I'm going to show a UV-purificator for water on science fair. How can I easily check if purificator kills most of the bacteria? I need a quick and possibly easy method.

  • $\begingroup$ How did you use to test for whether your UV-purificator works? Did you just assume it works? $\endgroup$ – Remi.b Feb 28 '16 at 19:42
  • $\begingroup$ Well... I will also check if it works using this method. It's a student project so we can just follow the descriptions on the internet and hope that it will kill bacteria. And even if it won't work with 100% efficacy, we just want to show killing effect of UV. $\endgroup$ – user46147 Feb 28 '16 at 19:58
  • $\begingroup$ There are not many quick inexpensive methods that can show the number of dead/viable cells. Staining+microscopy is a quick method but I am not sure if you can obtain a microscope and the stain (whether the school will fund you for that). Moreover counting bacterial cells directly would require some patience and expertise. Colony based methods (plating) are convenient but would take time. $\endgroup$ – WYSIWYG Feb 29 '16 at 5:12

The medical field uses Limulus Amebocyte Lysate (LAL) strip technology for parentarel pharmaceutical and medical devises. The LAL strip will detect PAMPS,Bacterial endotoxins and substances with elicit inflammatory responses in mammals. The technology derived from the special blood of the Horseshoe crab AKA "The Living Fossil" it lived as we see it today without evolving 450 million years ago.


  • $\begingroup$ Do these strips distinguish between dead and alive bacteria? And how quick is the assesment? I like your answer since I never knew about such strips :) $\endgroup$ – user46147 Mar 7 '16 at 18:40
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    $\begingroup$ They work instantaneously and dead bacteria can not regenerate and is not of any harm. The only way to determine before the test would be to buy a bottle of Fiji water pore it on a strip and see if the color has changed. Just make sure all the equipment you use are sterile before the test. Do about three or four full practice trials before the final observation. Good luck to you and your beautiful mind. $\endgroup$ – user5434678 Mar 7 '16 at 19:18
  • $\begingroup$ Can you tell me example of commercial name of said bacteria detectors? I just want to be sure to check for things you actually mention to avoid potential confusion. $\endgroup$ – user46147 Mar 7 '16 at 19:38
  • $\begingroup$ I can make the plans for your entire experiment scaled to proportion with every measurement on Auto CAD for you too, Free of charge I am a civil engineer $\endgroup$ – user5434678 Mar 7 '16 at 22:23

Not having the equipment needed to actually count bacteria, you can use things you know bacteria does to find out how much they were affected.

My approach would be thus:

  1. Select a model bacteria. There are a few candidates, but probably the most readily available is lactobacilli, which can be found in yogurt and (loosely translating terms of art) is a Gram-stainable fermentor-sometimes-breather.
  2. Infect the water with a known quantity of bacteria and a food source (lactobacilli eat lactose - ie from milk) and equally divide it into a control and experimental group...
  3. You'll need to find an alternative way to estimate the bacterial content in the volume of water used compared to a control colony. Since lactobacilli produce lactic acid, you could use the pH value of the water (either with an indicator or a meter) to detect bacterial metabolism, compare that to the control's pH value.
  4. Record the UV exposure time from the experimental group, and isolate the control group from any potential UV light source, such as the sun.

Even if you find that, for example, your particular strain of lactobacilli is not sensitive to your UV light source, that's a perfectly scientific result! How did you obtain it? The source likely knows what kind of bacteria it is. Perhaps the UV light merely interferes with bacterial metabolism, and doesn't kill it. Unanswered questions are wonderful, you may have provided a place for further research to take place. You could then use the two groups to infect a food source (ie milk) and compare post-exposure growth, depending on how much time is available.

  • $\begingroup$ I think the main point that is left unaddressed is regarding the technique that can be used to quantify viable cells in the sample after treatment. Your answer as of now focusses only on how to set up the experiment properly. $\endgroup$ – WYSIWYG Feb 29 '16 at 8:46
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    $\begingroup$ OP is looking to "check if purificator kills most of the bacteria" - viable cell count is probably overkill for that. I suggest a simple comparative approach. Dead bacteria don't behave like live bacteria, so one only need compare the chemical content of the water containing "possibly dead" and "known live." Say there's no difference - it wouldn't make sense to do a cell count at all. You would expect the known live bacteria to do more than a dead one, thereby changing the chemistry of the water. $\endgroup$ – jzx Feb 29 '16 at 13:50
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    $\begingroup$ OP asked whether most bacteria are killed. So you count the live ones remaining to determine that. That's how it is usually done. The dead ones can also possibly cause harm because of endotoxins. Even in that case an explanation of an existing methodology to quantify the "chemical content" is, IMO, essential. $\endgroup$ – WYSIWYG Feb 29 '16 at 17:57

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