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Intro: Colony Morphology and Hemolysis

Characteristics of bacterial colonies grown on specific agar based media can contribute greatly to the identification process.

Classic morphologic descriptions of colonies include the following features:

• presence or absence of growth
• size of isolated colonies (24 hour growth is assumed)
• pigment of colonies and adjacent agar
• surface contour and texture
• edge contour
• hemolytic pattern

The hemolytic pattern arises from the lysis of red blood cells (erythrocytes) in the blood agar medium. The patterns are classified as follows:

• Alpha-hemolysis: partial hemolysis, usually with greening of the medium due to the reduction of hemoglobin in the erythrocytes to methemoglobin
• Beta-hemolysis: complete hemolysis; clear zone around colonies on blood agar medium. Sometimes organisms produce "double zone" beta-hemolysis, where there is both an inner zone of complete hemolysis, and an outer zone of incomplete hemolysis.
• Gamma-hemolysis: no hemolysis

The first image shows hemolysis as observed when the plate is sitting on the lab bench and the second image shows hemolysis when the plate is held up to the light.