Unless you're a working scientist, your exposure to the petri dish has probably been limited to using them high school biology class or seeing them in mad scientist horror films . But the lowly petri dish has played an important role in modern science.
The "New Penguin Dictionary of Science" (Penguin Books, 2004), gives a barebones definition of petri dish as "a shallow circular or square plastic or glass dish with an overlapping lid used particularly for growing bacteria or fungi under laboratory conditions." That's a decent, if dry, definition, but there's a bit more to understanding its importance to science.
The dish was named after Julius Richard Petri (1852-1921), a German bacteriologist who invented the apparatus while working with Robert Koch, whose work on isolating tuberculosis strains earned him a Nobel Prize in 1905.
In modern times, the idea of keeping an area clean and sterile is nothing special. But in the late 1800s, creating a sterile environment especially for high-quality scientific experiments was not easy.
A scientist trying to grow a specific type of bacteria under laboratory conditions would be faced with a very difficult task. Invisible microbes in the air could easily drift onto a "clean" surface. By sterilizing a glass dish and placing a tight-fitting lid over it, Petri dramatically reduced the level of contamination in such experiments. Petri's idea was both simple and vitally important to microbiology.
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