Science Buddies: "Ask an Expert"

I have been doing a sodis experiment for my science fair. The problem is that it is not working. I took samples from 2 ponds put them in clear Pete1 bottles, placed them on foil in the sun for 7 hours, used sterile swabs to streak on a nutrient agar petri dish and now there is lots of bacteria growing, what could I have done wrong? The bottles weren't totally full, would that make a difference, should I not have put them on foil and is it alright to have used nutrient agar? Also I just want to know how to tell the difference if it is bacteria or algae growing, will algae grow on nutrient agar, and how to tell the difference? I really need help - Thanks!

Hello there,

I think it will be helpful for me and all of the other experts if you could provide more details about your project. For example, what is the main question you want to answer, what is your hypothesis, and what kind of experiment did you set up to test your hypothesis? From what you had written, I'm guessing you want to see whether leaving water in clear bottles in the sun for 7 hours would be able to sterilize the water. Here is a Science Buddies project that follows a very similar idea and I think you would be able to get some ideas on how to troubleshoot your experiment to try to get it to work. https://www.sciencebuddies.org/science- ... ml#summary

For example, the introduction of that project mentions, "The SODIS process is easy to follow. The first step is to find a PET bottle with a lid and clean it well. PET bottles are recyclable and have a "1" surrounded by a triangle symbol on the bottom. Then fill the bottle ¾ full with water. The water should not be too turbid. Highly turbid water protects the pathogens from the Sun's radiation. Shake the bottle for 20 seconds to aerate it. Now fill the bottle fully and close the lid. Place the bottle on a black iron sheet or on your roof, where it will have access to sunlight. Leave it in the sunlight. After a certain amount of time (which you'll be investigating in this science fair project), the water will be ready to drink." Did you follow these steps when you were preparing your water samples? One thing I could think of is that perhaps your pond water was really turbid, and that may affect the time it takes for the sun to actually sterilize it? Perhaps you need to leave the water samples out in the sun for longer than 7 hours to fully sterilize it? Was the sun out the entire time of the 7 hour treatment? Why did you decide to try 7 hours and not any other times?

You should be able to grow algae on nutrient agar--the only problem is whether the "nutrients" in the agar you're using is something that the algae can use to grow. I don't know much about how you can distinguish between algae and bacteria on an agar plate, but I would think that green or other colored algae (if they are able to grow on your plate) would be pigmented, and their morphology would probably look different from bacteria. Other experts have any other ideas? Regardless, I don't think it's too important whether you distinguish whether the things on your plates are algae or bacteria, but the important thing is whether or not there are still microbes in your water after the sterilization process. If there aren't any microbes growing on those nutrient agar plates, then that suggests that your sterilization process likely worked well, but if there are still microbes, I would think not. I think an important thing to do is to have a negative control. For example, you can take samples from the same pond and put them in two different bottles. One will be exposed to the sun, whereas the other would not. Afterward the sun exposure, you can plate samples from both bottles onto a plate and compare the numbers of microbes that grow on the plates. The water that hasn't been exposed to the sun should have plenty of microbes growing, whereas the sun-treated water should have none to a decrease in microbe numbers compared to the unexposed one if your sterilization process worked.

Hope that helped! Read through the Science Buddies project link that I posted--I think you would find it very useful!

Best,
Connie

Thanks for your answer. I have followed all the steps including the negative control. My experiment is basically to check that the Sodis method does work, to be able to be used in times of natural disaster. I think my main question is: even after using the Sodis method, will there still be a small amount of bacteria growing because this is what I found after doing the experiment. I guess I was expecting no bacteria to grow and there was, obviously not as much as than in the negative control. But this is showing me that bacteria will still be present and how do we know that this is not bad bacteria that could still make you sick? The reason I was asking about the algae was that I thought that maybe it was algae that survived and this was what was growing, but on closer inspection I do believe it is bacteria because it looks very much like the negative sample bacteria, except for one colony that looks very weird, with fibers growing out.
Anyway, just need to check that even after doing the Sodis method there will still be bacteria that survives? Thanks!

Hi there,

Interesting. So the fact that you have bacteria still growing could be due to one of several reasons: 1. You needed to leave the bottles of water in sunlight longer. 2. Your pond water was too turbid. 3. The intensity of the sunlight you're getting could affect the amount of UV light your bottle is receiving. 4. Given the conditions you've tried, there are still some bacteria that couldn't be killed off. However, the fact that you see less bacteria after the treatment compared to the negative control is a good thing! But, your question about whether these bacteria are harmless or harmful is a great one--it's hard to tell by morphology alone and you would probably need more sophisticated molecular techniques in order to identify what these bacteria are. Interesting that you also have a colony with fibers growing out--that could be fungi.

You can probably do a bit of research on the SODIS method and see if anyone has done anything in regards to checking whether bacteria are still present in water after treatment with the method, and if there is, how did they do that? Did they also identify what species of bacteria are present? This could perhaps give you more insight as to what you are seeing.

Let me know if you have anymore questions!

Connie