Petri plates are incubated upside down to prevent contamination.
Explanation:
The plates are always incubated inverted or turned upside down on their covers during storage. This is done to prevent evaporation if plaque is stored for long periods, which can affect the growth efficiency of bacteria, or allow contamination by multiplying unwanted organisms such as mold.
Once the plaques have been filled with a damp suspension of bacteria, it should be allowed to evaporate shortly before overnight incubation. However, a moderate amount of moisture will still be present. If plaque is not inverted during incubation, bacteria will not be able to attach to the culture medium properly, which will either prevent them from growing and forming colonies properly or will encourage the growth of undesirable microorganisms.
Any of these results will invalidate the experiment. In addition, any condensation or moisture can cause streaking, which will make it difficult to select and analyze separate colonies. After bacteria form colonies, plaque is also stored upside down to maintain moisture levels.
