Differential Media

Isolation of bacteria is accomplished by growing ("culturing") them on the surface of solid nutrient media. Such a medium normally consists of a mixture of protein digests (peptone, tryptone, etc.) and inorganic salts, hardened by the addition of 1.5% agar. Examples of standard general purpose media that will support the growth of a wide variety of bacteria include nutrient agar, tryptic soy agar, and brain heart infusion agar. A medium may be enriched, i.e., by the addition of blood or serum. Examples include sheep blood agar and chocolate (heated blood) agar.

Selective media contain ingredients that inhibit the growth of some organisms but allow others to grow. For example, mannitol salt agar contains a high concentration of sodium chloride that inhibits the growth of most organisms but permits staphylococci to grow.

Differential media contain compounds that allow groups of microorganisms to be visually distinguished by the appearance of the colony or the surrounding media, usually on the basis of some biochemical difference between the two groups. Blood agar is one type of differential medium, allowing bacteria to be distinguished by the type of hemolysis produced (see below). Some differential media are also selective, i.e., most of the standard enteric agars such as MacConkey and EMB agars, which are selective for gram-negative coliforms and which differentiate lactose-fermenting and non-lactose-fermenting bacteria.

Hemolysis Observation of the hemolytic reactions on blood agar is a very useful tool in the identification of bacteria, particularly streptococci. The types of hemolysis are defined as follows:

alpha (α) hemolysis: An indistinct zone of partial destruction of red blood cells (RBCs) appears around the colony, often                                accompanied by a greenish to brownish discoloration of the medium. Streptococcus pneumoniae and many                                oral streptococci are α hemolytic.

beta (β) hemolysis: A clear, colorless zone appears around the colonies, in which the RBCs have undergone complete lysis. S.                               pyogenes, S. agalactiae, and several other species of streptococci are β hemolytic. Many other bacteria                               besides streptococci can be β hemolytic, including Staphylococcus aureus, Pseudomonas aeruginosa, and                               Listeria monocytogenes, and hemolytic reactions can also be a useful diagnostic tool for these organisms.

no (γ) hemolysis: No apparent hemolytic activity or discoloration is produced by the colony (also called gamma hemolysis).


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