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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