How to Perform a Mannitol Salt Agar (MSA) Test: Selective and Differential Properties

Mannitol Salt Agar (MSA) is a selective and differential culture medium used to isolate and presumptively identify staphylococci, particularly Staphylococcus aureus, from mixed microbial populations. The medium selects for halotolerant organisms through its high sodium chloride concentration (7.5%) and differentiates based on mannitol fermentation, which is detected by a pH indicator color change from red to yellow. This method is most useful in food microbiology, environmental monitoring, and educational laboratory settings where distinguishing S. aureus from other staphylococci and gram-positive cocci is required.

At a Glance

Scientific Principle

Mannitol Salt Agar operates on two fundamental microbiological principles: selective inhibition and differential detection. The high salt concentration (7.5% NaCl) creates a hyperosmotic environment that inhibits the growth of most gram-negative bacteria and many gram-positive organisms, while allowing halotolerant and halophilic staphylococci to thrive [1]. This selective pressure is critical because clinical and food samples typically contain diverse microbial communities where staphylococci may be present in low numbers relative to other bacteria.

The differential component relies on mannitol fermentation. Mannitol is a sugar alcohol that can be catabolized by certain bacteria through the phosphotransferase system and subsequent glycolysis. When mannitol is fermented, acidic end products (primarily lactic acid) are produced, lowering the pH of the medium. The pH indicator phenol red, which is red at neutral pH (approximately 7.4), turns yellow when the pH drops below approximately 6.8. This color change is visible both in the colony and the surrounding agar, creating a characteristic yellow halo around mannitol-fermenting colonies.

Staphylococcus aureus is the most clinically significant mannitol-fermenting staphylococcus, though other species such as S. epidermidis (non-fermenter) and S. saprophyticus (variable) may also grow. The medium does not differentiate between coagulase-positive and coagulase-negative staphylococci; mannitol fermentation alone is presumptive evidence for S. aureus but requires confirmatory testing (e.g., coagulase test, catalase test, or molecular methods) for definitive identification [4].

Materials and Instrumentation

Essential Materials

• Mannitol Salt Agar plates (commercially prepared or dehydrated medium)
• Sterile inoculating loops (10 µL calibrated loops recommended for quantitative work)
• Bunsen burner or microincinerator for loop sterilization
• Incubator set to 35–37°C
• Sterile saline or phosphate-buffered saline (0.85% NaCl) for sample dilution
• Sterile pipettes and tips (for liquid samples)
• Vortex mixer for homogenizing samples
• Colony counter (optional, for quantitative analysis)
• pH meter or pH strips (for quality control of prepared medium)
• Refrigerator (2–8°C) for plate storage

Instrumentation Considerations

The choice of incubator temperature and atmosphere depends on the target organism. For S. aureus, 35–37°C under aerobic conditions is standard. However, some staphylococci grow optimally at 30°C, and certain food safety protocols may specify 35°C for 24–48 hours. If anaerobic incubation is required (e.g., for S. saccharolyticus), an anaerobic jar or chamber with gas-generating sachets is necessary, though this is uncommon in routine MSA testing.

For quantitative analysis (e.g., food samples), a stomacher or blender for sample homogenization and a spiral plater or serial dilution equipment may be needed. The choice of plating method (spread plate vs. pour plate) affects colony morphology and enumeration accuracy. Spread plating on MSA is preferred because it allows better visualization of the color change around individual colonies.

Medium Preparation

If preparing MSA from dehydrated powder, follow the manufacturer's instructions precisely. Typical preparation involves suspending 111 g of dehydrated MSA powder in 1 L of distilled water, heating to boiling with agitation, and autoclaving at 121°C for 15 minutes. After cooling to 45–50°C, pour approximately 20 mL into sterile Petri dishes. The final pH should be 7.4 ± 0.2 at 25°C. Prepared plates can be stored at 2–8°C for up to 2 weeks in sealed plastic bags to prevent desiccation.

Controls

Proper controls are essential for validating MSA performance and interpreting results. Include the following controls with each batch of tests:

Positive Control

• Staphylococcus aureus (e.g., ATCC 25923): Expected result—yellow colonies with yellow zone (mannitol fermentation), good growth (salt tolerance).

Negative Control (Growth)

• Staphylococcus epidermidis (e.g., ATCC 12228): Expected result—red/pink colonies with red/pink zone (no mannitol fermentation), good growth (salt tolerance).

Negative Control (No Growth)

• Escherichia coli (e.g., ATCC 25922): Expected result—no growth or very weak growth (inhibited by 7.5% NaCl).

