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If I grow microorganisms on agar plates and I expose them to UV light. How exactly will I know that the microorganisms have been killed? I was watching a video that showed spots in the petri dish where the microorganisms were killed when viewed under microscope but will this be visible to the naked eye? If not, is there any way that I can show without the use of a microscope the fact that the organisms have been killed?

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  • $\begingroup$ It would help if you speficied the microorganisms in question. Some fungi grows when exposed to certain wavelengths of UV light $\endgroup$ – Chimango Chisuwo Jan 12 at 12:15
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Looks like this is a follow up to this question on this forum.

The short answer to your question is you will not be able to indicate that a cell or colony on an agar plate is dead even with a microscope unless you are using fluorescent dyes (i.e., some form of live/dead staining). There are quite a few ways to do this, but they will all require that you have access to a microbiology laboratory.

Now, if your objective is to simply show that UV light kills microorganisms, this is easily done as an experiment with one target microorganism (i.e., one species). I'm going to obfuscate details aplenty here, but I'll offer you a description anyway.

You'll need replicate plates of agar colonies inoculated with the same quantity of the microorganism--this is typically achieved by propagating the microorganism in liquid medium and depositing equal quantities of the medium onto multiple agar plates. Once your plates have been incubated to obtain some colonies, retain one plate as control (this will not be exposed), and expose the remaining plates to your UV source. [Ensure that you've counted the number of colonies in every plate--including the control--before you subjected them to UV irradiation.]

Now comes the hard part. You will need to re-suspend the colonies from your plates in liquid media and re-plate them on agar. You can either choose a single colony from each plate (ideally, colonies that are similar in size across plates) for re-suspension, or you can re-suspend all colonies visible to the naked eye. Either way, re-suspend your colonies in liquid media, and deposit them on agar thereafter, and incubate the agar plates.

If everything went right, the number of colonies you would observe in the re-plated control plate should be higher than the number of colonies in the UV-exposed "re-plates." What that indicates is that at least some of the colonies that were exposed to UV irradiation died, and thus did not show up as colonies upon re-plating. This would loosely "prove" that exposure to UV irradiation kills microorganisms (with many, many caveats).

All that being said, I would not attempt this at home. To the best of my knowledge, this shouldn't even be doable at home without spending a significant amount of money to obtain some of the necessary equipment (which I have not described here). Further, doing this experiment in the format I described would likely yield very noisy results--the way I've described it is more back of the envelope experimentation than something proper.

If you are keenly interested in this topic, find the nearest microbiology department, and talk to a subject expert. They might even let you try this experiment out for yourself in their lab.

Edit - the core methodology I've described here is CFU counting (CFU - colony forming units).

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    $\begingroup$ Why bother inoculating liquid media when you can just replica plate onto fresh plates? $\endgroup$ – Cell Jan 12 at 19:07
  • $\begingroup$ You could of course, but picking colonies is already messy enough, and I just wanted to offer a solution where the user didn't inadvertently end up streak-plating and end up with even more noisier data. $\endgroup$ – Dunois Jan 12 at 19:17
  • $\begingroup$ There is no picking colonies with replica plating though nor streaking? And you can easily calculate proportion of viable cells by looking at the original plate and the replica. $\endgroup$ – Cell Jan 12 at 20:07
  • $\begingroup$ Sorry, I misread (and also misunderstood) your comment. I read that as "replicate", and didn't catch on (and I thought you were referring to colony picking). Would this actually work though? Per my understanding irradiation would not uniformly inactivate entire colonies, and it looks to me that replica plating an irradiated plate would yield an erroneous CFU count. $\endgroup$ – Dunois Jan 12 at 22:00
  • $\begingroup$ I'm not sure, that's why I asked you the question. If the UV radiation does not kill off all the cells then how would culturing colonies work? Wouldn't you just be growing up the survivors? $\endgroup$ – Cell Jan 12 at 22:31

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