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: Concept Overview:
Agar, a common solidifying agent in microbiological media, requires identifying the false statement regarding its properties.
Step 2: Detailed Analysis:
Evaluating each statement:


(A) Agar is obtained from seaweed: Correct. Agar is a polysaccharide extracted from red algae cell walls, mainly Gelidium and Gracilaria.

(B) It has virtually no nutritive value: Correct. Agar is advantageous because most microorganisms cannot metabolize it, making it a solidifying agent, not a nutrient.

(C) It melts at 98\(^\circ\)C and usually sets on 42\(^\circ\)C...: Correct. Agar exhibits hysteresis, melting around 95-100\(^\circ\)C and solidifying around 40-45\(^\circ\)C, enabling heat-sensitive component addition before solidification.

(D) For solid media 0.2\% to 0.5\% agar concentration is employed: Incorrect. 0.2-0.5\% agar is for semi-solid media, used in motility testing. Solid media (agar plates/slants) require 1.5\% to 2.0\% agar for a firm gel.

Step 3: Conclusion:
The claim that 0.2\% to 0.5\% agar is for solid media is false; this concentration is appropriate for semi-solid media.