A microbiological medium is a medium that provides the substances a microorganism needs to grow. To grow microorganisms successfully, different media must be used, according to their differing needs.
In the past, microbiologists used the foods they got in restaurants to feed their microbes. Added to foods were blood, fruit juices and different other substances that they found in the same environment as the microorganisms they were interested in.
To be usable, a microbiological medium and environment must contain all the substances the specific species needes to grow. This means the right amount of a specific reductant (for example oxygen), water, a source of nitrogen, a source of carbon, energy source, vitamins, minerals and microminerals (trace elements).
There are different types of energy source. Often glucose and carbon sources are used. Certain bacteria can conserve some energy of light in their energy-rich compounds. Some species oxidize sulfur to get energy.
The source of nitrogen are mostly organic compounds, especially protein, peptides or amino acids. Some species are able to use inorganic substances, for example nitrates, nitrites or ammonia. In certain cases, bacteria are even able to fixate nitrogen from the atmosphere.
Vitamins and other factors are also needed. Microorganisms differ in their ability to produce them independently and in many examples, they must be obtained from the environment.