In the complex medium nutrient agar, what is the primary source of nutrients that most bacteria metabolize for growth?

• A. Only defined amounts of glucose and ammonium sulfate as the sole carbon and nitrogen sources
• B. Digests of proteins such as peptones and beef extract that provide amino acids, peptides, vitamins, and minerals
• C. Only agar, which serves as the main carbon and energy source for most bacteria
• D. A chemically defined mixture of purified amino acids, salts, and vitamins with known exact composition

Answer: (B)

Verified step by step guidance

1. Understand that nutrient agar is a complex medium, meaning it contains ingredients that are not chemically defined but provide a rich source of nutrients.
2. Recognize that in nutrient agar, the main components supplying nutrients are digests of proteins, such as peptones and beef extract.
3. Know that peptones and beef extract provide a mixture of amino acids, peptides, vitamins, and minerals, which bacteria can metabolize for growth.
4. Recall that agar itself is a solidifying agent and is not metabolized by most bacteria as a nutrient source.
5. Conclude that the primary source of nutrients metabolized by most bacteria in nutrient agar are the protein digests (peptones and beef extract), not defined carbon or nitrogen sources or agar.