Differential and Selective Bacterial Growth Media

Apr 24, 2010
Tami Port

Differential and Selective Media - T. Port

Agar media are used to grow bacteria in the laboratory. There are also specialized media that provide information on the identity of the microbes growing.

Bacterial growth media are used in labs to provide nutrients, moisture and a surface for bacteria to grow on. Some media will grow just about any type of bacteria, whereas other types are specialized to only grow certain microbes and to often provide additional information that can help identify microbes.

Selective Bacterial Growth Media

Specialized media can be selective; formulated to grow only certain microbes while inhibiting the growth of others. For example MacConkey’s (MAC) selective bacterial growth medium will only grow Gram-negative bacteria. Mannitol Salt medium (MSA) will only grow halophilic (salt-loving) bacteria, such as Staphylococcus and Micrococcus. While selective media do not typically identify bacteria down to the species level, they do help narrow down the search.

Differential Bacterial Growth Media

Differential media typically display some type color change in the presence of certain bacteria. Some specialized media are both selective and differential, others are either or. The MacConkey’s and Mannitol Salt described above are both selective and differential bacterial growth media.

In addition to MacConkey’s only growing Gram-negative bacteria (the selective aspect of the medium), MAC has special additives that cause lactose fermenting bacteria (microbes that can metabolize the sugar lactose) to grow in pink colonies, whereas Gram-negative non-lactose fermenting bacteria will grow in colorless colonies.

Mannitol Salt is also both selective and differential. This medium only grows salt-loving bacteria (so it is selective). In addition, bacteria that grow on MSA that can ferment mannitol, a sugar alcohol, will turn the medium from its original pink color to a bright, neon yellow. This color change is clinically significant in that Staphylococcus aureus, a disease-causing bacterium, is a mannitol fermenter, while S. epidermidis, a beneficial bacteria that normally grows on the surface of the skin and mucous membranes, is not a mannitol fermenter.

Blood Agar (BAP) is another type of differential medium. BAP is rich in nutrients and contains sheep blood. It is not selective, and will grow many different types of microbes. Blood agar medium is, however, differential. It will display a color change in the presence of bacteria that can lyse (break down) the red blood cells in the medium. Bacteria that grow on this medium will produce one of three hemolytic patterns:

beta-hemolysis = complete lysing of the red blood cells, which turns the agar from red to clear
alpha-hemolysis = partial lysis of the red blood cells, which gives the medium a bruised appearance
gamma-hemolysis = no lysis of the red blood cells and no change to the appearance of the mediums color

The beta-hemolytic bacteria Streptococcus pyogenes, is a pathogen that will completely break down the red blood cells in this agar, causing clear areas to form around its colonies growing on BAP.

For more information on microbiology, see the SPO Virtual Microbiology Classroom or Todar's Online Textbook of Bacteriology.