Microbiology Practice Exam

What is the appearance of the surrounding area of a culture exhibiting full lysis?

1. Green halo

2. Clear halo

3. Red halo

4. Pink halo

The correct answer is: Clear halo

The appearance of the surrounding area of a culture exhibiting full lysis is characterized by a clear halo. This phenomenon is typically observed in the case of bacterial species that produce enzymes such as hemolysins, which can lyse red blood cells. When a bacterium exhibits complete lysis of red blood cells, it results in the breakdown of hemoglobin, leading to a clear zone around the growth of the bacteria on blood agar plates. This clear zone indicates that red blood cells have been completely destroyed and no longer provide any coloration to the surrounding medium, thus creating a distinct contrast that is readily observable when compared to areas where hemolysis is partial (which would present a green halo) or absent (where no halo would appear).