Mannitol Salt Agar (MSA)
Bacterial Growth Media
Differential + Selective Medium to Grow & ID Staphylococcus Bacteria
LAB NOTES from Science Prof Online
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.
What is Media?
A growth medium (plural: media) is a mixture of nutrients, moisture and other
Article Summary: Mannitol Salt Agar is a special bacterial growth medium that is selective for halophiles and can differentiate pathogenic and nonpathogenic Staphylococcus.
Mannitol Salts Agar (MSA)
Page last updated: 4/2013
Portions of this article originally appeared on Suite101 online magazine.
Two plates of Mannitol Salt agar. On left, plate is growtin Staphylococcus aureus, a pathogenic mannitol fermenter. On right, plate is growting non-mannitol fermenter S. epidermidis. Click here for more images of Mannitol Salt Agar.
You have free access to a large collection of materials used in a college-level introductory microbiology course. The Virtual Microbiology Classroom provides a wide range of free educational resources including PowerPoint Lectures, Study Guides, Review Questions and Practice Test Questions.
SPO is a FREE science education website. Donations are key in helping us provide this resource with fewer ads.
(This donation link uses PayPal on a secure connection.)
SPO VIDEO: How to Interpret Mannitol Salt Agar (MSA) Specialized Bacterial Growth Medium
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. There are also liquid media broth that allows 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.
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 growth medium is considered differential if, when specific microbes are present, the medium, or bacterial colonies themselves, exhibit a color change that provides information about their identity.
Mannitol Salt Is Selective
Mannitol Salt is a selective bacterial growth 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 bright 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 (a.k.a. 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.