Petri dish, what is it and how to choose it?

November 2, 2021

All laboratories need a series of instruments that allow them to carry out all kinds of experiments and evaluations. The Petri dish is one of the essential equipment within a laboratory , especially those of microbiology. There it is used to study seed germination or explore microorganisms.

It is known as a Petri dish or box. This is a round container, either plastic or glass, accompanied by a cover in the same way as the plate, only with a slightly larger diameter. This allows it to be placed on the plate and closed. This closure is not hermetic.

History of the Petri Dish

At the end of the 19th century, Julius Richard Petri accomplished what by then was impossible: studying bacteria. His method (or a container) quite simple, would prove that it was the ideal device to develop a field of cultivation that could be observed without coming into direct contact with it , avoiding the danger of contagion. Thanks to this method the plates became popular, baptizing them with his surname, today giving rise to the Petri dishes.

Who would believe that a double crystal disc, created by a doctor and microbiologist, would make it possible for man after 1877 to have the power to create an artificial universe where he could examine processes that until now were impossible for man to see? As he did? It created an optimal microclimate of bacteria to be able to evaluate their behavior.

Before the invention of Julius Petri, the cultivation of microorganisms was carried out in a liquid broth . As a result, the studio became contaminated, rendering it useless. The few advances in microbiology were aimed at very complex methods to perform that, in most cases, did not give accurate results.

Seeing such a problem, Petri had the brilliant idea of ​​placing two glass discs, with a minimum difference in size in diameter, to form a box without a hermetic closure and thus allow oxygen to enter without atmospheric debris entering it. This is how this German doctor devised a circular mechanism (which today is an essential device in any laboratory) which he baptized by adding his surname.

For more than a century, Julius Richard Petri’s unique disc has been the portal to a new universe , making it possible to examine and evaluate bacteria, especially animals and humans. Thanks to this, after studying the behavior of the cells in the Petri dish, it is possible to find vaccines against diseases that in the past were fatal to people.

More than a great inventor

In addition to inventing the Petri dish, Julius Richard Petri worked as a laboratory assistant to Dr. Robert Koch , the Nobel Prize in Medicine. This scientist showed Julius Petri the bacteriological universe, where together they conducted various studies in this field of science that they were passionate about.

In this way, it became a fundamental reference for the methodological development for microbiological practice, which allowed it to discover the tuberculosis bacillus . Koch had previously tried to find her, but was not very successful. Robert tried to study the bacteria in gelatin plates on glass jars, but the experiment failed. It was there that Petri, taking inspiration from his mentor’s idea, placed molten agar (a substance extracted from seaweed) to the bottom of a dish and covered it with an easily removable lid.

Before the Petri dish became popular, the few experiments that were performed on solid media were conducted under Dr. Koch’s primitive system. Gelatin was spread out to one side in a test tube, yet the process of obtaining separate colonies was an almost impossible task. To this was added an almost null vision into the tube. This was an inspiration for Petri.

The plate he designed was about four inches in diameter, and its edges ranged from 1 to 1.5 centimeters high. His plate had a thickness of about 0.5 to 0.7 centimeters and was covered with a round lid, with a slightly larger diameter that allowed to cover the base without making a hermetic seal. Thanks to this, an ideal environment was created for a suitable culture, with oxygen input, where it was possible to separate the colonies by dilution.

At the same time, Julius Petri’s invention made it possible to appreciate the entire surface of the culture without having to remove the lid to study the behavior of microorganisms. Regarding the study of yeasts and bacteria, it was necessary to place the Petri dish (and it continues to be done until now) resting on the incubator on the lid (or rather, upside down). This causes the gelatin to stay on top and, once the water vapor produced by microbial metabolism condenses, it falls onto the shell. This method prevents microorganisms from breaking up and continuing to develop independent colonies.

Petri dishes and microbiology

The most common use of Petri dishes is within microbiology , specifically, to make agar plates for studies.

The Petri dish is partially filled with a warm liquid. To this is added the agar and other specific ingredients for the study, which are usually: antibiotics, blood, dyes, nutrients, carbohydrates, salts, indicators or amino acids.

After the agar has cooled, it solidifies. Here the plate is ready for inoculation (or ‘plated’) with a load of microbes on a sample. The phage or virus cultures are inoculated in two stages: after being prepared with agar, bacteria are grown on the dish that develop hosts for the viral inoculum.

Incubation with Petri dishes is done upside down . This is to minimize the risk of contamination by any particle suspended in the air. It also seeks to avoid the accumulation of water condensation, as this could alter or compromise the crop.

Although Petri dishes are widely used in microbiological studies, smaller dishes are used in large-scale investigations where developing cells within Petri dishes require a high amount of labor as they are very expensive.

There is a special type of Petri dish known as a rolled plate or contact plate . This is used for the detection and counting of replicated organisms.

These Petri dishes have the peculiarity of protruding in the middle (raised agar level) the edges of the plate. This makes it easier to sample solid objects . The boxes are also divided into squares, the purpose of this is to facilitate CFU counting. They are regularly used to control the cleanliness of certain areas, such as the kitchen.

Other uses of the Petri dish

In addition to their use in microorganism cultures, Petri dishes are often used when it is necessary to weigh solids on a laboratory balance. It is also worth noting that glass Petri dishes can be sterilized and reused , while plastic ones are single use. These are discarded after use as they do not require any sterilization. These plastic plates tend to contain residues of the samples already deposited in it, so you would contaminate a next sample, the study being ineffective, losing the used materials.

These plates are similar to a small round plate. Its diameter is about 10 centimeters, and being transparent they facilitate the observance of the growth of deposited cultures with unmatched accuracy. In addition, they are very comfortable to observe from a microscope. Its use is specific to study and analyze the growth of microorganism cultures.

At the same time, they are essential equipment to be able to analyze the cultures that have been isolated. This makes a much more precise analysis possible, ensuring that they have not been exposed to external agents . Isolation is possible thanks to the lid that covers each Petri dish , allowing the cultures to be safely protected.

How to use the Petri dish

The Petri dish does not work under an established mechanism that makes it work. You only need it to be glass or plastic and that its closure is not hermetic in order to keep living organisms in a neutral environment. So that they can develop and notice the changes.

When carrying out a culture it is necessary to place the plate upside down , so that the culture grows downwards, towards the lid. After completing the time established by the study, you can turn it over and appreciate the entire microbial environment that has developed inside the Petri dish.

Precautions when using the Petri dish

If you are thinking of doing microbial culture studies, you should take into account the following:

  • Before using the plates, you must submit them to an oven heating process. This process sterilizes them and will prevent your sample from becoming contaminated.
  • Pour the liquids and compounds to use in the study, but only halfway, you should not fill the Petri dish.
  • Do not rush, allow the time necessary for the condensation of the components to pass. If you used agar-agar, then you should turn the plate upside down since the evaporation of the water causes it to solidify.
  • When the time is up, incubate the organisms you want to study on the plate.

It is very easy to carry out cultures with the Petri dish. You just have to pour the liquids and microorganisms, cover and wait for results.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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