Wild-type bacteria are prototrophic i.e., they can produce colonies on minimal medium (a solution containing only inorganic salts, a source of energy such as a sugar, and water). From these simple substances, the bacteria synthesize all the molecules of life. Auxotrophs are organisms that are unable to produce a particular organic compound required for their growth.
Related Theory
Auxotrophic mutants are a special case of conditional lethal mutants, in which the condition is presence or absence of a specific nutrient in the media.