In microbiology, culture refers to the method of multiplying microbial organisms by letting them reproduce in a predetermined culture medium under controlled laboratory conditions. Microbial cultures are used to determine the type of organism, its abundance in the sample being tested, or both, and are one of the primary diagnostic methods of microbiology. Culture media are essentially composed of basic elements, such as water and nutrients, to which different growth factors are added that are specific to each bacterial species. There are many different formulations of culture media available to suit the differing nutritional needs of bacterial species. The type of medium chosen will depend on the purpose of the culture, such as isolating specific strains of microorganisms, identifying disease-causing pathogens, preparing pure cultures, and testing antibiotic sensitivity. Once a microorganism has been isolated in pure culture, it is necessary to preserve it in a viable state for further study and use. Stock cultures have been developed to preserve microorganisms in a viable state for long periods of time.