Hemolysis is a concept in microbiology, referring to the breakdown of red blood cells. When bacteria are grown on a blood agar plate, which contains red blood cells, they can exhibit different patterns of hemolysis. These distinct patterns are tools for identifying bacteria. One such pattern, known as alpha hemolysis, provides clues about bacterial species.
Appearance of Alpha Hemolysis
Alpha hemolysis on a blood agar plate appears as a greenish discoloration around bacterial colonies. This green hue is not due to complete clearing of the blood, but rather a partial breakdown of the red blood cells. Bacteria produce hydrogen peroxide, which oxidizes hemoglobin into methemoglobin, a greenish-brown derivative.
The zone of alpha hemolysis often appears somewhat cloudy or turbid, indicating that the red blood cells have not been fully lysed. Instead, they have undergone a chemical change that alters their color and integrity. This partial lysis results in a translucent, greenish halo that contrasts with the surrounding red agar. Observing this specific color change and cloudiness is a primary indicator of alpha-hemolytic activity.
Comparing Hemolysis Types
Alpha hemolysis is understood by comparing it to other hemolytic reactions on blood agar. Beta hemolysis is the complete lysis of red blood cells. This complete breakdown results in a clear, transparent zone around the bacterial colonies, where the red color of the agar has entirely disappeared. The agar in this zone becomes completely see-through, resembling a clear window.
Gamma hemolysis is the absence of hemolytic activity. With gamma-hemolytic bacteria, there is no change in the blood agar around the colonies. The agar remains uniformly red, indicating that the red blood cells are unaffected and intact. These distinct visual outcomes—a partial greenish change, a clear zone, or no change—are important for differentiating bacterial species in laboratories.
Why Alpha Hemolysis Matters
Alpha hemolysis is an important preliminary step in identifying certain bacterial species. It is particularly relevant for distinguishing specific Streptococcus bacteria. For example, Streptococcus pneumoniae (pneumococcus) is an alpha-hemolytic bacterium. This characteristic helps in its presumptive identification during diagnostic procedures.
Many members of the viridans streptococci group also exhibit alpha hemolysis. These bacteria are often found as normal flora in the human mouth and upper respiratory tract. Identifying alpha-hemolytic patterns guides further testing to differentiate between pathogenic species and commensal organisms. This visual cue provides valuable information for clinical diagnostics and microbiological research.