You have some old plastic Petri dishes that you would like to use for pouring bacteriological agar plates. The only method of physical sterilization is ultraviolet radiation sterilization with your UV light (you cannot use the autoclave because it will disfigure and melt the plastic). After sterilizing the plates, pouring the sterilized agar medium, and then leaving the plates out at room temperature for a day or two to let them solidify and dry, you find contaminating bacterial colonies growing on the agar. Predict what has happened in this situation.
A. The room temperature enhanced the growth of normal biota of the agar.
B. The agar had bacteria in it, even after sterilization in the autoclave.
C. Air got into the poured agar plates, contaminating them.
D. The lids of the Petri dishes were inadvertently left on while being sterilized.
Answer: D
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