Sterility Control

• Uninoculated MSA plate incubated alongside test plates: Expected result—no growth, medium remains red.

Environmental Control

• Expose an open MSA plate to the laboratory air for 15–30 minutes during sample processing: Expected result—few or no colonies (demonstrates aseptic technique).

Control organisms should be fresh (less than 24–48 hours old) and grown on non-selective media (e.g., Tryptic Soy Agar) before inoculation onto MSA. Document control results in the laboratory notebook, including lot numbers of media and control strains.

Conceptual Workflow

Step 1: Sample Preparation

For solid samples (e.g., food, environmental swabs), aseptically weigh 10 g or 10 mL into 90 mL of sterile diluent (0.85% saline or Butterfield's phosphate buffer). Homogenize using a stomacher for 1–2 minutes or vortex for 30 seconds. For liquid samples, direct plating or serial dilution may be appropriate. Prepare serial ten-fold dilutions in sterile saline to achieve countable plates (25–250 colonies per plate).

Step 2: Inoculation

Using a sterile loop, streak the sample onto the MSA plate using the quadrant streak method for isolation. For quantitative analysis, spread 0.1 mL of appropriate dilutions onto the surface of MSA plates using a sterile spreader. Allow the inoculum to absorb into the agar for 5–10 minutes before incubation.

Step 3: Incubation

Invert plates and incubate aerobically at 35–37°C for 24–48 hours. Examine plates after 24 hours; if no growth is observed, re-incubate for an additional 24 hours. Some staphylococci, particularly stressed cells from food samples, may require extended incubation.

Step 4: Observation and Recording

After incubation, examine plates for:

• Growth presence/absence: Record whether colonies are present.
• Colony morphology: Note size, color, texture, and hemolysis (if blood agar is also used).
• Color change: Record colony and agar color (yellow = mannitol fermentation; red/pink = no fermentation).
• Quantitative count: Count colonies on plates with 25–250 CFU and calculate CFU/g or CFU/mL.

Step 5: Presumptive Identification

Colonies that are yellow on MSA are presumptive S. aureus (mannitol fermenters). However, confirmatory tests are required for definitive identification, as other staphylococci (e.g., S. saprophyticus, S. xylosus) may also ferment mannitol. Common confirmatory tests include:

• Catalase test: Positive (bubbles with 3% H₂O₂)
• Coagulase test: Positive for S. aureus (tube coagulase or slide coagulase)
• Gram stain: Gram-positive cocci in clusters
• DNase test: Positive for S. aureus
• MALDI-TOF MS: Rapid species-level identification [4]

Quality Checks

Medium Performance Testing

Before using a new lot of MSA, perform quality control testing with the control organisms listed above. Document the following:

• Lot number and expiration date of dehydrated medium or prepared plates
• Date of preparation (if prepared in-house)
• pH of prepared medium (should be 7.4 ± 0.2)
• Sterility check results
• Control organism growth and reaction results

Incubation Monitoring

Record incubator temperature daily using a calibrated thermometer. The acceptable range is 35–37°C. If using a CO₂ incubator, verify CO₂ concentration (5% is standard for some protocols, though aerobic incubation is typical for MSA).

Sample Integrity

Ensure samples are collected aseptically and processed within 2 hours of collection (or held at 2–8°C for no more than 24 hours). Document sample source, collection date/time, and processing date/time.

Reagent Expiration

Check expiration dates of all reagents, including saline, hydrogen peroxide (for catalase test), and coagulase plasma. Expired reagents may give false-negative results.

Result Interpretation

Positive Result (Mannitol Fermentation)

Appearance: Yellow colonies (typically 1–3 mm in diameter) surrounded by a yellow zone in the agar. The yellow color indicates acid production from mannitol fermentation.

Presumptive identification: Staphylococcus aureus (most common), but also possibly S. saprophyticus, S. xylosus, S. cohnii, or S. sciuri. Confirm with coagulase test or other methods.

Quantitative interpretation: Count yellow colonies and calculate CFU/g or CFU/mL. For food samples, regulatory limits vary by jurisdiction (e.g., <100 CFU/g for ready-to-eat foods in some standards).

Negative Result (No Mannitol Fermentation)

Appearance: Red or pink colonies (typically 1–2 mm in diameter) surrounded by a red/pink zone. The medium remains red, indicating no acid production.

Presumptive identification: Coagulase-negative staphylococci (e.g., S. epidermidis, S. haemolyticus, S. hominis). These are generally considered non-pathogenic in healthy individuals but may be opportunistic pathogens.

