A growth medium is a substance in which plants, fungi, or bacteria may be grown. Growth media are used as standardized environments in which controlled experiments can be done with microorganisms or cell cultures. Accordingly, they are kept sterile to prevent contamination. The most common growth medium is agar infused with a nutrient broth, but some organisms require specialized environments (selective media), such as viruses and certain bacteria. For example, since viruses are obligatory intracellular parasites, they require a growth medium composed of living cells.
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