Agar Powder

In microbiology, agar powder is the absolute backbone of the lab. It is a gelatinous substance derived from red algae (specifically from genera like Gelidium and Gracilaria) used to solidify liquid nutrient broths so that bacteria and fungi can grow in distinct, isolated colonies.

What makes agar so perfect for microbiology is its unique thermal property known as hysteresis: it melts at around 85°C (185°F) but doesn’t solidify again until it cools down to about 32–40°C (90–104°F). Plus, almost no microorganisms can actually digest it, so it stays perfectly solid while they eat the nutrients mixed into it.

Why Agar for Microbiology?

• It melts at ~85–95°C and solidifies at ~32–40°C, making it ideal for pouring plates without killing heat-sensitive microbes.
• It is non-nutritive — it doesn’t provide nutrients itself, allowing you to add specific nutrients (e.g., for Nutrient Agar, Blood Agar, etc.).
• High gel strength, clarity, and stability.
• Resistant to most microbial enzymes (unlike gelatin).

Typical Usage

• Concentration: 1–2% (10–20 g per liter) for standard plates.
• 0.5% for motility tests.
• Lower (≤0.1%) for some anaerobic work.
• Dissolve in media by heating, autoclave to sterilize, then pour into Petri dishes.