Mannitol Salt Agar its principle and Uses

Mannitol Salt Agar is a selective and differential media that are commonly used in microbiology to isolate and identify certain types of bacteria.

It contains a high concentration of salt (7.5%), which inhibits the growth of many other types of bacteria, and mannitol, a sugar that is fermented by some bacteria, which produces acid as an end product that can be detected by a pH indicator.

The purpose of Mannitol Salt Agar in microbiology is to isolate and identify certain types of bacteria that are of clinical or environmental importance. It is often used to isolate and identify S. aureus, a gram-positive bacterium that is a common cause of infections, such as skin infections and respiratory tract infections.

Principle of the Mannitol Salt Agar

Mannitol salt agar contains peptones and beef extract, which provide nitrogen, vitamins, minerals, and amino acids that are essential for the growth of bacteria. The media also contains a high concentration of sodium chloride (7.5%), which helps to inhibit the growth of many other types of bacteria and provides essential electrolytes for transport and osmotic balance.

Mannitol is a sugar that is fermented by some bacteria, such as S. aureus, which produces an acid end product that can be detected by a pH indicator.

The pH indicator in the media, such as phenol red, changes color depending on the acidity of the media, allowing for the visual detection of the acid end product produced by the fermentation of mannitol.

The addition of 5% v/v Egg Yolk Emulsion to Mannitol Salt Agar enables the detection of lipase activity of staphylococci in addition to mannitol fermentation. Lipase is an enzyme produced by some bacteria that breaks down fats.

The salt in the media clears the egg yolk emulsion, and lipase production is detected as a yellow, opaque zone around the colonies. This helps to further differentiate between different types of staphylococcal species.

Coagulase-positive staphylococci, such as S. aureus, produce yellow colonies and a surrounding yellow medium, while coagulase-negative staphylococci produce red colonies and no color change of the pH indicator.

Composition of Mannitol Salt Agar

IngredientsGms / Litre
Pancreatic Digest of Casein5.0 gm
Peptic Digest of Animal Tissue5.0 gm
Beef Extract1.0 gm
Sodium Chloride75.0 gm
D-Mannitol10.0 gm
Phenol Red0.025 gm
Agar15.0 gm
Total111. 025 gm
  1. Pancreatic Digest of Casein. Pancreatic Digest of Casein is a partially hydrolyzed protein that is derived from the digestion of casein, a protein found in milk, with enzymes from the pancreas. It is used as a source of nitrogen and carbon for the growth of bacteria.
  2. Peptic Digest of Animal Tissue. Peptic Digest of Animal Tissue is a partially hydrolyzed protein that is derived from the digestion of animal tissue with enzymes from the stomach. It is used as a source of nitrogen and carbon for the growth of bacteria.
  3. Beef Extract. Beef Extract is a concentrated extract of beef that is used as a source of nitrogen, vitamins, minerals, and amino acids for the growth of bacteria.
  4. Sodium Chloride. Sodium Chloride, also known as table salt, is used in Mannitol Salt Agar at a concentration of 7.5%. It serves as an osmotic agent that helps to inhibit the growth of many other types of bacteria and provides essential electrolytes for transport and osmotic balance.
  5. D-Mannitol. D-Mannitol is a sugar that is fermented by some bacteria, such as S. aureus which produces an acid end product that can be detected by a pH indicator.
  6. Phenol Red. Phenol Red is a pH indicator that changes color depending on the acidity of the media. It is used in Mannitol Salt Agar to detect the acid end product produced by the fermentation of mannitol by certain bacteria.
  7. Agar. Agar is a gelatinous substance derived from seaweed that is used as a solidifying agent in media. It provides a solid surface for the growth of bacteria.

Uses of Mannitol Salt Agar

Mannitol Salt Agar is a widely used media in microbiology for the selective isolation and differentiation of S. aureus from clinical samples, such as wound swabs or throat swabs.

It is also used for the enumeration of staphylococci in food and dairy products, as well as in the bacteriological examination of cosmetics, food samples, and environmental samples (such as water or soil).

In addition, it is often used in the bacteriological examination of swimming pool water, spas, and drinking water using membrane filtration. Its selective and differential properties make it useful for isolating and identifying certain types of bacteria in a variety of settings.

