Petri dishes have a variety of uses, but they are most commonly used for culturing bacteria. Petri dishes are small, clear, plastic or glass containers that can also securely contain small animals or germinating seeds. A growth medium is often placed in the bottom of a petri dish, which provides a food source for the bacteria, enabling them to reproduce rapidly.
To study bacteria, scientist begin by taking a swab or sample of the bacteria in question. For example, if a scientist was studying salmonella bacteria, he may drag a cotton swab along some spoiled food. The swab is then wiped on the food source in the petri dish and sealed. Depending on the bacteria in question, the petri dish may be incubated in a heated box to accelerate the growth of the bacterial culture. After a few hours, days or weeks, the scientist will open the petri dish and examine the bacteria growing inside.
Scientists may view the bacteria in a petri dish microscopically, or they may count the number of bacterial cells in the culture. Scientists may compare the results of different food sources by using petri dishes with different types of food. Alternatively, scientists may treat each petri dish with a different chemical to see which ones have the desired effect on the bacteria.