No Growth

Appearance: No colonies visible on the plate.

Interpretation: Either the sample contained no halotolerant organisms, or the organisms present were inhibited by the high salt concentration. This does not rule out the presence of non-halotolerant pathogens (e.g., E. coli, Salmonella).

Mixed Results

Some plates may show both yellow and red colonies, indicating a mixed population of mannitol-fermenting and non-fermenting staphylococci. In such cases, enumerate each colony type separately and report as "presumptive S. aureus" and "coagulase-negative staphylococci."

Edge Cases

• Weak yellow color: May indicate weak mannitol fermentation or delayed reaction. Re-incubate for an additional 24 hours.
• Yellow colonies with red zone: May indicate mannitol fermentation only in the colony, not diffusing into the agar. This is less common but can occur with certain strains.
• Pinpoint colonies: May indicate stressed or slow-growing organisms. Extended incubation may be needed.
• Spreading colonies: May indicate Proteus species (though usually inhibited by salt) or Bacillus species. Gram stain to confirm.

Troubleshooting

Limitations

Mannitol Salt Agar has several important limitations that users must understand:

1. Not definitive for S. aureus: Mannitol fermentation is presumptive only. Other staphylococci (e.g., S. saprophyticus, S. xylosus) and some micrococci can also ferment mannitol. Confirmatory testing (coagulase, DNase, or molecular methods) is required for definitive identification [4].

2. False negatives possible: Some strains of S. aureus may be weak or delayed mannitol fermenters, particularly if stressed (e.g., from food samples). Extended incubation (48 hours) may be necessary.

3. False positives possible: Non-staphylococcal halotolerant organisms (e.g., Bacillus species, some yeasts) may grow on MSA and produce yellow colonies. Gram stain and catalase test help differentiate.

4. Not suitable for all staphylococci: Some staphylococci (e.g., S. aureus subsp. anaerobius) require anaerobic conditions and may not grow under standard aerobic incubation.

5. Quantitative limitations: The high salt concentration may inhibit some viable but non-culturable (VBNC) cells, leading to underestimation of bacterial load.

6. Matrix interference: Food samples with high fat or protein content may interfere with colony visualization or cause false color changes.

7. No differentiation of enterotoxin production: MSA does not indicate whether S. aureus isolates produce enterotoxins. Additional testing (e.g., ELISA, PCR) is required for enterotoxin detection [4].

8. Not for clinical diagnosis: This protocol is intended for educational and food microbiology applications. Clinical specimens require different media and protocols under appropriate biosafety levels.

Documentation

Maintain a laboratory notebook or electronic record with the following information for each MSA test:

Pre-Analytical

• Sample identifier and source
• Collection date and time
• Sample condition (e.g., temperature, appearance)
• Dilution scheme used
• Medium lot number and expiration date
• Date of medium preparation (if in-house)

Analytical

• Incubation start and end times
• Incubator temperature (recorded at start and end)
• Control organism results
• Any deviations from standard protocol

Post-Analytical

• Colony counts (for each dilution)
• Colony morphology description
• Color change observations
• Presumptive identification
• Confirmatory test results (if performed)
• Final interpretation
• Technician signature and date

Quality Control Records

• Monthly quality control testing results
• Incubator temperature logs
• Medium performance records
• Corrective actions taken (if any)

Biosafety Considerations

Mannitol Salt Agar testing with non-pathogenic control strains (e.g., S. aureus ATCC 25923, S. epidermidis ATCC 12228) is considered BSL-1 work and can be performed in standard teaching laboratories [6]. However, when processing unknown samples (e.g., food, environmental swabs), treat all cultures as potentially hazardous.

Standard Precautions

• Perform all work in a biosafety cabinet (BSC) if processing samples with unknown pathogen content.
• Wear appropriate personal protective equipment (PPE): lab coat, gloves, and safety glasses.
• Decontaminate work surfaces before and after use with 10% bleach or 70% ethanol.
• Autoclave all waste (plates, loops, pipette tips) before disposal.
• Wash hands thoroughly after handling cultures.

Specific Considerations

• S. aureus is a BSL-2 organism when handling clinical isolates or known toxin-producing strains. For teaching labs using ATCC strains, BSL-1 is acceptable.
• Do not use MSA for clinical specimens unless appropriate BSL-2 facilities and training are available.
• Avoid aerosol generation during loop sterilization and plate opening.
• If using a Bunsen burner, ensure it is placed away from flammable materials and never left unattended.

