Agar is a natural polysaccharide extracted from red algae (primarily Gelidium and Gracilaria species). Chemically, it’s composed of agarose and agaropectin. When dissolved in hot water and then cooled, agar forms a firm, translucent gel that remains stable at typical incubation temperatures. Unlike gelatin (which melts around body temperature), agar stays solid up to about 85 °C—making it ideal for culturing heat‑sensitive organisms.
How Is Agar Prepared for Culture Media?
To make agar media, the dry powder is mixed with distilled water and any required nutrients (like dextrose, peptone, or potato extract). The mixture is heated until the agar fully dissolves, then sterilized—usually by autoclaving at 121 °C for 15–20 minutes. Once cooled to around 50–55 °C, you can pour the liquid into Petri dishes or tubes. As it cools further, the gel sets, creating a stable surface for microbial growth.
Common Types of Agar Media
| Agar Type | Primary Use | Key Ingredients |
|---|---|---|
| Potato Dextrose Agar (PDA) | Cultivating fungi and yeasts | Potato infusion, dextrose, agar |
| Malt Extract Agar (MEA) | General‑purpose fungal media | Malt extract, peptone, agar |
| Sabouraud Dextrose Agar (SDA) | Isolation of pathogenic fungi and dermatophytes | Peptone, dextrose, agar |
| Cornmeal Agar (CMA) | Morphological studies of fungi and yeasts | Cornmeal infusion, agar |
| Nutrient Agar (NA) | Bacterial culture | Peptone, beef extract, agar |
Glossary of Related Terms
Agarose
The purified fraction of agar that forms the gel matrix. It’s commonly used in electrophoresis gels due to its uniform pore size.
Agaropectin
The non‑gel‑forming fraction of agar, rich in sulfated galactans and responsible for water retention.
Gel Strength
A measure (in grams per square centimeter) of how firm an agar gel is. Higher gel strength means a more rigid surface.
pH Indicator
Some agar formulations include dyes (like phenol red) that change color if the culture’s pH shifts—useful for detecting acid or alkali production.
Autoclave
A pressurized steam sterilizer essential for eliminating contaminants in agar and other culture media.
Pour Plate vs. Spread Plate
Pour plating involves mixing microbes into molten agar before it solidifies; spread plating deposits organisms on the gel surface for colony isolation.
Selective vs. Differential Media
Selective media contain inhibitors favoring certain organisms, while differential media include indicators that visually distinguish species or metabolic traits.
Putting It All Together
Understanding agar—and the specific formulations built on it—is fundamental whether you’re working in a professional lab or setting up a DIY mycology project. From choosing the right media type to mastering sterilization and gel strength, the terms above form the vocabulary you’ll use every time you prepare a new plate. Enjoy exploring the world of fungal and microbial cultures!
Frequently Asked Questions
What’s the difference between PDA and MEA?
Both are nutrient media for fungi. PDA uses potato infusion and dextrose; MEA uses malt extract. Either can be appropriate for basic observation. Pick one and keep it consistent for comparability.
Do I need a flow hood to use agar?
No. Many researchers use a Still Air Box successfully. Focus on clean technique, deliberate movements, and disciplined workflow.
How should agar plates be stored?
Seal plates to reduce desiccation and contamination. Store inverted when possible to limit condensation dripping onto the surface. Keep cool and dark per your lab’s standards.
Can I identify a strain by looking at a plate?
Agar can aid observation, but strain identity claims are unreliable without rigorous methods. Use agar to document morphology and cleanliness; avoid inferring attributes outside microscopy.