The aqueous environment containing a dissolved substrate and all the other necessary nutrients in which microorganisms are cultured such as in the process of fermentation. All microorganims require water and a source of carbon, nitrogen, minerals, salts, trace elements, and vitamins. Oxygen is also required for some types of microorganisms. A defined medium consists of pure compounds in which the composition and concentration of all the chemicals is known. Common media used to culture microorganims as colonies on petri dishes include agar and gelatine.