Can Gram-Negative Grow on MSA? What to Expect and Why

Most gram-negative bacteria will not grow on mannitol salt agar (MSA) under standard lab conditions. The 7.5% NaCl in the medium kills or inhibits the vast majority of them before they can form colonies. That said, "gram-negative" is not an automatic disqualifier. A small group of naturally salt-tolerant gram-negative organisms, including some halophiles like Halomonas and occasionally strains of Aeromonas, can survive and even form colonies on MSA. So if you are seeing unexpected growth on an MSA plate, the gram-stain alone does not tell you whether the result is real or a contamination problem. The salt tolerance of the specific organism is what actually matters.

What MSA actually is and why it works the way it does

Mannitol salt agar is both a selective and differential medium. The FDA BAM formula (M97) specifies 75 g/L NaCl, 10 g/L mannitol, 0.025 g phenol red, and a final pH of 7.4 ± 0.2, autoclaved at 121°C for 15 minutes. That 75 g/L NaCl works out to roughly 7.5%, which creates a high-osmotic-stress environment that wipes out most non-halotolerant bacteria regardless of their gram stain.

The differential part comes from the mannitol and phenol red combination. Phenol red has a pK of around 7.8, so it sits in the red-to-yellow transition zone at physiological pH. When an organism ferments mannitol, it produces acidic end-products that drop the pH and shift the indicator from red to yellow, either in the colonies themselves or as a halo in the surrounding agar. Organisms that grow but do not ferment mannitol leave the agar red or produce colorless colonies against a red background.

The intended purpose of MSA in food safety and clinical microbiology is isolation and presumptive identification of Staphylococcus aureus. Merck’s technical data sheet describes mannitol salt phenol-red agar as a medium for the blank" rel="noopener noreferrer">isolation and presumptive identification of Staphylococcus aureus in food and related contexts, with yellow colonies indicating mannitol fermentation. It is used in food testing workflows (FDA BAM, Merck, Remel/Thermo) specifically because S. aureus is halotolerant and a mannitol fermenter, giving it a clear signature: growth plus yellow colonies. Related organisms like Staphylococcus epidermidis also grow on MSA but do not ferment mannitol, so they produce colorless or pale colonies on a red background.

Can gram-negative organisms actually grow on MSA?

Under standard incubation at 35 to 37°C for 18 to 24 hours, most gram-negative bacteria are inhibited on MSA. E. coli is the textbook example: ASM's own MSA protocol uses E. coli as a control organism specifically to demonstrate that it should not grow. If you see robust E. coli-like colonies on MSA, that is a red flag for a media or contamination problem.

The exceptions are organisms that have evolved to tolerate high salt concentrations. Halomonas, a gram-negative halophilic genus, can form colonies on high-NaCl solid media and would not be suppressed the way E. coli would. Some Aeromonas strains have also shown growth at NaCl concentrations approaching the range MSA provides, depending on the strain. These are not common food-safety pathogens you would expect to encounter in routine testing, but they are real biological exceptions to the "gram-negatives don't grow on MSA" rule.

The practical takeaway: if you are doing routine food safety or clinical microbiology and you see unexpected growth on MSA, your first assumption should not be that you have found a salt-tolerant gram-negative organism. Your first assumption should be contamination, satellite colonies from a nearby heavy growth area, or a media preparation issue. Genuine halophilic gram-negative growth is the explanation of last resort, not the first.

How the selective agents in MSA affect gram-negative bacteria

The 7.5% NaCl is the main gatekeeper. At that concentration, osmotic pressure exceeds what most gram-negative cell walls and membranes can manage. Gram-negative bacteria have an outer membrane that does provide some protection against certain agents, but it does not confer halotolerance. Salt tolerance is a physiological trait tied to osmotic regulation mechanisms, not gram-stain category.

Because the selectivity is entirely osmotic rather than based on something like bile salts or antibiotics, any organism with sufficient halotolerance, regardless of gram-stain result, can potentially survive. This is why VUMicro and other sources correctly point out that MSA is selective for halotolerant organisms, not exclusively for gram-positives. It happens that most halotolerant human-relevant pathogens are gram-positive staphylococci, which is why the medium is described in shorthand as "selective for gram-positives."

The mannitol and phenol red components have no selective function against gram-negative organisms. They only reveal fermentation behavior in organisms that already survived the salt barrier. So if a salt-tolerant gram-negative organism does grow, the yellow-vs-red color reading still tells you whether it fermented mannitol, which is useful differential information but does not identify it as S. aureus.

