 Culture medium

A culture medium is a prepared environment that supplies nutrients and suitable chemical conditions for growing cells outside their original setting. In bacterial culture, its composition can provide carbon, nitrogen, mineral ions and other requirements while helping control pH and the osmotic environment. A buffer solution and dissolved salts can affect growth even when neither is the main energy source.^[ref]

A defined medium has a specified chemical composition. A complex medium contains ingredients such as biological extracts whose individual constituents are incompletely specified. These are classifications by composition; liquid versus solid is a different distinction. Adding agar can give a solid surface on which separated growth produces a colony.^[ref][ref]

The medium is part of the experiment. In a hypothetical nutrient-dependence test, compare a complete defined medium, an otherwise identical medium lacking nutrient X, and the deficient medium with X restored. Growth in the first and third conditions but not the second supports a requirement for supplied X under those conditions. Equal initial cell numbers and matched observation times make that comparison interpretable. The observation does not establish the same requirement in every environment.

A failed bacterial culture can consequently reflect an unsuitable culture medium rather than absence of living bacteria in the starting sample. Cultivation measures growth under the selected conditions; it does not directly inventory all microorganisms present.