Question

Agar is used for preparing solid media for culture. Which of the following is not true about agar media?

• A. Agar is obtain from a sea weed
• B. It has virtually no nutritive value.
• C. It melts at 98\(^\circ\)C and usually sets on 42\(^\circ\)C depending on agar concentration.
• D. For solid media 0.2\% to 0.5\% agar concentration is employed.

Hint:

Remember the standard concentrations for agar media: \textbf{Solid} media (plates, slants) \(\rightarrow\) \textbf{1.5\%}. \textbf{Semi-solid} media (motility) \(\rightarrow\) \textbf{0.4-0.5\%}. \textbf{Broth} (liquid) \(\rightarrow\) \textbf{0\%}.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Agar is the most common solidifying agent used in microbiological culture media. The question asks to identify an incorrect statement about its properties.
Step 2: Detailed Explanation:
Let's evaluate each statement:


(A) Agar is obtain from a sea weed: This is true. Agar is a polysaccharide extracted from the cell walls of certain species of red algae (seaweed), primarily from the genera Gelidium and Gracilaria.

(B) It has virtually no nutritive value: This is true. One of the key advantages of agar is that it is a complex polysaccharide that is not metabolized or degraded by the vast majority of microorganisms. It serves only as a solidifying agent, not a nutrient source.

(C) It melts at 98\(^\circ\)C and usually sets on 42\(^\circ\)C...: This is true. Agar exhibits hysteresis; it melts at a high temperature (around 95-100\(^\circ\)C) but solidifies at a much lower temperature (around 40-45\(^\circ\)C). This allows heat-sensitive components (like antibiotics or blood) to be added to the molten agar before it solidifies.

(D) For solid media 0.2\% to 0.5\% agar concentration is employed: This is not true. This concentration range (0.2-0.5\%) is used to prepare semi-solid media, which is used for motility testing. For a standard solid medium (like an agar plate or slant), a concentration of 1.5\% to 2.0\% agar is required to produce a firm gel.

Step 3: Final Answer:
The statement that 0.2\% to 0.5\% agar is used for solid media is incorrect; this concentration is for semi-solid media.