Introduction
Before any biochemical or serological test, nutrient agar is, in fact, a great medium for verifying purity, since it includes nutrients that are appropriate for subculturing a diverse array of microorganisms. Additionally, adding agar to nutrient agar makes it more stable; consequently, it is ideal for growing microbes. Moreover, you may include up to 10% of blood or other biological fluids, which, in turn, are appropriate for your experiment.
Table of Contents
Principle of nutrient agar
A versatile, no selective medium that fosters the development of a large variety of nonfastidious bacteria. It is frequently used for isolating and maintaining pure cultures; consequently, it provides fundamental elements (peptone, beef extract, NaCl) for bacterial development.
How to make nutrient agar?
- In 1 litre of distilled water, mix 28 grams of (NA) powder.
- Thoroughly mix and heat till agar dissolve completely.
- Autoclave for 15 minutes at 121°C to sterilize.
- Wait for the medium to set after pouring the liquid into the petri-dish.
- To prevent any contamination, make sure you prepare the agar in a aseptic environment.
- The agar is ready for use after getting solid.
Composition of nutrient agar
Components | Function |
0.5% peptone | Provide nitrogen |
0.3% yeast extract | Provide nitrogen, salts & vitamins |
1.5% bacteriological Agar | Solidifying agent |
0.5% sodium chloride | Provide NaCl |
Distilled water | Transport medium for the various substances in agar |
The storage requirements for nutrient agar are as follows:
- The dried medium must be kept at 10–30°C and used before the expiration date listed on the package.
- Keep the NA media at 28°C after you’ve made it in the petri-dish.
Uses of nutrient agar
- Cultivating a diverse array of bacteria and fungi.
- Monitoring the colony’s appearance.
- Determining the microbial load (colony counting).
- Keeping bacterial cultures alive.
- Simple antibiotic testing.
Examples of microorganisms on nutrient agar
Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, and Micrococcus luteus

Colony morphology on nutrient agar
Bacterium | Colony appearance |
Escherichia coli | Smooth, circular, moist, cream-colored colonies. |
Staphylococcus aureus | Round, convex, opaque, golden-yellow colonies. |
Bacillus subtilis | Large, irregular, dry, rough, cream to light brown colonies. |
Pseudomonas aeruginosa | Flat, irregular colonies with metallic sheen and fruity odor. |
Micrococcus luteus | Small, round, convex, bright yellow pigmented colonies. |
Precautions
- Before preparing and using agar, verify that the medium is adequately sterilized by autoclaving at 121°C for 15 minutes to remove any contaminants.
- Before sterilization, check and adjust the pH to roughly 7. 0 to ensure optimum microbial growth.
- Avoid overheating, since it can deplete vital components in the media. To avoid contamination during media pouring and inoculation, always work in a sterile atmosphere, such as a laminar airflow cabinet.
- Clearly identify the plates with the date and kind of medium, and store them at 2–8°C if not used right away.
- Use aseptic practices at all stages to ensure the culture’s integrity.
Limitations
- Non-selective and non-differential: It is unable to discriminate or suppress any particular population of species.
- Not suitable for fussy organisms: It lacks the enriched nutrients that organisms such as Neisseria and Haemophilus need.
- Poor for diagnostic identification since it offers minimal to no biochemical information.
Reference and Sources
Also Read
- Sabouraud Dextrose Agar: Introduction, Principle, Example & Uses
- Mannitol salt agar (MSA): Introduction, Principle, Composition and Uses
- Techniques for Identifying Mutants
- Antimicrobial Chemotherapy: History, Characteristics and Tests
- Milk: Composition, Processing, Pasteurization, Pathogens and Spoilage
- Citric acid: Introduction, Fermentation, Recovery and Uses
- Cyanobacteria: occurrence, morphology, structure, reproduction
- Introduction of Plant-Microbe Association and the Mycorrhizae
- Microbial Interactions in the Environment