Which step must be completed before pouring a liquid agar gel solution into the gel casting tray during microbiological media preparation?

• A. Seal the casting tray with parafilm.
• B. Allow the agar solution to cool to about 50°C to prevent cracking of the tray and ensure even solidification.
• C. Add bacterial inoculum directly to the hot agar solution.
• D. Place the tray in a refrigerator to pre-chill it.

Answer: (B)

Verified step by step guidance

1. Understand the purpose of pouring agar gel into a casting tray: it is to create a solid medium for microbial growth.
2. Recognize that agar is initially heated to dissolve it completely, but pouring it while too hot can damage the tray or cause uneven solidification.
3. Identify the critical temperature step: allow the agar solution to cool to approximately 50°C before pouring. This temperature is cool enough to prevent damage and hot enough to keep the agar liquid.
4. Avoid adding bacterial inoculum directly to the hot agar, as high temperatures can kill the bacteria.
5. Do not seal the tray with parafilm or pre-chill the tray before pouring, as these steps are not necessary and can interfere with proper gel formation.