Interpreting what you see on the plate: growth, color, and what they mean

The two variables to read are presence/absence of growth and whether the phenol red indicator has shifted. Here is how to think through each combination:

One thing worth stressing: yellowing on MSA tells you that mannitol was fermented, not that the organism is S. does staphylococcus aureus grow on emb agar is a closely related question to ask when you see yellow or robust colonies on MSA yellowing on MSA. aureus. A salt-tolerant gram-negative that ferments mannitol would also produce blank" rel="noopener noreferrer">yellow colonies. This is why confirmatory testing (coagulase, DNase, MALDI-TOF, or similar) is required before calling a yellow MSA colony S. aureus in a food safety context. The color just gets you to the next test, not to a final answer.

Conversely, if an organism fails to grow on MSA, you cannot conclude it is mannitol-negative. As VUMicro correctly notes, growth failure on MSA only means the organism could not tolerate the salt. Its fermentation behavior is simply untestable on this medium.

Troubleshooting when your MSA results do not look right

MSA plates that give you unexpected results, whether that is no growth where you expected some, growth where you expected none, or wrong colony colors, are usually pointing to one of a small number of fixable problems. Work through these in order before re-interpreting the biology.

Check your media preparation first

NaCl concentration is everything on MSA. If your weigh-out or dilution was off, the selective pressure changes dramatically. A batch prepared at 5% NaCl instead of 7.5% will allow gram-negative organisms that would otherwise be inhibited to grow. Always verify the lot and confirm autoclave conditions: 121°C for 15 minutes per FDA BAM M97. Over-autoclaving can degrade phenol red and give you plates that will not shift color correctly even when mannitol is fermented. A Reddit discussion documented exactly this kind of failure, where plates stayed red despite confirmed S. aureus inoculation, pointing to degraded indicator.

Verify your incubation conditions

Standard incubation for MSA is 35 to 37°C for 18 to 24 hours. Himedia's technical document and FDA BAM confirmatory steps both use this window. Incubating at lower temperatures can slow or prevent growth even for halotolerant organisms and delay the phenol red shift. Extended incubation beyond 48 hours can sometimes allow marginally tolerant organisms, including some gram-negatives, to form small colonies as the medium ages, which can create confusion. Read plates at the recommended time.

Confirm your organism's identity and viability

If you are testing whether a specific organism grows on MSA, make sure your stock culture is correctly identified and viable. If you are trying to determine where Staphylococcus aureus grows best, it helps to consider both its preferred growth temperature and the nutrients and salt levels in the environment If you are testing whether a specific organism grows on MSA. Use ATCC-recommended QC strains: S. aureus ATCC and S.

epidermidis ATCC are the standard positive and negative controls listed in Remel's IFU. Running E. coli as a negative control (expected: no growth) is also standard practice per ASM's MSA protocol. If your positive or negative controls are not performing as expected, the problem is the media or the organism, not the biology of gram-stain selectivity.

Look at your streaking technique and inoculum load

Too heavy an inoculum can lead to satellite growth around the main streak, where cells that are not actually tolerating the salt are surviving in nutrient-rich zones created by neighboring lysed or growing cells. This can look like unexpected gram-negative growth. Streak for isolation so that individual colonies are well separated, and use a moderate inoculum rather than a heavy one.

Rule out contamination before assuming unusual biology

Unexpected yellow colonies or unusual growth patterns on MSA should always prompt a contamination check. A mixed culture or an environmental contaminant introduced during plating can produce misleading results. Gram-stain any unexpected colonies directly. If the result is gram-negative, repeat the test from a fresh pure culture before drawing any conclusions about the organism's salt tolerance. As one Reddit discussion noted about erroneous yellowing on MSA, mixed cultures and environmental contamination are the most common explanation for results that do not fit the expected pattern. As another Reddit discussion about erroneous yellowing on MSA points out, mixed cultures and environmental contamination are the most common explanation for results that do not fit the expected pattern.

The bottom line for your MSA work

Gram-negative bacteria as a group do not grow on MSA under standard conditions, but that is because most gram-negatives are not salt-tolerant, not because of anything specific to the gram-negative cell wall. Salt-tolerant gram-negative exceptions do exist in nature. In routine food safety or clinical microbiology, unexpected gram-negative growth on MSA almost always means a media, technique, or contamination problem rather than a genuine halophilic organism.

Micrococcus luteus is sometimes discussed in relation to whether it can grow on mannitol salt agar, so results depend on the strain and its salt and mannitol tolerance. Use your controls consistently, verify your media prep, and always follow up any MSA presumptive result with a confirmatory test before making a final identification call.