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.

The lids of the Petri dishes were inadvertently left on while being sterilized

Explanation:

The process of eliminating any microbe from a particular object to achieve a sterile aseptic environment is termed as sterilization. Sterilization mainly aims to either remove, kill or make any microbe inactive by using various means such as irradiation, heat, pressure, etc. In the given question radiation is used to sterilize the Petri dishes and thus, if sterilization is successful there should not be any contamination. However, since there is contamination present amongst the bacterial colonies growing on agar plated in the Petri dishes this shows that the sterilization has not been done successfully and a possible reason for this could be that while sterilizing the lids of the Petri dishes were kept on.