Nutrient Agar: composition, preparation and uses

Nutrient Agar

Nutrient agar is used as a general purpose medium for the growth of a wide variety of non-fastidious microorganisms. It consists of peptone, beef extract and agar. This relatively simple formulation provides the nutrients necessary for the replication of a large number of non-fastidious microorganisms.

Nutrient Agar/broth is used for the cultivation and maintenance of non-fastidious organisms as well as enumeration of organisms in water, sewage, dairy products, feces and other materials.

Composition of Nutrient Agar

Nutrient Agar
Nutrient Agar

Beef Extract……………………..…………..3.0 g
Peptone……..…………………….…………5.0 g
Agar………………………………………….15.0 g
Distilled Water………………………………….……1000 ml
Final pH 6.8 +/- 0.2.
Composition of Nutrient Broth: Nutrient broth contains same ingredients except agar.

Characteristics of the components used in nutrient agar/broth

  1. Beef extract is an aqueous extract of lean beef tissues. It contains water-soluble substances of animal tissue, which include carbohydrates, organic nitrogen compounds, water soluble vitamins, and salts.
  2. Peptone is made by digesting proteinaceous materials e.g., meat, casein, gelatin, using acids or enzymes. Peptone is the principal source of organic nitrogen and may contain carbohydrates or vitamins. Depending up on the nature of protein and method of digestion, peptones differ in their constituents, differing in their ability to support the growth of bacteria.
  3. Agar is a complex carbohydrate obtained from certain marine algae. It is used as a solidifying agent for media and does not have any nutritive value. Agar gels when the temperature of media reaches 45°C and melts when the temperature reaches 95 °C.

Preparation of Nutrient Agar

Nutrient agar and broth are available commercially in powdered (free-flowing, homogeneous) form.

  1. Dissolve the dehydrated medium in the appropriate volume of distilled water  i.e., 23 gm dehydrated nutrient agar (see the manufacturer instruction) in 1000 ml distilled water.
  2. Heat with frequent agitation and boil for 1 minute to completely dissolve the powder
  3. Sterilized the medium by autoclaving (121°C for 15 min)
  4. Dispense the medium in to tubes  or plates. Left the agar medium to solidify and store.
  5. Determine the pH of the medium (pH 6.8 +/- 0.2) with a pH meter and adjust if necessary.

Uses of Nutrient Agar/broth

Uninoculated Nutrient Agar
Un-inoculated Nutrient Agar

1. For the enumeration of organisms in water, sewage, dairy products, feces and other materials.
2. For the cultivation and maintenance of non-fastidious species.

Quality Control

  • The color of prepared Nutrient Agar will be light amber, very slightly to slightly opalescent
  • pH of the prepared media should be  6.8 ± 0.2.
  • Media should be checked for their performance using stable, typical control cultures.  When the prepared media are inoculated with Escherichia coli (ATCC 25922), Psuedomonas aeruginosa (ATCC 27853) and incubate at 35 ± 2°C for 18-48 hours, media should show good growth of these organisms.

About tankeshwar 362 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion, I am working as a Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. Hi, How can you prepare nutrient agar from nutrient broth? As per your recipe above, is it simply adding 15g of agar to 13g of nutrient broth in 1l of water? Really appreciate your response.

    • This is not unusually long and depends on ambient temperature. Another problem may be the agar you are using. Agar absorbs water, so unless it is kept very dry, weighing can become inaccurate. Agar also varies from one supplier to another, I have used anything from 1-2% agar.

  2. autoclaving and sterilization is different. Our lecturer instructed us to first sterilize the media by warming it to 121 degrees Celsius for 15 minutes followed by autoclaving. Would you please explain why you said sterilize the medium by autoclaving.


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