Some of the best inventions seem so obvious after the event. The Petri dish, a circular flat dish with a lid that is used for growing bacteria on a bed of nutrient jelly, is one such innovation. It can be found in any immunology laboratory in the world and its basic design has not changed since it was first invented in 1887 by a German microbiologist called Julius Richard Petri.
Petri worked in the laboratory of the bacteriologist Robert Koch, the titan of microbiology who was the first to prove the link between specific bacterial infections and certain diseases, namely cholera and tuberculosis. Koch was able to do this by growing or “culturing” bacteria on plates of nutrient gelatin, a kind of solidified broth, which were kept under heavy bell jars to keep out dust and airborne contaminants.
Koch first demonstrated the technique in 1881 at the International Medical Congress in London. The meaty gelatin, however, proved problematic. It tended to breakdown enzymatically, was prone to discoloration and turned to liquid when heated to the kind of temperatures best suited for the growth of bacterial cultures.
A year later, the wife of one of his lab workers, Fannie Hesse, suggested replacing the gelatin with agar, which was used to make fruit jellies. Agar is a polysaccharide derived from red seaweed and proved a superior gelling agent: it only melts when heated to about 85 Celsius and yet when cooled does not gel until it reaches about 35C-42C. It also retains its clarity and resists digestion by bacterial enzymes.
But for another six years, the Koch team continued to use the heavy bell jars to keep their agar plates dust-free. That is until Petri came up with the deceptively simple idea of coving the circular glass culture plates with another slightly larger plate that acts as a transparent lid. It is perhaps remarkable that it took six years for them to devise this simple alternative to the cumbersome bell jars, but its simplicity and ease of use were immediately obvious. It meant that these pioneering bacteriologists could observe their cultures under a microscope or with the naked eye without having to expose the agar jelly to the open air each time they lifted a bell jar to get near them, with all the inherent risks of contamination.
Little has changed to the basic design of the Petri dish in more than a 100 years of use, apart from them being made of disposable plastic and the introduction of small “lugs” or tabs on the lid to allow for the limited amount of air exchange needed for bacteria to grow. Indeed, whenever something in medical science or immunology is said to have been performed “in vitro” or “in the test tube”, in reality this often means it was carried out in a culture plate known as a Petri dish.