Nutrient agar: Principle, Composition, Examples & Uses

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.

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.