Nutrient broth is a liquid culture medium containing peptone, beef or yeast extract and sodium chloride, formulated to support the growth of a wide variety of non-fastidious bacteria and some yeasts.
Composition and preparation
Like nutrient agar, nutrient broth is made from an enzymatic digest of animal proteins (peptone) that supplies nitrogen in the form of amino acids and peptides, and beef extract or yeast extract that provide water-soluble vitamins, minerals and carbohydrates. Sodium chloride is included to maintain osmotic balance. The ingredients are dissolved in distilled water and the pH is adjusted to approximately neutral (around 7.0). Because the medium does not contain agar, it remains fluid after heating and sterilization. The prepared broth is dispensed into flasks, bottles or test tubes, stoppered or capped and sterilized by autoclaving at 121 °C for 15 min. After cooling, the broth can be stored at room temperature or refrigerated until use. Variants of nutrient broth may include additional components such as glucose for carbohydrate metabolism or specific salts for studying physiological responses.
Applications and limitations
Nutrient broth is commonly used for propagating pure cultures, preparing inocula for biochemical tests, and producing large quantities of bacterial biomass. In a laboratory, small volumes are inoculated with bacteria and incubated with shaking or aeration to ensure oxygenation, producing uniform turbidity as the cells proliferate. Broth cultures are useful for determining cell concentration by measuring optical density, and for harvesting cells for DNA extraction, protein production or metabolic assays. However, because it is a rich medium without selective agents, nutrient broth permits the growth of diverse organisms and does not differentiate among them. It does not support fastidious or obligately anaerobic bacteria that require specific growth factors or redox conditions. Overgrown cultures may exhaust nutrients and accumulate waste products, so timing and agitation must be controlled. Careful aseptic technique is essential to avoid contamination.
Nutrient broth is a staple liquid medium in microbiology laboratories, offering a simple means to cultivate non-fastidious organisms and generate cell suspensions for downstream analyses.
Related Terms: Liquid medium, Turbidity, Culture flask, Inoculum, Optical density