The study of microorganisms is foundational to modern biology and medicine, leading to advancements in treating diseases and in various industrial applications. Scientific progress often hinges not on complex machinery, but on simple tools that solve a basic problem. The development of a reliable way to grow and isolate microorganisms is a primary example of such an advancement, paving the way for the field of microbiology.
Early Culturing Challenges
In the late 19th century, the laboratory of German physician Robert Koch was a hub of bacteriological research. Scientists at the time faced the challenge of how to separate and grow individual types of bacteria. Early methods using liquid broths resulted in a mixture of different microbes, making it impossible to study a single species. The first attempts at a solid surface involved using potato slices or solidified egg albumen, but these had limited nutrient sources.
A step forward was using gelatin to solidify nutrient broths, which could then be poured onto flat glass plates. This technique had a flaw, as the gelatin would liquefy at 25°C, a temperature too low for incubating many human bacteria. The solution came from Fanny Hesse, the wife of Koch’s associate Walther Hesse. She suggested using agar, a substance from seaweed that remained solid at higher temperatures. While agar solved the growth medium problem, contamination from airborne particles remained a challenge.
Julius Petri’s Contribution
Julius Richard Petri, an assistant in Koch’s laboratory, solved the contamination and usability problem. In 1887, he introduced a modification to the plating technique: a shallow, circular glass dish with a slightly larger lid that fit over the top. This design created a barrier to the outside environment.
The new dish prevented airborne microbes from settling on the nutrient agar while still allowing gas exchange for bacterial growth. The transparency of the glass allowed researchers to observe colony growth without exposure. The flat, stackable design also made the dishes efficient for laboratory use, an improvement over the previous bell jar method. Petri’s contribution was not the growth medium, but the container that made the method practical.
A Revolution in Microbiology
The combination of the agar-based medium and Petri’s dish created the culture plate method, which revolutionized microbiology. Scientists could now reliably isolate microorganisms from a mixed sample. By spreading a diluted sample onto the agar surface, individual bacterial cells could grow into distinct, visible colonies, with each colony representing a “pure culture” descended from a single ancestor cell.
This allowed researchers to study one type of bacterium at a time, documenting its unique characteristics. This breakthrough helped establish the germ theory of disease, providing Koch and his contemporaries the means to link a specific microbe to a specific disease. Using this method, Koch isolated the causative agents of illnesses like tuberculosis and cholera, providing proof that microscopic organisms caused sickness.