemolysis.jpg

The second method of classifying Streptococcal bacteria goes beyond the hemolytic characteristic, and is based on the type of carbohydrate contained in the bacterial wall. The term used for this designation is “group” (or more technically, “serotype”), and letters currently ranging from A-H are used to classify which group the species belongs to. Thus we can have “group A strep” (abbreviated GAS), “group B strep” (GBS), etc.

The following Streptococcus species lists are based on the most usual hemolytic showings, but there can be variance.

Alpha (α) hemolytic species of Streptococcus bacteria

1. The pneumococcus species:

  • Streptococcus pneumoniae (common respiratory tract pathogen). Can cause ear infection, sinusitis, pneumonia, meningitis, peritonitis.

2. The viridans group of species (can be alpha hemolytic or non-hemolytic); it includes many of the oral (mouth) strep species. Note that “viridans” is a group name, but not a species name. The Latin word “viridis” means “green”. 

  • Streptococcus anginosus group (can show alpha, beta, or non-hemolytic characteristics). See also “Group F strep” below.

  • Streptococcus mitis group.

  • Streptococcus sanguinis group (includes S. sanguinis, S. parasanguinis, S. gordonii).

  • Streptococcus bovis group (includes S. equines, S. gallolyticus, S. infantarius).

  • Streptococcus salivarius group (includes S. salivarius, S. vestibularis, S. thermophilus [a species used in culturing dairy products]).

  • Streptococcus mutans group (includes S. mutans [causes cavities], S. sobrinus).

Beta (β) hemolytic species of Streptococcus bacteria

  • These groups are based on serotypes (that is, the specific type of carbohydrate(s) contained in the bacterial cell wall).

  • These bacteria generally achieve complete lysis of RBCs in blood agar culture media.

  • They can produce exotoxins called “streptolysins” O and S, which cause membrane lysis (rupture) of cells.

  • Streptolysin O interacts with cholesterol in the cell membrane; it is oxygen-sensitive, which means it degrades in the presence of oxygen.

  • Streptolysin S is oxygen-stable; it attacks leukocytes & lymphocytes (types of white blood cells), and thus hinders clearance of the infection.

  • Most group A streptococcal species [GAS], produce streptolysins O and S.

 1. Group A strep  [GAS]

  • The most common species is Streptococcus pyogenes.

  • Non-invasive infection causes strep throat, impetigo, scarlet fever.

  • Invasive (systemic) infection can cause toxic shock syndrome, necrotizing fasciitis, pneumonia, bacteremia.

  • Can also trigger rheumatic fever, which is caused by an auto-immune reaction to the antibodies produced by the body; it also can cause PANDAS, especially in children, caused by the same auto-immune reaction at the basal ganglia nerves.

 2. Group B strep  [GBS]

  • The most common species is Streptococcus agalactiae.

  • Infects the female reproductive system; causes pneumonia and meningitis in neonates and the elderly; and occasionally goes systemic.

 3. Group C strep 

  • Streptococcus equi (causes strangles in horses).

  • Streptococcus zooepidemicus (infects some mammals including horses and cattle).

  • Streptococcus dysgalactiae, causes pharyngitis, pneumonia, meningitis, endocarditis in humans.

 4. Group D strep

  • Most species in this group are now reclassified as Enterococcus bacteria, which sometimes cause hemolysis.

  • Includes beta-hemolytic forms of Streptococcus bovis, Streptococcus equinus.

 5. Group F strep

  • Includes beta-hemolytic forms of Streptococcus anginosus.

 6. Group G strep

  • A group that is usually but not always beta-hemolytic.

  • Includes Streptococcus dysgalactiae, Streptococcus canis (which can also infect humans).

7. Group H strep

  • Includes some canine species.

The material in this article is summarized from the following online resources about streptococcus and hemolysis. For more complete information, please consult those websites, other reliable medical resources, or a medical professional. The information in this article is not intended to be diagnostic.






Copyright © 2019 by Charlene Boehm