Mannitol salt agar (MSA) is selective because it contains 7.5% sodium chloride, a concentration high enough to inhibit the growth of most bacteria. Only salt-tolerant organisms, primarily Staphylococci, can survive in this hypertonic environment. That single ingredient is what transforms a standard growth medium into one that filters out the vast majority of bacterial species.
How 7.5% Salt Creates a Bacterial Filter
Most bacteria thrive in environments with salt concentrations well below 7.5%. When they encounter a medium this salty, water is pulled out of their cells through osmosis. The cells shrink, their internal machinery stalls, and they die before they can form visible colonies. This process, called plasmolysis, is lethal to most gram-positive and gram-negative organisms. In lab demonstrations, when Escherichia coli is streaked onto MSA alongside Staphylococci, the E. coli simply fails to grow.
Staphylococci survive because they have built-in defenses against osmotic stress. Their cells contain specialized potassium channels that help regulate internal salt and water balance even when the surrounding environment is extremely salty. Some species of Staphylococcus can tolerate salt concentrations as high as 25%, making the 7.5% in MSA a manageable challenge for them. This is why MSA reliably selects for Staphylococci while shutting out most other bacteria you’d encounter in a clinical or environmental sample.
What’s Actually in the Medium
MSA contains six components per liter of water: 75 grams of sodium chloride (the selective agent), 10 grams of peptone and 1 gram of beef extract (nutrients), 10 grams of mannitol (a sugar), 0.025 grams of phenol red (a pH indicator dye), and 15 grams of agar to solidify the plate. The sodium chloride dominates the recipe, making up roughly two-thirds of the dry ingredients by weight. Everything else supports bacterial growth and provides the tools for identifying which species are present.
Selective vs. Differential: Two Jobs at Once
MSA is often called both selective and differential, and those are two distinct functions. The selectivity comes entirely from the salt, which controls which organisms can grow. The differential capability comes from the mannitol and phenol red, which reveal what those surviving organisms are doing metabolically.
When a bacterium ferments mannitol, it produces acid as a byproduct. That acid lowers the pH of the surrounding medium, and phenol red responds by shifting from its default pinkish-red color to yellow. So a yellow halo around a colony means that organism is fermenting mannitol. A colony that grows but leaves the medium pink or red is surviving the salt but not breaking down the sugar.
Reading the Plate: Color Tells the Story
Staphylococcus aureus, the species most clinicians care about, ferments mannitol. On MSA, it produces large colonies surrounded by a bright yellow zone. This color change is one of the quickest visual clues that S. aureus may be present in a sample.
Staphylococcus epidermidis, a common skin bacterium that rarely causes serious infections, grows on MSA but does not ferment mannitol. Its colonies appear with a pink or unchanged medium around them. The contrast between yellow and pink makes it straightforward to distinguish these two species on the same plate.
Staphylococcus saprophyticus, which is associated with urinary tract infections, can sometimes produce a small yellow halo, though only about 10% of strains ferment mannitol. This variability means that a yellow zone is a strong indicator for S. aureus but not an absolute guarantee, and further confirmatory testing is standard practice.
Why This Matters in Clinical Labs
MSA has been used as a selective medium for isolating pathogenic Staphylococci since 1945. It remains a workhorse in microbiology labs because it’s simple, inexpensive, and effective across a range of specimen types. It’s used to screen samples from skin, nasal swabs, respiratory secretions, water, and milk. It has also proven particularly useful for isolating S. aureus from sputum samples in patients with cystic fibrosis, where the bacterial landscape is complex and overgrowth from other species can obscure results on standard media.
One of its more important modern applications involves screening for MRSA (methicillin-resistant Staphylococcus aureus). Labs can add an antibiotic to standard MSA to create a modified version that selects for both salt tolerance and antibiotic resistance simultaneously. If colonies grow on this modified plate and turn the medium yellow, that’s a strong signal for drug-resistant S. aureus. This approach has been used in national and international surveillance studies tracking MRSA prevalence.
Organisms That Break the Rules
While MSA is designed to select for Staphylococci, it isn’t a perfect barrier. A few other salt-tolerant organisms can occasionally grow on it. Micrococcus species, which are closely related to Staphylococci and commonly found on skin, can survive the 7.5% salt concentration and form colonies. Enterococcus species are another group with notable salt tolerance that may appear on MSA plates. These organisms won’t necessarily produce the same color reactions, which helps experienced lab workers identify them as something other than the target species, but their presence is a reminder that growth on MSA alone isn’t enough to confirm a Staphylococcus identification. Colony morphology, color changes, and follow-up tests like the coagulase test are all part of accurate interpretation.