Petri dishes are inverted during incubation to prevent condensation from falling into the microbes, thereby contaminating samples. Condensation in Petri dishes causes bacterial samples to spread and potentially mix with each other. Warm incubators tend to attract more condensation, so the dishes are inverted. Inversion allows water to drip down onto the lid, away from the agar, rather than onto bacteria.
Inversion also prevents contamination of other bacteria in the samples, in case Petri dishes were not thoroughly cleaned or other microbes happened to contaminate samples. Petri dishes can be stored up to three months in cold storage before use in an incubator.
Inverted Petri dishes also minimize the evaporation of water used to grow the samples sealed in the containers. Dishes are sealed with tape so that lids do not fall off when the inversion occurs, and the tape can be written on for labeling purposes. The apparatus can also be stacked for easy storage in an incubator.
Petri dishes were invented in 1887 by German physician Julius Petri. He improved upon the design of his mentor, Dr. Robert Koch, by inventing a small plate that has a slightly larger plate that fits over it to create smaller samples for bacterial growth. The plate allowed samples in growth media to be more easily observed under a microscope without cross-contamination from other samples. Previously, glass plates were kept under a bell jar.