The Petri Dish and It’s Story

December 1, 2020

From the first antibiotics to the growth of stem cells, from observing micro- organisms to the testing of new medicines — it’s all possible thanks to the Petri dish, a short, flat cylinder cove- red with a transparent lid of a cor- responding shape. This humble glass plate has its own fascinating story.

ISSUE: #01 DECEMBER 2020

Topic Last Updated on 04-07-2024

A Pure Culture

Post on topic: Petri Dish

In 1872, Robert Koch, who would later become a renowned microbiologist, was named the District Medical Officer in Wollstein (now Wolsztyn in Poland), where an anthrax outbreak was running rampant at the time. The disease was known even back in antiquity when it was called “sacred fire”: people thought that only furious gods could have sent such a scourge to Earth. Anthrax was a threat to all agricultural settlements — more often than not, livestock were infected. However, animals were not the only ones to die: the illness also claimed the lives of farmers, shepherds, and milkmaids.

Robert Koch (1843–1910) was a German microbiologist. He discovered Bacillus anthracis, Vibrio cholerae, and Mycobacterium tuberculosis (Koch’s Bacillus). In 1905, he was awarded the Nobel Prize in Physiology and Medicine “for his investigations and discoveries in relation to tuberculosis.”

Looking through a microscope at the blood of the deceased animals, Koch discovered that the development of the disease could be blamed on a single Bacillus microbe (Bacillus anthracis). The researcher successfully isolated the bacteria and was able to grow a pure culture from it — a culture that is composed of a single type of microbe. He infected an entirely healthy animal with this pure culture, thus giving it anthrax. The scientist understood that the creation of a pure culture was the key to successfully determining the cause of an infection.

Koch’s Quest for Pure Bacterial Cultures

Koch, like his predecessors, cultivated bacterial cultures in liquid broths made from meat or cereal grains. While he successfully isolated Bacillus anthracis in a pure culture, he sought a more reliable method. Contamination by multiple bacterial species made separation challenging. To isolate a single species, Koch repeated the process of transferring a small drop from the most concentrated solution to a fresh broth several times, gradually reducing foreign bacteria.

Before Robert Koch, researchers believed that microorganisms were colorless, which led to numerous mistakes. Koch used aniline dyes, which selectively stained microbes. After Koch’s experiments, researchers around the world began to use this technique of dyeing bacteria. Then, in 1884, Dr. Hans Christian Gram developed a method of staining that became the primary approach to determining the presence of a type of bacteria in a given substrate

WHY DO WE NEED PURE CULTURES?

Microbes are very small (0.5–5 μm on average) and do not differ much in appearance, which causes difficulties in studying them. For research, it is necessary to single out one microbe from the surrounding world, which is filled to the brim with radically different microorganisms. Microbial cells, after landing in the growth medium, produce offspring — a cluster of identical cells called a colony, which can be studied like a single microorganism. It turns out that, by manipulating the conditions of cultivation, it is possible to obtain a pure form of the cells of any microorganism. Thereafter, you give it a name, describe its properties, and classify it. Thanks to Koch’s discovery, microbiology has been recognized as a science in its own right.

The Petri Dish | A Tough World

After his report on the discovery of the agent that causes anthrax, Robert Koch was invited to head a laboratory at the Hygiene Institute in Berlin, and he was offered the post of Advisor to the Imperial Health Office. Koch suddenly had good equipment, talented assistants, and the ability to solve a mystery that had long haunted him. It was known that tuberculosis was also caused by some kind of microbe: the tissues of a sick person could infect healthy animals. Koch managed to find a technique for staining prepared tissues to discern the pathogen bacteria under a microscope. But his joy didn’t last for long — the bacteria did not want to grow in normal growth media.

One day, the scientist noticed that a pile of moldy potatoes left on the table had an abundance of colored spots—colonies—of gray, yellow, and green. He collected samples from each colony, and he noticed under the microscope that each spot was a colony of a single species of microbe! In liquid media, the microbes mixed and separated, making them extremely difficult to work with. But in a solid medium, they stayed in a single spot, multiplying and giving birth to a pure culture!

Koch’s accidental observation fomented a revolution: fresh potatoes became one of the first solid media for cultivating microorganisms. However, such a nutritious substrate was not suitable for all microbes, and, therefore, the search for an alternative solid medium continued.