= 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.<Bacteriology Culture Guide by ATCC>[Growth Media: Bacterial Culture Media, nutrient composition, buffering and osmotic conditions]{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 (biology)>.<Bacteriology Culture Guide by ATCC>[Growth Media: defined and complex media; liquid cultures and solid agar-based media]{ref}<The Streak Plate Protocol by D. Sue Katz>[page 1, History and isolation rationale]{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.
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