Mannitol salt agar (MSA) tests for the presence of staphylococci, specifically distinguishing pathogenic species like Staphylococcus aureus from non-pathogenic ones like Staphylococcus epidermidis. It does this by combining two functions in a single plate: it selectively allows only salt-tolerant bacteria to grow, then uses a color change to reveal which of those bacteria can ferment the sugar mannitol.
How MSA Works as a Selective Medium
The “salt” in mannitol salt agar is the key to its selectivity. MSA contains 7.5% sodium chloride, roughly 15 times the salt concentration found in standard growth media (about 0.5%). Most bacteria cannot survive at that salt level. Staphylococci are naturally salt-tolerant, so they grow just fine on MSA while most other organisms are inhibited. This means that if you streak a mixed sample onto an MSA plate and see colonies after incubation, those colonies are very likely staphylococci or closely related salt-tolerant species like Micrococcus.
How MSA Works as a Differential Medium
Growth alone doesn’t tell you which Staphylococcus species you’re looking at. That’s where the differential component comes in. The medium contains mannitol, a type of sugar alcohol, along with phenol red, a pH indicator dye. Phenol red is red-orange at a neutral pH (around 7). When a bacterium ferments mannitol, it produces acid as a byproduct, dropping the pH of the surrounding agar and turning the phenol red from red-orange to yellow.
So you’re reading two things on an MSA plate: whether something grew (selective result) and whether the agar turned yellow around the colonies (differential result).
Interpreting MSA Results
There are three basic outcomes you can see on an MSA plate:
- Yellow colonies with a yellow zone around them: The organism is salt-tolerant and ferments mannitol. This is the classic result for Staphylococcus aureus, the species most commonly associated with staph infections.
- Small pink or red colonies with no color change: The organism is salt-tolerant but does not ferment mannitol. This is typical of Staphylococcus epidermidis, a common skin bacterium that is generally non-pathogenic.
- No growth at all: The organism is not salt-tolerant and was inhibited by the 7.5% NaCl. This rules out staphylococci.
Why MSA Is Used in the Lab
The primary clinical value of MSA is as a screening tool for S. aureus. Because S. aureus is a major cause of skin infections, wound infections, food poisoning, and more serious conditions, quickly identifying it from a mixed sample matters. MSA gives a presumptive identification: yellow colonies on a high-salt plate strongly suggest S. aureus. In educational microbiology labs, MSA is one of the standard plates students use to practice identifying unknown bacteria from a mixed culture.
It’s also useful for screening specimens that are expected to contain many different types of bacteria, like nasal swabs or skin samples. The high salt concentration clears away most of the background organisms, making the staphylococci much easier to find.
Limitations to Keep in Mind
MSA is not a confirmatory test. One important limitation is that some coagulase-negative staphylococci (species other than S. aureus) can also ferment mannitol and produce yellow colonies that look identical to S. aureus on MSA. A study examining 171 strains across 13 different coagulase-negative species found that all of them grew on MSA and fermented mannitol. This means a yellow result narrows suspicion toward S. aureus but does not confirm it on its own.
For definitive identification, additional testing is needed. The coagulase test (which checks for a clotting enzyme produced by S. aureus) is the standard follow-up. Other methods like mass spectrometry or genetic sequencing provide even more precise identification. MSA gives you a strong starting point, but it’s one step in the identification process rather than the final answer.