What is the Difference Between Alpha and Beta Hemolytic Streptococci

The main difference between alpha and beta hemolytic Streptococci is that alpha-hemolytic Streptococci undergo partial or “greeny” hemolysis, reducing the red blood cell hemoglobin whereas beta-hemolytic Streptococci undergo complete lysis of red blood cells surrounding the colony.

Alpha and beta hemolytic Streptococci are two types of Streptococci bacteria that react differently to the hemolytic reaction. Generally, the hemolytic reaction is displayed on blood agar. It is important to classify different types of Streptococci bacteria according to the antigenic types.

Key Areas Covered

1. What are Alpha Hemolytic Streptococci
     – Definition, Features, Importance
2. What are Beta Hemolytic Streptococci
     – Definition, Features, Importance
3. Similarities Between Alpha and Beta Hemolytic Streptococci
     – Outline of Common Features
4. Difference Between Alpha and Beta Hemolytic Streptococci
     – Comparison of Key Differences

Key Terms                                                                                                                  

Alpha Hemolytic Streptococci, Beta Hemolytic Streptococci

What are Alpha Hemolytic Streptococci

Alpha hemolysis refers to the partial hemolysis of red blood cells shown by the greenish discoloration surrounding the bacterial colony in the blood agar. It is caused by several Streptococcus species such as Streptococcus pneumonia and Streptococcus viridans. Generally, alpha hemolysis is a type of partial hemolysis in which the iron molecules in the hemoglobin are oxidized by hydrogen peroxide produced by the bacteria. This generally gives a greenish color to the surroundings of the bacterial colony as hemoglobin is converted into methemoglobin. Normally, alpha hemolysis is not a complete rupture of red blood cells; the red blood cells remain intact during the infection.

Figure 1: Alpha and Beta Hemolysis

Furthermore, the prolonged incubation allows the formation of clear areas in alpha hemolysis. However, green or brown shades remain in the medium. Since alpha hemolysis occurs by Streptococcus pneumonia, it can be used as a diagnostic feature during the identification of the bacterial strain.

What are Beta Hemolytic Streptococci

Beta hemolysis refers to the complete breakdown of red blood cells, demonstrated by a clear zone surrounding the bacterial colony in the blood agar. It is involved in the true or complete lysis of red blood cells. In general, the toxic by-products of bacteria cause the complete breakdown of red blood cells. Streptococcus pyogenes produces a hemolysin called Streptolysin O, which is only active in low oxygen conditions. Anaerobic pockets can be produced on the agar plate by stabbing an inoculating loop into the agar vertically, after streaking the plate. Some Streptococcus pyogenes species produce an oxygen-stable hemolysin called Streptolysin S.

Figure 2: Streptococci

Moreover, some beta hemolysis reactions are very subtle. These weak hemolysis reactions also occur in Streptococcus agalactiae or Listeria monocytogenes.  

Similarities Between Alpha and Beta Hemolytic Streptococci

• Alpha and beta hemolytic Streptococci are two antigenic types of Streptococci bacteria with different types of hemolysis on blood agar.
• Generally, Streptococci are Gram-positive cocci bacteria that are non-motile, non-spore forming, and catalase negative.
• They occur in pairs or chains.
• In general, they are facultative anaerobes and some are obligate anaerobes.
• They require enriched media for their growth.

Difference Between Alpha and Beta Hemolytic Streptococci

Definition

Alpha hemolytic Streptococci refers to a type of Streptococci that undergo alpha-hemolysis while beta hemolytic Streptococci refers to a type of Streptococci that undergo beta-hemolysis.

Hemolysis

Alpha hemolytic Streptococci undergo partial or “greeny” hemolysis, reducing the red cell hemoglobin, while beta hemolytic Streptococci undergo complete lysis of red blood cells surrounding the colony.

Examples

Moreover, alpha hemolytic Streptococci are S. pneumoniae and S. viridans while beta hemolytic Streptococci are S. pyogenes and S. agalactiae.

Hemolysins

Alpha hemolytic Streptococci use hydrogen peroxide produced by the bacterium for hemolysis while beta hemolytic Streptococci use toxic by-products that destroy red blood cells to hemolyze cells.

Change

While alpha hemolytic Streptococci produce greenish black zones, beta hemolytic Streptococci produce clear zones.

Width of the Zone

Alpha hemolytic Streptococci produce zones with the width of 1-2 mm while beta hemolytic Streptococci produce zones with the width of 2-4 mm.

Location

Alpha hemolytic Streptococci occur in the oral cavity while beta hemolytic Streptococci occur in the throat.

Conclusion                                                     

In brief, alpha-hemolytic Streptococci and beta-hemolytic Streptococci are two types of Streptococci bacteria that undergo hemolysis. Generally, alpha-hemolytic Streptococci undergo partial hemolysis reducing the hemoglobin of red blood cells. It also produces a greenish-black zone. In comparison, beta-hemolytic Streptococci undergo complete hydrolysis of hemoglobin in red blood cells. Therefore, it produces clear zones. Hence, the main difference between alpha and beta hemolytic Streptococci is their type of hemolysis.

References:                

1. Patterson MJ. Streptococcus. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 13.

Image Courtesy:

1. “Streptococci” By User:Graham Beards – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “OSC Microbio 09 02 listumbrel” By CNX OpenStax – Own Work (CC BY 4.0) via Commons Wikimedia