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Agar or agar plate is nothing but a growth media composed of certain nutrients that allow the growth of microbial organisms needed for scientific experiments. A Blood Agar is a special and more complex kind of agar that is composed of the requisite rich nutrients along with blood. The blood (RBC only) is usually taken from animals like sheep and horse. Human Blood Agar (HuBA) is also used sometimes. Blood agar plates are used to culture those bacteria or microbes that do not grow easily. Such bacteria are called “fastidious” as they demand a special, enriched nutritional environment as compared to the routine bacteria. Blood agar is, predictably, red in colour and opaque. Besides being a rich base for the fastidious bacteria to grow, blood agar also has the function to help differentiate and classify bacteria into three types based on their hemolytic activity on the agar. They are alpha, beta and gamma-hemolytic bacteria. The gamma-hemolytic bacteria leave the RBC and the medium as it is (no hemolysis occurs). The alpha-hemolytic bacteria make the medium green in colour because of partial hemolysis. Beta-hemolytic bacteria cause complete hemolysis, which makes the medium clear.
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