Final answer:
Nutrient agar is a complex media containing peptone, beef extract, agar, and sometimes sodium chloride. It is not chemically defined because the exact amounts of the multiple nutrients present in peptone and beef extract are not specified, therefore providing a medium that supports the growth of a variety of non-fastidious organisms.
Step-by-step explanation:
Nutrient agar is a complex medium used for culturing bacteria in laboratory settings. The main components of nutrient agar include peptone, beef extract, agar, and sometimes sodium chloride. Peptone is a partially digested protein source, which provides amino acids and peptides necessary for bacterial growth. Beef extract supplies a mixture of water-soluble substances, such as vitamins, carbohydrates, organic nitrogen compounds, and salts. Agar, a polysaccharide extracted from red seaweed, acts as a solidifying agent to provide a surface for bacterial growth. Unlike chemically defined media, where all the chemicals used are known and their exact concentrations stated, the compositions of peptone and beef extract in nutrient agar are not chemically fully defined, as they are complex mixtures of many nutrients.
Due to the inclusion of these ingredients, nutrient agar is classified as a complex media because it contains an undefined mixture of amino acids, salts, vitamins, and other nutrients. Its exact chemical composition is unknown, differing from chemically defined media where each individual component is known and its concentration is measured accurately. Therefore, nutrient agar is especially useful for growing a wide variety of non-fastidious organisms, not tailored for specific bacterial groups that might require precise nutrient conditions.