Nutrient Agar

Introduction image

Nutrient agar is a standard media that is used for growing and isolating of a broad class of microorganisms. It is the most popular media that are generally available in all laboratories for enumeration of different bacteria and their maintenance.

The enumeration of different bacteria from different samples like soil, water, sewage, food etc. can be carried out by using such media by employing different planting methods. One thing we should keep in mind that it is a supportive media that provides a base for the development of non-fastidious organisms.

Content: Nutrient Agar

  1. Definition
  2. Composition
  3. Principle
  4. Media Preparation
  5. Uses
  6. Quality Control
  7. Conclusion

Definition of Nutrient Agar

Nutrient agar can define as the ordinary basal media that provides a suitable medium for the cultivation of non-fastidious microbes that have no special growth requirements. It is one of the functional types of culture media, which differs from the enriched, selective and differential media.

Nutrient agar can be turned into an enriched medium by the addition of serum and other biological fluids. The formulation of supportive nutrient agar can function as a selective media by the incorporation of some selective agents like antibiotics. Similarly, if the nutrient agar becomes saturated with specific dyes or chemicals, then it will function as a differential media.

Composition of Nutrient Agar

The composition of nutrient agar media includes the following ingredients per 1000 ml of distilled water:

Composition of nutrient agar media

  • Peptone: 5.0 gm
  • Beef extract: 3.0 gm
  • Sodium chloride: 5.0 gm
  • Agar: 15.0 gm
  • pH: 7.4 ± 0.2 at 25°C

The composition of nutrient broth is the same as above, but one should skip the addition of solidifying agar. Nutrient broth can be easily prepared from the readymade media by taking 13 grams of nutrient broth powder per litre of sterile water.


Nutrient agar is used in culturing and isolation of various non-fastidious bacteria by providing the principal sources like peptone and beef extract for their growth.

Peptone provides the principal source of organic nitrogen from the enzymatic digest of proteinaceous materials, e.g., meat, casein, gelatine etc. It supplies nitrogen and comprises of pure amino acids or nitrogenous compounds that can be readily utilized by the bacteria. Different bacteria utilize peptone differently, depending upon the constituents of peptone and method of digestion.

The beef extract is a water-soluble extract of the animal tissue, which provides key elements like vitamins, carbohydrates, nitrogen, and salts necessary to obtain bacterial growth on media. Sodium chloride or common salt maintains the osmotic equilibrium by keeping the osmotic concentration of the media similar to that of bacterial cytoplasm.

Agar is a complex carbohydrate obtained from certain marine algae. It is a chief material used in the culture media as a solidifying agent. It is the component that is not utilized by the bacteria, as it does not have any nutritive value. The gelling and liquefying temperature of agar is 45°C and 95 °C, respectively.

Distilled or sterile water provides is free off impurities like calcium, chlorine, fluorine etc. and provides pure water source essential for the growth of and development of bacteria. It also functions as the medium through which various nutrients translocate into the bacterial cells.

Media Preparation

Add the above ingredients into the conical flask and then add one litre of distilled water. Stir the components well until homogeneity and then autoclave the solution at 15lbs (121 degrees Celsius) for 30minutes. After autoclaving bring the media to the laminar airflow.

Then, one can perform plating methods of their own choice by the inoculation of the bacterial culture. Then one should follow the incubation of the culture plates up to 24-42 hours at an optimum temperature of 35 degrees Celsius. The left-over media can be set aside in the refrigerator.

Uses of Nutrient Agar

Following are the applications of the nutrient agar media:

Uses of Nutrient agar

  1. Nutrient agar is a standard media indispensable for cultivating non-fastidious organisms.
  2. It also requires in the subculturing and maintenance of the pure culture.
  3. Nutrient agar is the standard media which composition can be altered to make it function like enriched, selective or differential media.
  4. It can consider as the prilimnary testing method that checks the presence or absence of any harmful microbes before biochemical or serological testing.
  5. Its usefulness is not limited to the areas mentioned above, but also aids in the preparation of bacterial lawns to perform antibiotic sensitivity test.
  6. It is also applicable for the bacteriological examination from a variety of materials or samples.

Quality Control

To maintain the quality of the nutrient media and to obtain the best results, the pH of the nutrient agar should be 6.8 ± 0.2. The prepared nutrient agar should appear as a clear homogeneous solution, without any lumps and hazziness in it.

The mixtures of nutrient agar should be autoclaved until it obtains slightly opalescent or light amber colour. While performing the plating method, one should always prepare a control or standard plate to compare the growth with the culture plates.

The incubation of the bacteria requires 35 ± 2°C temperature and a time period of 18-48 hours, depending upon the different kinds of bacteria and their growth requirements.


Therefore, we can conclude that the nutrient agar is a basic media for the culturing various non-fastidious bacteria and their isolation based on their culture characteristics like growth pattern, texture, margin and elevation of the bacterial colonies developed.

Leave a Comment

Your email address will not be published. Required fields are marked *