A Petri dish is a shallow nesting dish made of glass or plastic that is typically used in laboratories. These dishes also have clear lids that allow scientists to view the contents without removing the lid. Scientists use them to grow bacteria, mold and other microscopic organisms that they can then examine under a microscope.
History of the Petri Dish
In 1887, a German scientist named Julius Richard Petri developed a shallow nesting dish to use in laboratory experiments. Before this invention, scientists used test tubes, empty liquor bottles and other types of glass jars to grow cultures. Unlike these glass containers, Petri's dish had a lid that fit securely over the dish to protect the culture growing inside.
The Contamination Problem
Petri's dish provided a way for his boss to grow bacteria more efficiently, it also solved the problem of culture contamination in laboratories. Growing cultures in an open container allowed contaminants from the air to interfere with the bacteria or mold. Switching to covered glass jars provided some protection. However, lifting the top to check inside the jar allowed air and sometimes condensation to enter.
Uses of Petri Dishes in a Lab
Petri dishes are one of the most useful pieces of equipment in a laboratory because of their versatility. Not only is it an excellent environment for growing bacteria, yeast, mold and viruses, but it has other applications as well. They can be used to observe the behavior of insects, watch the germination of seeds, store samples and evaporate solvents.
Petri dishes played a role in one of the most important discoveries of the twentieth century. Alexander Fleming was growing bacteria in Petri dishes when he noticed that a type of mold appeared to prevent the bacteria from reproducing. This observation eventually led to the development of penicillin.
How to Use a Petri Dish
In some cases, the Petri dish must be prepared for the experiment. The first step is to sterilize the dish with a bleach solution before filling it with the agar medium. It can then stay in the refrigerator until it's time to start the experiment. During the experiment, it's important to leave the dish covered. In some labs (including classrooms), the Petri dishes may be sealed with tape to prevent accidental opening.
All About Agar
When scientists grow bacteria, yeast or mold in Petri dishes, they do so on an agar plate. Agar comes from a type of algae. This jelly-like substance provides important nutrients like glucose and salt to the microorganisms. It's packaged in a bottle or comes in the form of a tablet or powder. When cooled, the prepared agar sets and has the consistency of gelatin.
Smart Petri Dishes
In 2011, researchers at Caltech used the first smart Petri dish. It uses a smartphone camera instead of a microscope to provide views of how the culture inside is growing. Instead of sliding the dish under a microscope, they get real-time views of the sample as the camera transmits images directly to a laptop.