Emergency Procedures

• Spill: Cover with absorbent material, apply 10% bleach for 30 minutes, then clean with fresh bleach solution.
• Exposure: If culture contacts skin or mucous membranes, wash immediately with soap and water for 15 minutes. Report to supervisor and seek medical attention if needed.
• Needlestick: Follow institutional sharps injury protocol immediately.

Frequently Asked Questions

1. Why is 7.5% NaCl used in MSA instead of a lower concentration?

The 7.5% NaCl concentration is specifically chosen to inhibit most gram-negative bacteria and many gram-positive organisms while allowing staphylococci to grow. Staphylococci are naturally halotolerant due to their ability to accumulate compatible solutes (e.g., proline, glycine betaine) to counteract osmotic stress. Lower concentrations (e.g., 5%) would allow more background organisms to grow, reducing selectivity. Higher concentrations (e.g., 10%) might inhibit some staphylococci, particularly stressed cells from food samples.

2. Can MSA be used for quantitative enumeration of S. aureus in food?

Yes, MSA is commonly used for quantitative enumeration of presumptive S. aureus in food samples, following standard methods such as those from the FDA Bacteriological Analytical Manual (BAM) or ISO 6888. However, results are presumptive and require confirmation. The high salt concentration may inhibit some injured or stressed cells, so pre-enrichment in non-selective broth (e.g., Buffered Peptone Water) may improve recovery [1]. For accurate enumeration, use the spread plate method with appropriate dilutions and count only yellow colonies after 24–48 hours.

3. Why do some S. aureus colonies appear pink or red on MSA?

This can occur for several reasons: (1) The strain may be a weak or delayed mannitol fermenter, requiring extended incubation (48 hours) to show a yellow color. (2) The organism may be stressed (e.g., from frozen or heat-treated samples) and unable to ferment mannitol efficiently. (3) The medium pH may be too high (above 7.6), requiring more acid production to trigger the color change. (4) Some S. aureus variants (e.g., small-colony variants) may have altered metabolism. If pink colonies are suspected to be S. aureus, perform a coagulase test or subculture to a fresh MSA plate.

4. How does MSA compare to other selective media for staphylococci?

MSA is the most commonly used selective and differential medium for staphylococci. Alternatives include:

• Baird-Parker Agar: More selective for S. aureus (uses tellurite and egg yolk), but more complex to prepare and interpret.
• Staphylococcus Medium No. 110: Similar to MSA but uses different carbohydrate sources.
• CHROMagar Staph aureus: Chromogenic medium that gives specific colors for S. aureus (mauve) and other staphylococci (blue), allowing direct identification without confirmatory testing.
• Mannitol Salt Agar with egg yolk: Modified MSA that adds egg yolk for lipase detection (clear zone around colonies).

MSA is preferred for teaching labs and routine food testing due to its simplicity, low cost, and well-established performance characteristics.

References and Further Reading

1. Villamizar-Rodríguez G, Fernández J, Marín L, Muñiz J, González I, Lombó F. Multiplex detection of nine food-borne pathogens by mPCR and capillary electrophoresis after using a universal pre-enrichment medium. 2015. https://pubmed.ncbi.nlm.nih.gov/26579100/

2. Briaud P, Camus L, Bastien S, Doléans-Jordheim A, Vandenesch F, Moreau K. Coexistence with Pseudomonas aeruginosa alters Staphylococcus aureus transcriptome, antibiotic resistance and internalization into epithelial cells. 2019. https://pubmed.ncbi.nlm.nih.gov/31719577/

3. Wang Y, Zhao L, Li Z, Xi Y, Pan Y, Zhao G, Zhang L. A generative artificial intelligence approach for the discovery of antimicrobial peptides against multidrug-resistant bacteria. 2025. https://pubmed.ncbi.nlm.nih.gov/41044364/

4. Mairi A, Ibrahim NA, Idres T, Touati A. A Comprehensive Review of Detection Methods for Staphylococcus aureus and Its Enterotoxins in Food: From Traditional to Emerging Technologies. 2025. https://pubmed.ncbi.nlm.nih.gov/40711131/

5. Magalhães CRP, de Aquino NSM, Vieira JM, Gonçalves CTH, Tondo EC. Assessing the behavior of food handlers wearing face masks and the passage of bacteria through disposable masks. 2025. https://pubmed.ncbi.nlm.nih.gov/39621293/

6. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. 2020. https://www.cdc.gov/labs/bmbl/index.html

7. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules/

8. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. https://www.ncbi.nlm.nih.gov/books/

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