Microbiology Practice Exam 2026 - Free Microbiology Practice Questions and Study Guide

Question: 1 / 400

Alpha hemolysis is characterized by what type of halo?

Clear halo

Red halo

Green halo

Alpha hemolysis is characterized by a green halo around bacterial colonies on a blood agar plate. This occurs due to the partial lysis of red blood cells, leading to the reduction of hemoglobin to methemoglobin, which imparts a greenish hue to the surrounding agar. This change in color indicates that the bacteria possess certain enzymes (like alpha-hemolysin) that can partially digest the hemoglobin found in red blood cells.

In contrast, a clear halo would suggest complete lysis of red blood cells (beta hemolysis), while a red halo implies no lysis (gamma hemolysis). A yellow halo typically indicates a different metabolic process, often associated with the fermentation of specific sugars and changes in pH, rather than hemolytic activity. Thus, the presence of a green halo is a defining characteristic of alpha hemolysis, confirming that option C is indeed the correct response.

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Yellow halo

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