For best results, use water that is free of impurities, especially chlorine. Types of purified water include distilled, reverse osmosis, deionized, and ultrafiltered. Distilled water is available for purchase at most drug stores and grocery stores.
Agar vs. Gelatin
Many people are tempted to replace agar with gelatin as a low-cost alternative. But gelatin has some drawbacks.
- Gelatin is less firm than agar, so the media will be fragile. Students may poke through the surface while trying to spread a sample.
- You will need to store and incubate your plates media-side down, and they will melt in a 37 °C incubator.
- Some bacteria can digest gelatin (they can't digest agar), turning the plates to mush.
- Gelatin doesn't set properly in media with 10% or more salt.
Types of Agar
Less expensive, food-grade agar is available at Asian markets or the international section of specialty grocery stores (look in the Asian food section). Food-grade agar usually works fine, but consider spending a little more money and buying lab-grade agar. It produces more consistent and higher quality plates, and it is less often contaminated with bacteria or mold spores.