What type of media is defined by having a known chemical composition?

Defined media is characterized by having a precise and known chemical composition. This means that all the ingredients in the media are specifically identified, and their concentrations are measured out accurately. Such media are used when the exact nutritional requirements of an organism are known, enabling researchers to study how the organism behaves under controlled conditions. This level of precision is crucial for experiments that require reproducibility and clarity about the nutrients being provided.

In contrast, complex media contain ingredients that are not precisely defined, such as extracts from yeast or beef, which provide a rich blend of nutrients but with variability in composition. Selective media are designed to favor the growth of specific organisms while inhibiting others, and their composition may or may not be known, but their primary function is to select for particular types of bacteria rather than to provide a defined nutrient environment. Therefore, defined media stands out for its utility in studies requiring exact knowledge of the growth conditions.