Mannitol Salt Agar

When bacterial cultures are grown in a laboratory environment, they are essentially captives, part of a microbial zoo dependent on people to provide the proper environment for their survival and growth. A nutrient-rich media is required to grow bacteria in the lab. Mannitol Salt Agar is a special bacterial growth medium that is selective for halophiles (bacteria that can grow in a salty environment) and can differentiate pathogenic and nonpathogenic species of Staphylococcus.

What is Media?

A growth medium (plural: media) is a mixture of nutrients, moisture and other chemicals that bacteria need for growth. Media aren’t used to directly examine individual bacteria, but rather to grow colonies (millions of bacteria having arisen through the binary fission of a single progenitor).

Some media are solid, such as Jell-O-like agar. When hot, agar is liquid that can be poured into the bottom half of a Petri dish; once cooled, it solidifies, providing a surface for bacterial growth. Media can also be liquid, a broth to allow for bacterial growth suspended in test tubes.

Using Media to Identify Bacteria

Like the differential staining of bacteria, special types of media can be used to provide clues about a microbe’s identity. There are many types of solid media that are specific about what they grow, or that provide information about the type of microbes present.

Differential and selective media can exclude certain types of bacteria and even test for certain bacterial metabolic capabilities that are the result of a species’ unique genetic makeup. MacConkey’s (MAC), Blood agar (BAP), and Mannitol Salt (MSA) are three examples of these specialized types of media.

Selective and Differential Media

If a bacterial growth medium is selective, that means that it grows only certain types of microbes while inhibiting the growth of other types of microbes. A differential growth medium exhibits a color change when certain microbes are growing on it, providing additional information about their identity.

Mannitol Salt Is Selective

Mannitol Salt is a selective medium because it has a very high concentration of NaCl (7.5%). Most bacteria cannot survive in this highly saline, hypertonic environment. But the genus Staphylococcus is well adapted to saline environments and grows well in this media.

Mannitol Salt Is Differential

This growing medium is also differential because it contains a dye that identifies types of Staphylococcus that produce an organic acids from mannitol fermentation (eating mannitol, a type of alcohol).

The bacterial waste products generated, organic acid metabolites, change the pH indicator in MSA from red to yellow. Pathogenic staph, such as Staphylococcus aureus, are mannitol fermenters, and when growing on Mannitol Salt Agar, their wastes turn the MSA a bright yellow color.

In contrast, nonpathogenic Staph such as Staphylococcus epidermidis (aka Staph epi), the normal flora that grows on human skin, does not ferment mannitol. When Staph epi grows on Mannitol Salt, the naturally orange-pink color of the agar doesn’t change, since S. epidermidis doesn’t eat mannitol or produce the resulting organic acid wastes.