Petri dish, and Richard Julius Petri the scientist

At some point, you may have heard about a Petri dish and have the image that it is some sort of lab tool to grow things, usually bacteria. If you haven't, that's about the simplest description that can be given. By "dish", the term actually means a flat circular container that has a cover which rests on it to keep out airborne contaminants.

The man credited with its design, Richard Julius Petri, was born in Germany on May 31, 1852. Both his father and grandfather were professors of various subjects. After high school, he studied at Kaiser Wilhelm Academy for Military Physicians from 1871 to 1875, did some work as a military physician, then finished his doctoral degree in 1876 in Berlin. The title of his dissertation was "The Chemistry of Protein Urine Tests". At some point in his life, he joined the Freemasons.

He continued his military physician work until 1882. During 1877-1879, Petri was assigned to a research facility called the Kaiserliches Gesundheitsamt (Imperial Health Office) in Berlin, where he became a laboratory assistant to the famous bacteriologist and physician Robert Koch. Koch's research team was producing great results in developing revolutionary laboratory methods to study the causes of infectious diseases and find their causes.

Petri and an example of his self-named dish with a bacterial culture on it

As an example of their research, they studied such things as tuberculosis, anthrax, and cholera, all devastating diseases at the time. To grow samples from humans, animals, or the environment in order to detect bacteria in them or to measure their growth properties, researchers like Koch's team would put them in rich nutrient solutions like beef broth. It had been only 17 years earlier when Louis Pasteur created the first actual broth recipe to grow bacteria, using a mixture of yeast, ash, sugar, and ammonium salts in 1860. Other recipes were soon developed to grow bacteria or to identify them based on color changes or production of gas as the germs grew.

These broths were commonly sterilized by boiling then put into glass test tubes with cotton plugs after they had been inoculated with test samples. If certain dyes were in the broth, bacteria that made acid would change the pH and color of the broth. If they also made gas as they ate nutrients, the tiny bubbles could be trapped inside a Durham tube flipped upside down in the tube.

Color changes and gas production in bacterial broth tubes (from asm.org)

Some bacteria need oxygen in different concentrations or not at all. Growing bacteria in a tube of broth was sometimes convenient to identify this property and add to the list of characteristics of what a scientist was examining. A non-inoculated tube would be clear, but based on the location of bacteria growing in it, the scientist could tell if the bacteria required atmospheric levels of oxygen (growth only at the top), a little oxygen (near the top), or no oxygen at all (growth only at the bottom). Some bacteria were able to grow with or without oxygen, so a tube would show cloudiness through its length.

Tube growth (from Northern Arizona University microbiology course study guide)

Pigments made by the bacteria were also an important tool to distinguish between some bacteria grown on solid or semi-solid surfaces. Before looking at them under a microscope, if a scientist could see their color in large colonies growing on potato slices, bread, coagulated egg whites, etc., it would help identify them more quickly. For example, Bartolomeo Bizio studied "blood spots" on communion wafers in 1832, and the distinctive red was one trademark of the bacteria Serratia marcescens, later grown in the lab on bread chunks. Molds have also been commonly seen in prominent colors from snow white to blue-green to jet black.

"bloody bread" with bacteria; moldy bread

But using foods was not always convenient for lab work, nor did it show colorless bacteria very well. Some bacteria didn't even grow on food surfaces. Robert Koch's lab developed many carefully designed nutrient recipe broths mixed with gelatin which hardened into a semi-solid jelly that could be smeared on glass slides and inoculated with test samples then examined under microscopes. But this presented a few problems.

• Incubating them at body temperature melted the gelatin and made it fall off the slides.
• Some bacteria digested gelatin, and the result was changing it to a cloudy liquid that ran off slides.

In 1881, Koch made a "moist chamber" with a glass container that contained a thin layer of gelatin containing the bacteria smeared onto a microscope slide resting on wet filter paper. (Today's incubators provide humidified air for all cultures, so the filter paper is no longer necessary.)

Koch's moist chamber

While working in Koch's lab, Richard Petri noticed the problems people had with growing bacteria in these moist chambers or in stacks of glass plates under heavy glass bell jars. 

Bell jar and one glass tray to go inside stacked on others

So, he suggested pouring the agar nutrient mixture recently devised by Fanny and Walther Hesse (also in Koch's lab) into the glass dishes to make a layer covering the entire bottom, and then covering them to prevent airborne contamination. The agar was harder than gelatin, would not melt as easily, and was not eaten by bacteria. The dish with bacteria growing on it had an additional benefit; it could be comfortably put on microscopes for closer examination or counting. Petri modestly published this in a 300-word paper in 1887, "A minor modification of the plating technique of Koch".

Similar work with covered dishes had been done at nearly the same time by other people. In 1885, William Nicati and Maximilien Rietsch grew the bacteria Vibrio cholerae in what they called godets (“jars”) with nutrient gelatin. They described these as similar to parts of a pill-box, but they were much smaller than Koch's moist chamber, and they didn't publish any pictures of them. Researchers like Percy Frankland and Walther Hesse were interested in studying bacterial contaminants of the air. In 1886, Frankland published a paper where he showed a deep glass dish that had a thin layer of gelatin (not agar) coating its bottom. This was exposed to air in various locations to collect bacteria, then the cover was placed on it before incubation. 

Frankland's dish, 1886

Romanian Victor Babes and Frenchman Victor Cornil collaborated on studies in bacteriology and wrote a definitive book on the topic. In their 1890 edition, they claimed to have invented the culture dish before Petri, but they had never mentioned it in their 1885 or 1886 editions, so this passed unnoticed. Moreover, their 1890 drawing showed a different design than Petri's with a slanted body to the dish, plus the need to use rubber bands to hold the cover in place. So, the credit has gone to Richard Petri simply because he took the time to write a short article to describe his idea.

A year after Petri's publication, Isidor Soyka and Frantisek Král described another type of dish with a cover which they used to grow potato slices and then seal up the cover with wax to prevent it from drying out.

Soyka and Král dish for potato slices, 1888

Today, Petri dishes are used for a variety of laboratory experiments. They may still be glass, but the majority are plastic. They may be built in several diameters, and some even contain sections in them for various media use at the same time. Not only are bacteria and mold grown in them, but they may be used to test antibiotic resistance. Some Petri dishes even have grids on them to aid in counting. Some are used as dissection containers. 

Petri dishes today

From 1882 to 1885, Petri oversaw the Brehmerschen Göbersdorf tuberculosis sanatorium in what is now Poland. He was considered quite strict to everyone around him, perhaps as a result of working for an equally strict Koch or his own military training. He enjoyed wearing his military uniform of chief army doctor whenever he could, including a sash that accentuated his large belly in later years.

For a short time, he was director of a Berlin museum of hygiene, and then he returned to the Imperial Health Office until he retired in 1900. During his lifetime, Petri published 150 papers. One of the most interesting titles (and topics?) was "The microscope from its beginnings up to the present perfection for all lovers of this instrument", published in 1896. Petri was married twice and died on December 20, 1921.