Nutrient agar is a general-purpose solid medium containing peptone, beef or yeast extract, sodium chloride and agar, used to cultivate a wide range of non-fastidious bacteria and fungi.
Composition and preparation
The basic formulation of nutrient agar consists of an enzymatic digest of casein or animal proteins (peptone) that supplies amino acids and peptides, beef extract or yeast extract that provides vitamins and trace elements, sodium chloride to maintain osmotic balance, and agar derived from seaweed as a solidifying agent. To prepare the medium, the dry ingredients are weighed, dissolved in distilled water and the pH is adjusted to around 7.0. The mixture is heated to melt the agar, transferred to suitable containers such as bottles or flasks and autoclaved at 121 °C for 15 min to sterilize it. After cooling to about 50 °C, the sterile molten medium can be poured into sterile Petri dishes to solidify as plates or into tubes to make slants. Additives such as carbohydrates, salts or pH indicators can be incorporated before sterilization to tailor the medium for specific applications.
Applications and limitations
Because nutrient agar supplies basic carbon and nitrogen sources without inhibitory substances, it supports the growth of many heterotrophic bacteria and fungi isolated from soil, water, food and clinical specimens. It is commonly used to isolate and maintain laboratory strains, to estimate viable cell counts by plating serial dilutions and counting colonies, and to observe colony morphology such as size, color and texture. However, it is not suitable for fastidious organisms that require complex nutrients or growth factors, such as many pathogenic bacteria that grow on blood agar or chocolate agar. Nutrient agar is also not selective or differential, so contaminants can overgrow the organisms of interest, and it cannot be used to culture viruses or obligate intracellular bacteria that require living host cells. The medium must be stored properly to prevent dehydration and contamination, and plates should be used within a reasonable time to ensure quality.
Nutrient agar remains a fundamental tool in microbiology for routine cultivation and enumeration of microbes, providing a simple and versatile platform for observing and isolating colonies.
Related Terms: Solid medium, Agar plate, Peptone, Colony-forming unit, Colony morphology