In routine preparation of solid microbiological media, which solidifying agent is most commonly used because it is largely inert to most microbes and melts/solidifies at convenient temperatures?

Introduction / Context:
Solid media are indispensable for isolating colonies and assessing colony morphology. The choice of solidifying agent affects melting point, clarity, and microbial compatibility. A widely adopted gelling agent enables robust, routine culturing across diverse organisms.

Given Data / Assumptions:

• The question asks for the commonly used solidifying agent.
• Agent should be generally non-nutritive and stable across incubation temperatures.
• It should not be degraded by most microbes.

Concept / Approach:
Agar, a polysaccharide from red algae, is the standard. It melts near 85–95 °C and solidifies around 40–45 °C, remaining solid at typical incubation temperatures (for example, 25–37 °C). Most bacteria cannot metabolize agar, preserving plate integrity. While silica gel has niche uses (for example, some specialized chemoautotroph cultures), it is not the common, general-purpose choice.

Step-by-Step Solution:

Verification / Alternative check:
Standard media recipes (Nutrient agar, LB agar, MacConkey agar) universally employ agar at about 1.5–2.0% w/v, supporting the answer.

Why Other Options Are Wrong:

• Silica gel: rarely used in routine plate media.
• Both (a) and (b): overbroad; only agar is the common standard.
• None: incorrect; agar is widely used.

Common Pitfalls:
Confusing “common” with “possible.” Although alternatives exist, the question focuses on typical microbiology practice.

Final Answer:
agar