Contamination forms on an unwashed fingerprint in a Petri dish left in an incubator for one week because bacteria and other microorganisms from the environment are able to grow and multiply on the surface of the fingerprint. The warm and moist environment of the incubator provides ideal conditions for the growth of these microorganisms.
When a person touches a surface with their fingers, they transfer skin cells, oils, and other substances that can provide a food source for bacteria and other microorganisms. In the absence of cleaning or disinfecting the surface, these microorganisms can continue to grow and multiply, leading to the formation of contamination.
In the case of an unwashed fingerprint in a Petri dish left in an incubator, the bacteria and other microorganisms are able to grow and form colonies on the surface of the fingerprint, leading to the formation of visible contamination over the course of a week. To prevent this, it is important to wash or disinfect surfaces before using them for scientific experiments or culturing microorganisms.