Nutrient Agar Composition Preparation Uses

4 September, 2025

Introduction

Nutrient Agar (NA) is one of the most commonly used basal solid culture media in microbiology laboratories. It supports the growth of a wide variety of non-fastidious bacteria and serves as a foundation for the preparation of many specialized media. Due to its simplicity and versatility, it is widely used in teaching, research, and clinical microbiology

Media Type

Category: Basal (simple) solid medium
Purpose: Supports the growth of non-fastidious bacteria; used for colony morphology, subculture, and maintenance.

Composition & pH of Nutrient Agar

A standard Nutrient Agar contains (per liter of distilled water): Final pH: 7.2 – 7.4 (at 25°C)

Component	Function
Peptone (5 g)	Provides nitrogen, amino acids, growth factors
Beef extract (3 g)	Provides vitamins, carbohydrates, salts, nitrogen
Sodium chloride (5 g)	Maintains osmotic balance
Agar (15 g)	Solidifying agent
Distilled water	Solvent

Sterilization

Sterilize by autoclaving at 121°C for 15 minutes at 15 psi.

Preparation of Nutrient Agar

1. Dissolve peptone, beef extract, and sodium chloride in distilled water.

2. Add agar powder and heat to dissolve completely.

3. Adjust pH to 7.4.

4. Dispense into conical flasks or bottles.

5. Autoclave at 121°C for 15 minutes.

6. For plates: pour molten medium into sterile Petri dishes (20-25 mL per plate).

7. Allow to solidify and store at 2-8°C.

Uses of Nutrient Agar

General-purpose medium: Supports the growth of non-fastidious bacteria.
Colony morphology: Observation of bacterial colonies (size, shape, color, texture).
Subculturing: Used to maintain pure cultures.
Viable count: Quantification of bacterial growth (spread plate/pour plate methods).
Antibiotic sensitivity testing (modified forms).
Base medium: For preparation of enriched media (e.g., Blood Agar, Chocolate Agar).

Limitations

Does not support growth of fastidious organisms (e.g., Haemophilus influenzae, Neisseria gonorrhoeae).
No selective or differential properties.

Conclusion

Nutrient Agar is a fundamental solid culture medium in microbiology. Its simple composition, ease of preparation, and wide applicability make it an essential tool for diagnosis, teaching, and research.

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