Blood Agar


What is this medium used for?


Some bacteria are more demanding in the nutrients they require for growth, a quality that results in their being called fastidious (or "picky"). One way to encourage the growth of medically-important fastidious bacteria is to grow them on a medium containing defibrinated blood (blood with clotting proteins removed). Blood agar is a bright red, opaque medium. The variety of complex nutrients found in blood supports the growth of most bacteria, fastidious and otherwise, that would be encountered by students. Such media are said to be complex (incapable of being chemically recreated) and enriched (containing an uncommonly rich array of nutrients).


In addition to its role in fostering the growth of difficult-to-culture organisms, blood agar has a differential function. Bacteria growing on blood agar can be classified in part on what they do to the red blood cells incorporated into the medium. Some bacteria produce hemolysins, enzymes that destroy red blood cells (hemo = blood, lysin = to split). Hemolysins can destroy the cells and release the hemoglobin into the medium. As the hemoglobin is exposed to the chemicals in the agar, its characteristic red color is altered. This type of hemolysis, alpha-hemolysis, turns the medium under the bacterial growth brown-green. Other bacteria are capable of digesting the hemoglobin released as they destroy red blood cells. The result of this complete hemolysis, termed beta-hemolysis, is clearing of the medium under the bacterial colonies. The medium is altered from opaque to transparent. Other bacteria leave red blood cells essentially untouched. The medium is not discolored or cleared by growth. Such bacteria are said to be gamma-hemolytic.









How is hemolysis determined? 


Hemolysis is determined by streaking for isolation on a blood agar plate. In clinical settings, this might also include several stabs of the inoculum into the agar to encourage any anaerobic versions of the enzymes to digest blood cells. After incubation overnight, the medium is inspected for telltale signs of alpha- or beta-hemolysis. If the medium is discolored or darkened after growth, the organism has demonstrated alpha-hemolysis. If the medium has been cleared under growth, the organism is beta-hemolytic. No discernible change in the color of the medium constitutes gamma-hemolysis.



What is the content of this medium?  


The base medium is a nutrient agar that is autoclaved and allowed to cool to 45-50 C. Then, 5% defibrinated blood (usually from sheep) is added to the medium and it is dispensed into petri plates. Once it solidifies, it is ready for use.



How is the test performed?  


For information on how to test a microbe for hemolysis in VirtualUnknown Microbiology, refer to the hemolysis on blood agar test.