Introduction
The selective and differential medium known as mannitol salt agar (MSA) is mostly used to isolate and identify Staphylococcus species, notably Staphylococcus aureus. Chapman created it in 1945. The great salt concentration kills the majority of germs except staphylococci, and mannitol fermentation makes it possible to tell the difference.
Table of Contents
Principle of mannitol salt agar
A medium that is both selective and differential separates Staphylococcus species. A high salt content (7.5% NaCl) inhibits non-halotolerant organisms. Phenol red identifies the fermentation of mannitol, changing color to yellow in acid-producing colonies such as S. aureus.
Preparation of mannitol salt agar
- In 1 litre of distilled water, suspend around 111gm of MSA powder. To completely dissolve, heat to a boil.
- Sterilize in an autoclave at 121°C for 15 minutes.
- Cool to roughly 45-50°C and pour into sterile Petri-dishes. Streaking the organism on the surface using an aseptic method.
- Keep the temperature between 35 and 37 degrees Celsius during the 24 to 48-hour incubation period.
- Observe the changes in colony morphology and media color.
Shelf life: Dehydrated medium (in powder form): If kept properly in a cool, dry area, it may last up to three years.
When kept in sealed conditions (such as plastic sleeves or airtight containers) at 2–8°C to avoid dehydration, ready-made plates last four weeks.
Storage Requirements: Dehydrated powder: Keep in a dry, cool environment (15–25°C) with a tight seal to prevent moisture absorption.
Composition of mannitol salt agar
| Ingredients | Amount | Function |
| Mannitol | 10 gm | Fermentable carbohydrates |
| Beef meat extract | 1 gm | Provides vitamins, minerals, and nitrogenous compounds for growth |
| Sodium chloride | 75 gm | making the medium selective |
| Peptone | 10 gm | Supplies nitrogen |
| Phenol red | 0.025 gm | pH indicator |
| Agar | 15 gm | Solidifying agent |
Examples of microorganisms
Staphylococcus aureus, Staphylococcus epidermidis, and Micrococcus luteus
Colony appearance mannitol salt agar
| Bacterium | Appearance | Mannitol fermentation |
| Staphylococcus aureus | Yellow colonies with yellow surrounding zones | Positive |
| Staphylococcus epidermidis | White colonies with pink or red medium | Negative |
| Micrococcus luteus | Yellow pigmented colonies with pink or red medium | Negative |
Uses of (MSA)
- From clinical, food, and environmental specimens, Staphylococcus aureus was isolated and identified.
- Distinguishing between mannitol fermenters and non-fermenters in salt-tolerant species.
- Keeping an eye on pollution in food production, Pharmaceuticals and hospitals.
Precautions
- When working with (MSA), make sure that the high salt concentration (7.5%) is adequately dissolved to preserve its selectivity against non-halophilic bacteria, because the phenol red indicator is sensitive to pH changes and essential for detecting mannitol fermentation, the medium’s pH should be closely monitored.
- Use freshly created media since old media may loose efficacy.
- Inoculate and incubate under ideal conditions (35–37°C for 24–48 hours), avoiding over incubation, which may impair interpretation.
- Always practice proper aseptic procedures and include control strains to confirm the medium’s selectivity and differential function.
Limitations
- Only appropriate for Gram-positive cocci, notably Staphylococcus; excessive salt concentration inhibits Gram-negative and some Gram-positive bacteria.
- Not all Staphylococci ferment mannitol; hence, false negatives are possible with pathogenic strains that do not.
- Some bacteria may tolerate salt: rare species that tolerate salt can develop and people can misidentify them.
Assalamoalykum, Hi
Thank you for notes, sources and reference books. It’s very informative and hoping for more info.