When microorganisms are cultivated in the laboratory, a growth environment called a medium is used. The medium may be purely chemical, or it may contain organic materials, Microorganisms growing in or on such a medium form a culture. A culture is considered a pure culture if only one type of organism is present and a mixed culture if populations of different organisms are present. When first used, the culture medium should be sterile, meaning that no form of life is present before inoculation with the microorganism.