Selective Bacterial Growth Media

Using MacConkey’s Agar and Mannitol Salt Medium to ID Bacteria

Jul 16, 2008
Tami Port

Selective Bacterial Media Top Two (MAC and MSA) - Tami Port

Selective media inhibit the growth of certain microbes, providing general information regarding the bacteria that are able to grow on these specialized types of agar.

A growth medium (plural: media) is a mixture of nutrients, moisture and other chemicals that bacteria need for growth in a laboratory environment. Media can be solid, such as Jell-o-like agar that is poured into the bottom half of a Petri dish, or media can be liquid to allow for bacterial growth suspended in test tubes. Media aren’t used to examine individual bacteria, but rather to grow bacterial colonies; millions of bacteria having arisen through the binary fission of a single progenitor.

Types of Media

There are generalized media, like Tryptic Soy Agar (TSY) that will grow many different types of microbes. This all-purpose media is the type most often used to culture bacteria.

There are also specialized media that grow only certain microbes (selective) and/or that display a color change which provides information about a microbe’s identity (differential). These selective and differential growth media are useful in providing general information for identifying bacteria.

Selective Media

Think about what the word selective means. A club is selective if it only allows certain people to become members. A growth medium is selective if it allows only certain types of bacteria to grow, while inhibiting the growth of others. To make a medium selective, specific chemicals are added that are inhibitory to some groups of bacteria.

MacConkey’s Agar (MAC)

For example, MacConkey’s agar, a transparent light mauve-colored medium, is selective due to the presence of crystal violet and bile salts. These additives inhibit the growth of Gram-positive bacteria. So if something is growing on MacConkey’s agar it can only be Gram-negative bacteria. MacConkey’s is also a differential medium that provides information on the general type of Gram- bacteria present.

Mannitol Salt Agar (MSA)

This transparent orange-pink medium is selective due to a very high salt content; so high that it creates a hypertonic environment that most bacteria cannot survive in. Members of the genus Staphylococcus are specially adapted to live in salty environments.

Staphylococcus epidermidis (Staph epi) is part of the normal flora on the surface of our skin, a highly saline environment. Staphylococcus aureus (a virulent strain of which causes MRSA) is, as a pathogenic type of Staph, S. epi’s evil cousin. Both species grow well on Mannitol Salt Agar.

MSA is also differential, producing a color change when pathogenic Staphylococcus is present.

More Microbiology Information

For more information on bacteria and infectious disease see the SPO Virtual Microbiology Classroom, or Dr. Kenneth Todar’s online microbiology resource The Microbial World.