What is the Difference Between Alpha and Beta Hemolytic Streptococci

The key difference between alpha and beta hemolytic Streptococci is that alpha hemolytic Streptococci are a group of Streptococci bacteria that display partial hemolysis of red blood cells in the blood agar media, while beta-hemolytic Streptococci are a group of Streptococci bacteria that display complete hemolysis of red blood cells in the blood agar media.

Streptococcus is a genus of gram-positive coccus or spherical bacteria. These bacteria belong to the family Streptococcaceae within the order Lactobacillales and in the phylum Firmicutes. Most Streptococci bacteria are oxidase negative, catalase negative, and facultative anaerobes. Moreover, they are often pathogenic bacteria that cause several diseases in humans. The species of genus Streptococcus are classified based on their hemolytic (red blood cells breakdown) properties in blood agar. According to hemolytic properties, there are three groups of Streptococci bacteria: alpha, beta, and gamma hemolytic. Therefore, alpha and beta hemolytic Streptococci are two groups of Streptococci bacteria based on hemolytic properties.

CONTENTS

1. Overview and Key Difference
2. What is Alpha Hemolytic Streptococci
3. What is Beta Hemolytic Streptococci
4. Similarities – Alpha and Beta Hemolytic Streptococci
5. Alpha vs Beta Hemolytic Streptococci in Tabular Form
6. Summary – Alpha vs Beta Hemolytic Streptococci

What are Alpha Hemolytic Streptococci?

Alpha hemolytic Streptococci are a group of Streptococci bacteria that displays partial hemolysis of red blood cells in the blood agar media. Hemolysis is the breakdown of red blood cells. The ability of bacterial colonies to induce hemolysis in media such as blood agar is used to classify bacteria such as Streptococci. When alpha hemolysis occurs, the blood agar medium contents under the bacterial colonies are dark and greenish. This is because alpha hemolytic species cause the oxidation of iron in haemoglobin molecules within red blood cells. Thus, it gives a greenish colour on blood agar. Alpha hemolysis is also known as incomplete or partial hemolysis because the cell membranes of the red blood cells are still left intact after the hemolysis process.

Normally, Streptococcus pneumonia and a group of oral Streptococci (Streptococcus viridians) display this alpha hemolytic property. S. pneumonia is the leading cause of bacterial pneumonia. S. pneumonia can also cause otitis media, sinusitis, meningitis, and peritonitis. S. viridians is normally commensal but can be pathogenic. When pathogenic, S. viridians causes sepsis and pneumonia.

What are Beta Hemolytic Streptococci?

Beta hemolytic Streptococci are a group of Streptococci bacteria that displays complete hemolysis in red blood cells in the blood agar media. Beta hemolytic Streptococci cause complete lysis of red blood cells in the blood agar media around and under the bacterial colonies. The area in the media appears lightened yellow and transparent after hemolysis. The complete hemolysis is due to an exotoxin that is made by these bacteria called streptolysin. There are two types of streptolysins: streptolysin O (SLO) and streptolysin S (SLS). SLO is secreted by Group A Streptococccus (GAS) and Streptococcus dysgalactiae. SLS is only secreted by Group A Streptococccus.

Furthermore, Streptococccus pyogenes or Group A beta hemolytic Streptococcus (GAS) displays beta hemolysis. Sometimes, weak beta hemolytic species (Group B Streptococcus such as Streptococccus agalactiae) cause intense hemolysis of red blood cells when they grow together with a strain Staphylococcus. This is known as the CAMP test.

What are the Similarities Between Alpha and Beta Hemolytic Streptococci?

• Streptococci are two groups of Streptococci bacteria based on hemolytic properties.
• Bacteria of both groups are classified under genus Streptococcus, family Streptococcaceae, order Lactobacillales, and phylum Firmicutes.
• Bacteria of both groups show hemolysis of red blood cells in blood agar plates.
• They have specific hemolysins.
• They are gram-positive, oxidase negative, catalase negative, and facultative anaerobes.

What is the Difference Between Alpha and Beta Hemolytic Streptococci?

Alpha hemolytic Streptococci are a group of Streptococci bacteria that displays partial hemolysis of red blood cells in the blood agar media, while beta-hemolytic Streptococci are a group of Streptococci bacteria that displays complete hemolysis of red blood cells in the blood agar media. Thus, this is the key difference between alpha and beta hemolytic Streptococci. Furthermore, the area in the media appears green in colour after hemolysis by alpha-hemolytic Streptococci. On the other hand, the area in the media appears yellow colour and transparent after hemolysis by beta-hemolytic Streptococci.

The below infographic presents the differences between alpha and beta hemolytic Streptococci in tabular form for side by side comparison.

Summary – Alpha vs Beta Hemolytic Streptococci

Alpha and beta hemolytic Streptococci are two groups of Streptococci bacteria based on hemolytic properties. Alpha hemolytic Streptococci display partial hemolysis of red blood cells in the blood agar media, while beta-hemolytic Streptococci display complete hemolysis of red blood cells in the blood agar media. So, this is the key difference between alpha and beta hemolytic Streptococci.

Reference:

1.“Alpha Hemolytic Streptococcus.” An Overview | ScienceDirect Topics.
2. “Beta Hemolytic Streptococcus.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Streptococcal hemolysis” By Y tambe – Y tambe's file (CC BY-SA 3.0) via Commons Wikimedia
2. “Streptococcus agalactiae on blood agar” By 43trevenque – Own work (CC BY-SA 4.0) via Commons Wikimedia