Microbiology Practice Exam

Beta hemolysis results in what appearance around bacterial colonies?

1. Clear halo

2. Green halo

3. Red halo

4. Yellow halo

The correct answer is: Clear halo

Beta hemolysis is characterized by the complete lysis of red blood cells in the media surrounding bacterial colonies. This process occurs when certain bacteria produce hemolysins, which are enzymes that can break down the components of red blood cells, specifically hemoglobin. As a result of this lysis, the modified medium beneath and around the bacterial colonies becomes clear, indicating the absence of red blood cells. The clear halo appearance distinguishes beta hemolytic bacteria from other types of hemolysis. For instance, alpha hemolysis results in a greenish discoloration due to the partial breakdown of red blood cells, which is not observed in beta hemolysis. Therefore, the clear halo around beta hemolytic colonies is a definitive visual cue used in microbiological identification and differentiation.