I know that petri dishes with a growth medium are used to grow micro organisms. I guess this works as follows:

  1. The petri dish has to be kept sterile. To make it easier, I guess one could cool it.
  2. The microorganisms are somehow put on the petri dish.
  3. The petri dish is put in an incubator (e.g. 37 °C if one wants to simulate the human body).
  4. After some time (how long?) one can look at the colonies.

Does one only look at them with the naked eye or is there a way to make more detailed studies?

For example, when I have an illness, can the doctor (for some illnesses) simply use this petri dish / naked eye technique to confirm a hypothesis what kind of illness I have?

  • $\begingroup$ What exactly is your question? Please read biology.stackexchange.com/help/dont-ask Petri dish with appropriate medium can be used to count number of cells in liquid culture. You spread 100uL of liquid and after incubation count colonies. Each colony comes from exactly 1 cell if you dilute liquid culture enough. $\endgroup$ Dec 26, 2015 at 20:49
  • $\begingroup$ My answer to this question, biology.stackexchange.com/questions/40750/… , may give you an idea of the steps that might be taken to prevent contamination when working with plates, though in a diagnostics setting, it is unlikely that the plates will have an antibiotic on them. Also in a modern clinical laboratory, there would be other methods used, but the basic principles of asepsis are still the same, keep your work area clean and do your best to prevent any airflow onto the surface of an uncovered plate. $\endgroup$
    – AMR
    Dec 27, 2015 at 6:13
  • $\begingroup$ Also a trained technician or clinical scientist will recognize the morphology of the bacterial colonies that grow on the plate and whether or not they grew where the plates were streaked or on an unstreaked area. Contamination will pretty much cover the plate where as sample will only grow where the plate was streaked. $\endgroup$
    – AMR
    Dec 27, 2015 at 6:18
  • $\begingroup$ I think this is too broad, there are various uses depending on the objective which is probably an extremely long answer, if you have a question about how to test for specific diseases using petri dish methods that would be better. Petri dishes are just a piece of very broad use lab equipment. $\endgroup$
    – rg255
    Dec 27, 2015 at 11:08

1 Answer 1


I think the petri dishes you are referring to are also called agar plates. Agar is a 'growth medium' for microorganisms. It provides the nutrients that the microorganisms need to survive. It can also contain main other 'things', such as antibiotics or pH color change indicators. There are many types of agar, for example, tryptic soy agar is used to grow E. coli. I think the oven you are referring to is called an incubator. Incubators provide the atmosphere, particularly temperature, needed for the organisms to grow. There are many types of incubators. Some microorganisms actually cannot grow in oxygen rich environments, and some only grow at specific temperatures. These organisms must be incubated under their preferred conditions in order for them to grow.

And yes, a lot can be learned from simply looking at the agar with the organisms on it. What exactly can be learned depends on the growth medium (agar or other medium).

To summarize, petri-dishes are just the plastic dishes that contain growth medium. And incubators are usually refrigerator looking things that provide the proper temperature for the organisms to grow. The goal is generally to mimic the natural environment of the organism, but as I stated, scientists often add many other things to the environment for experimental and diagnostic purposes.

Here is a list of common agars used in bacteriology http://learn.chm.msu.edu/vibl/content/differential/

  • $\begingroup$ Oh, as for the time, this depends on the organism. Some grow very fast, E. coli replicates about every 20min under 'log growth' conditions, and some grow very slow, like Mycobacerium tuberculosis. The rate at which the organism replicates is highly dependent on the environment in which the organism is living. Some bacteria only grow at very high temperatures, like Thermus aquaticus, which was isolated from a hot spring in Yellowstone National park. $\endgroup$
    – DonJulian
    Dec 26, 2015 at 20:56
  • $\begingroup$ Also, cooling a petri dish will not sterilize it. In fact, low temperatures are often used to preserve bacteria. To sterilize a petri dish, one must autoclave the petri dish. An autoclave is like a big steaming pressure cooker. $\endgroup$
    – DonJulian
    Dec 26, 2015 at 20:58
  • $\begingroup$ I did not think that cooling a petri dish will sterilize it. I was talking about keeping the petri dish sterile. My thought was that when you produce a petri dish, it will probably get "contaminated" by just being surrounded by bacteria. Or that "sterile" probably only means that there are very few bacteria on the plate. $\endgroup$ Dec 26, 2015 at 21:02
  • $\begingroup$ Why is it called 'log growth' condition when the number of bacteria grows exponentially (with respect to time)? $\endgroup$ Dec 26, 2015 at 21:03
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    $\begingroup$ Hopefully Im not talking too much, but to answer the question of sterility, sterile by definition means no organisms. Sanitary is often used to describe what you were saying - very few organisms. $\endgroup$
    – DonJulian
    Dec 26, 2015 at 21:34

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