In a defined medium:
a. The exact chemical composition of the medium is known
b. Agar is available for microbial nutrition
c. Blood may be included
d. Organic chemicals are excluded

Verified step by step guidance

1. Step 1: Understand the definition of a defined medium in microbiology. A defined medium is a growth medium where all the chemical components and their exact amounts are known.
2. Step 2: Analyze option (a): 'The exact chemical composition of the medium is known.' This aligns with the definition of a defined medium.
3. Step 3: Analyze option (b): 'Agar is available for microbial nutrition.' Agar is typically used as a solidifying agent, not a nutrient source, so this is not a defining feature of a defined medium.
4. Step 4: Analyze option (c): 'Blood may be included.' Blood is usually added to complex or enriched media, not defined media, because its exact composition is not known.
5. Step 5: Analyze option (d): 'Organic chemicals are excluded.' Defined media often contain organic chemicals, but their exact amounts are known, so this statement is incorrect.

Key Concepts

Defined Medium

A defined medium is a growth medium in which all the chemical components and their exact concentrations are known. It allows precise control over the nutrients available to microorganisms, facilitating studies on microbial metabolism and nutritional requirements.

Complex vs. Defined Media

Complex media contain ingredients like blood, yeast extract, or peptones with unknown exact chemical compositions, while defined media have chemically pure components. This distinction is important for experiments requiring controlled nutrient conditions.

Role of Agar in Microbiology

Agar is a solidifying agent derived from seaweed, commonly used in culture media to provide a surface for microbial growth. It is generally inert nutritionally and does not serve as a nutrient source for microbes.