By Kyle Fiechter, Updated Mar 24, 2022
Agar is a gelatinous medium used for cultivating bacterial cultures. Agar plates combine this gelatinous base with essential nutrients such as nutrient agar, starch agar, milk agar, and egg yolk agar – examples provided by the University of Missouri‑St. Louis. To maintain optimal growth conditions, agar plates must be stored in a way that prevents bacterial contamination and preserves the medium’s integrity.
Place each agar plate on its lid side and stack them in their original storage bags. This orientation stops airborne contaminants from settling on the nutrient surface and provides an extra barrier against microbes.
Maintain a constant temperature of 4 °C (39 °F). Most bacteria have reduced metabolic activity below 10 °C, which preserves the agar’s consistency and inhibits unwanted growth. Keep plates sealed to avoid drying.
When a refrigerator cannot be used, a temperature‑controlled cold room (≤ 4 °C) is acceptable. After pouring, inspect plates for condensation within 2–4 hours. Condensation indicates heat exposure that may desiccate the agar. Flip the plates if moisture accumulates and monitor closely for further condensation.
Before use, examine each plate for visible colonies or cracks. A dry, intact agar surface indicates that the plates remain viable and ready for inoculation.
Antibiotic stability decreases over time. For example, ampicillin loses potency after 3–4 weeks of refrigerated storage. If the plates are stored at ambient temperature, the antibiotic’s activity may diminish within a week. Discard any antibiotic‑containing plates that exceed these time frames to avoid inaccurate results.