Procedure for using Mannitol Salt Agar

Step-by-step instructions for preparing and using Mannitol Salt Agar

Here are some general steps for using Mannitol Salt Agar:

  1. Sterilize the materials and equipment that will be used. This is important to prevent contamination of the media and the sample.
  2. Prepare the Mannitol Salt Agar according to the manufacturer’s instructions or a recipe. It is important to follow the correct proportions of ingredients and to sterilize the media before use.
  3. Inoculate the Mannitol Salt Agar with the sample that is being tested. This can be done by spreading a small amount of the sample over the surface of the media using a sterile loop or a swab.
  4. Incubate it at a suitable temperature (such as 35-37°C) for 18-24 hours. This allows for the growth of bacteria on the media.
  5. Examine for the presence of bacterial colonies. Look for the characteristic appearance of the bacteria that are being isolated, such as the size, shape, and color of the colonies.

Tips for obtaining accurate results when using Mannitol Salt Agar

Here are some tips for obtaining accurate results when using Mannitol Salt Agar:

  1. Use an aseptic technique to prevent contamination of the media and the sample.
  2. Check the expiration date of the media and follow the manufacturer’s instructions for storage and use.
  3. Follow the recommended incubation time and temperature to allow for proper growth of the bacteria.
  4. Confirm the identity of the bacterial colonies using a suitable biochemical test, such as coagulase testing or catalase testing.

Interpretation of results

The appearance of bacterial colonies on Mannitol Salt Agar depends on the type of bacteria being isolated. For example, S. aureus colonies typically appear as small, yellow or golden, circular, and smooth, while P. mirabilis colonies typically appear as large, pink or red, circular, and raised.

Tips for correctly identifying bacteria on Mannitol Salt Agar

Here are some tips for correctly identifying bacteria on Mannitol Salt Agar:

  1. Confirm the presence of bacterial colonies by using a suitable biochemical test, such as coagulase testing or catalase testing.
  2. Use an aseptic technique to prevent contamination of the media and the sample.
  3. Check the expiration date of the media and follow the manufacturer’s instructions for storage and use.
  4. Follow the recommended incubation time and temperature to allow for proper growth of the bacteria.
  5. Compare the appearance of the colonies on the media to descriptions and pictures of the expected bacterial colonies to help with identification.

Advantages and disadvantages of using Mannitol Salt Agar

Advantages:

  1. Selective: Mannitol Salt Agar is a selective media that is designed to favor the growth of certain types of bacteria, such as S. aureus and S. epidermidis, while inhibiting the growth of many other types of bacteria. This makes it useful for isolating and identifying specific types of bacteria.
  2. Differential: Mannitol Salt Agar is a differential media that is able to differentiate between different types of bacteria based on their ability to ferment mannitol, which produces an acid end product that can be detected by a pH indicator. This allows for the identification of specific types of bacteria, such as S. aureus.
  3. Ease of use: Mannitol Salt Agar is relatively easy to prepare and use, making it a convenient choice for many microbiological procedures.

Disadvantages:

  1. Limited sensitivity: Mannitol Salt Agar is not as sensitive as some other types of media, which means that it may not be able to detect all types of bacteria present in a sample.
  2. Limited specificity: Mannitol Salt Agar is not able to differentiate between all types of bacteria, and it may not be able to distinguish between closely related species.
  3. Inhibition of growth: The high concentration of sodium chloride in Mannitol Salt Agar may inhibit the growth of certain types of bacteria, which may affect the accuracy of the results.

Conclusion

Mannitol Salt Agar is a selective and differential media that are commonly used in microbiology to isolate and identify certain types of bacteria. It contains a high concentration of salt and mannitol, which favor the growth of certain bacteria and allow for the detection of the acid end product produced by the fermentation of mannitol.

It is important to use appropriate media and techniques in microbiology because this allows for the accurate identification and characterization of bacteria, which is essential for the proper diagnosis and treatment of infections.

Using the wrong media or techniques may result in the inaccurate identification or misidentification of bacteria, which can have serious consequences for patient care. It is also important to follow good laboratory practices, such as using an aseptic technique and following the manufacturer’s instructions for media preparation and use, to ensure the accuracy and reliability of the results.

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