If there are colonies after a period of time, it means the culture medium is contaminated; if there are no colonies, it means the culture medium is not contaminated.

To detect whether the culture medium is contaminated, a common method is to use a blank culture medium, that is, a culture medium that has not been inoculated with various fungi. After inoculation, culture the culture medium under the same conditions for a period of time and observe whether colonies appear. If colonies appear, it means the culture medium is contaminated.

The total number of bacterial colonies is mainly used as a marker to determine the degree of contamination of food. This method can also be used to observe the dynamics of bacterial reproduction in food, so as to provide a basis for hygienic evaluation of the inspected samples.