Why do alpha streptococci produce a green color on blood agar?

Alpha-streptococci produce a green color on blood agar due to the partial hemolysis of red blood cells.

Streptococci are classified into three groups based on their hemolytic activity on blood agar:

1. Alpha-hemolytic streptococci (α-hemolytic): These streptococci produce a partial hemolysis of red blood cells, resulting in a greenish discoloration of the agar around the colonies. The greenish pigment produced is due to the conversion of hemoglobin into methemoglobin by the enzyme NADH-dependent hydrogenase. The enzyme reduces NAD+ to NADH, which then reduces methemoglobin, an oxidized form of hemoglobin, to hemoglobin. This enzymatic reaction leads to the formation of a greenish pigment, giving alpha-streptococci their characteristic "greening" effect on blood agar.

2. Beta-hemolytic streptococci (β-hemolytic): These streptococci cause complete hemolysis of red blood cells, resulting in a clear zone of complete lysis surrounding the colonies on blood agar. Beta-hemolysis is caused by the production of potent hemolytic toxins, such as streptolysin O and streptolysin S, which lyse the red blood cell membranes.

3. Gamma-hemolytic streptococci (γ-hemolytic): Gamma streptococci do not cause any hemolysis of red blood cells. They produce colonies without any discoloration or change in the surrounding agar.