Petri dishes

The Petri dish is so simple, it’s a wonder no one thought of it sooner. The first versions were made from glass; today they’re largely made out of plastic, yet little else has changed since the Petri dish was first introduced in the 1880s. The key feature is a snugly fitted lid, which keeps lab samples from becoming contaminated and allows the entire dish to easily slide under a microscope for studying whatever is growing inside.

What’s behind the Petri dish’s enduring design? Let’s take a closer look.

In 1877, Julius Richard Petri was a German army physician who was assigned to the Imperial Health Office in Berlin under Robert Koch. Along with Louis Pasteur and Joseph Lister, Koch is considered to be one of the “fathers of microbiology” for his work identifying the specific bacteria that cause certain diseases. Koch also made improvements to the methods and equipment for studying bacteria, first by introducing beef-based gelatin as a culture medium and later seaweed-based agar.

At the time, the small glass dishes used for studying bacteria were typically covered with bell jars to prevent contamination. However, these jars were heavy, and had to be removed to fit the dish under a microscope. In 1887, Petri published a paper called “A minor modification of the plating technique of Koch” with the simple solution that would immortalize his name:

“Since the first of the year I have been using flat double dishes of 10-11 cm diameter and 1-1.5 cm height. The upper dish serves as a lid as usual and has a somewhat larger diameter…. Under these conditions contamination from airborne germs rarely occurs.”

300: Words in Julius Richard Petri’s paper describing his plating technique

150: Papers Petri wrote on hygiene and bacteriology

1-10 micrometers: Size of most bacteria

5×10^30: Estimated number of bacteria that live on Earth at any given time

$280 million: Value of the global Petri dish market in 2018

Julius Richard Petri may not actually have been the first to make—or even publish a description of—a lidded dish for studying bacteria. In 1885, Victor André Cornil and Victor Babeș included an illustration of a lidded dish in the textbook Bacteria and their Role in Pathological Anatomy and Histology of Infectious Diseases. Another 1885 textbook, Micro-Organisms and Disease by Emanuel Klein, references Robert Koch’s use of “a glass cell, [which] consists of a glass slide, in its centre a concave pit, not too large, and capable of being quite closed up by an ordinary cover-glass.”

So why is it called a Petri dish and not a Cornil and Babeș dish? Most likely Petri’s paper helped lead to the standardization of a dish with a slightly larger lid, and as more scientists began using them the name just stuck.

Julius Richard Petri may have gotten all the glory thanks to his eponymous dish, but another of Robert Koch’s assistants made an equally important contribution to the study of bacteria. Angelina “Fanny” Hesse was married to Koch’s assistant Walther and sometimes was a scientific illustrator for the lab. It was also her job to cook the beef-based gelatin Koch devised as a medium for growing bacteria.

As much as gelatin was an improvement on previous methods, it turned to liquid at 25°C (77°F) and could be digested by many of the bacteria the lab wanted to study. Thanks to a tip from a Dutch friend who had lived in Indonesia, Hesse suggested Koch try agar, a jelly-like substance made from red seaweed that is commonly used in Asian cooking. Agar’s melting point is 85°C (185°F) and most bacteria can’t break it down. It’s still used in labs today.

A 2001 study asked surgeons to fart into Petri dishes to determine whether flatulence is a risk to patients in the operating room. Turns out farts are fine, so long as everyone wears pants.

Though most associated with the study of bacteria, Petri dishes have also been important in other scientific discoveries and developments.

💊 Penicillin In 1928, Scottish scientist Alexander Fleming famously noticed some mold growing on a Petri dish full of the Staphylococcus aureus bacteria he was studying, leading to the discovery of the world’s first antibiotic.

👶 IVF Although colloquially called “test tube babies,” the children born via in vitro fertilization start life in a Petri dish, where an egg is fertilized outside the body.

👯 Cloning Similar to the process for IVF, but instead of engineering fertilization, researchers transfer the nucleus of one organism’s cell into the egg cell of another, and the new cell is allowed to develop. Despite ethical questions, the science behind cloning is saving lives.

🍗 Cultured meat Lab-grown meat involves taking animal cells and letting them multiply in a Petri dish until they grow into a large enough mass to make a burger—promising a more ethical (but so far very expensive) option for carnivores.

The American Society for Microbiology holds an annual contest for art created using agar and microbes on a Petri dish, and cites penicillin discoverer Alexander Fleming’s bacteria-based paintings as inspiration. In 2015, the winning submission was “Neuron,” created by Mehmet Berkmen of New England Biolabs and artist Maria Peñil.

“If you go to any natural history museum around the world, you’ll see dinosaurs, whales, lions, and plants, but there are no representation of microbes,” Berkmen told Quartz. “They are the most diverse organisms in the world, with the largest biomass, yet the only thing you hear in the media is bacteria do bad things and we want to change that.”

You won’t be able to create sterile lab conditions at home, but start by cleaning your kitchen and your hands thoroughly to get as close as possible. At-home culture medium recipes go for the old-school approach of beef gelatin rather than agar. In this video from the Sci Guys, they use two packages of unflavored gelatin, one teaspoon of sugar, one package of low-sodium beef bouillon, and 250 milliliters of water that’s been boiled for at least two minutes to make it sterile.

Once the dry ingredients have dissolved in the water, pour the mixture into shallow clear containers, cover tightly with a lid or plastic wrap, and stick them in the fridge overnight to set. Use cotton swabs to take samples from places where bacteria might grow in abundance, such as inside your mouth. Run the swab gently across the top of the culture medium and let it sit in a warm, humid spot. You should start to see bacteria colonies growing in a couple of days, and even more after a few weeks.

Petri’s original dish was made from glass, but today most of the vessels used in labs are made from plastic, typically polystyrene or polycarbonate. There are some advantages to this—they are inexpensive and come in presterilized sleeves for ease of use. But plastic Petri dishes are largely single-use as they cannot be sterilized in an autoclave like more durable lab equipment. There is a growing interest in returning to Petri dishes made from borosilicate glass to curb waste production.