By Joan Reinbold • Updated Aug 30, 2022

Nutrient agar is a versatile, general‑purpose medium that supports the growth of a broad spectrum of non‑fastidious bacteria, yeasts, and molds. Its composition delivers a balanced supply of organic nitrogen, carbon, vitamins, and minerals, making it the workhorse of many microbiology laboratories.

Microorganisms That Thrive on Nutrient Agar

Because nutrient agar contains no special growth factors, it is ideal for culturing organisms that do not require specific substrates. Typical colonies include Streptococcus, Staphylococcus, Escherichia coli, and a variety of environmental isolates. Fastidious organisms such as Treponema pallidum (the agent of syphilis) remain uncultivable on this medium due to their unique nutritional demands.

Researchers use nutrient agar for routine isolation, colony counting, and preliminary identification. It also serves in environmental monitoring—testing water, sewage, shellfish, meat, and dairy for contamination.

Key Ingredients of Nutrient Agar

The standard formulation is:

• 0.5 % peptone (protein hydrolysate)
• 0.3 % beef extract (or yeast extract)
• 1.5 % agar
• 0.5 % sodium chloride

These proportions may vary slightly by manufacturer, but the overall nutrient profile remains consistent.

Understanding Agar, Peptone, and Beef Extract

Agar

Agar is a complex polysaccharide derived from marine red algae. It solidifies the medium at 113 °F (45 °C) and melts at 203 °F (95 °C). While it provides structural support, agar offers no nutritional value to microbes.

Peptone

Peptone is a soluble protein fraction produced by partial hydrolysis of animal or plant proteins. It supplies organic nitrogen, some amino acids, and minor vitamins, fostering robust microbial growth.

Beef Extract

Beef extract is a water‑soluble mix of amino acids, peptides, nucleotides, vitamins, and minerals derived from boiled beef tissue. It enhances the growth of a wide range of bacteria. Yeast extract is an acceptable substitute, offering similar nutrients.

Preparing Nutrient Agar

1. Weigh 28 g of nutrient agar powder and dissolve in 1 L of distilled water.
2. Heat to a rolling boil while stirring to ensure complete dissolution (≈ 1 min).
3. Autoclave at 121 °C (250 °F) for 15 min.
4. Cool to ~45 °C (110 °F) before pouring.
5. Dispense into sterile Petri dishes or tubes; cover and allow to solidify.
6. Store plates upside down in a dark, refrigerated environment to prevent condensation.

The final pH should read 6.8, yielding a light amber, firm gelatinous medium. When stored at 4 °C, plates remain viable for up to two years, barring visible contamination.

Storage and Shelf Life

Keep plates refrigerated, covered, and upside down. Avoid exposing them to light or temperature fluctuations. Inspect for color changes, cloudiness, or microbial contamination before use.

Following these guidelines ensures reliable cultivation of non‑fastidious organisms and accurate microbiological assessments.