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Name: Nicole 
Status: student
Grade: 9-12
Location: Outside U.S.
Country: Canada
Date: Fall 2012

I am doing water purification for a science project. For a part of my project, I am going to test for natural substances that can kill bacteria in water. I have a few substances that I think may be able to kill bacteria, such as garlic, vinegar, etc. I want to try these substances on pond water. I want to be able to see exactly how much bacteria each substance can remove. I am thinking I can first measure the amount of bacteria in the original pond water sample, then add the substances into pond water that is taken from the same source, and then measure the amount of bacteria in the 'disinfected' water to see how much of an effect the substances have on bacteria. However I do not know how to measure bacteria at all. I have some ideas, and I think maybe using nutrient agar plates is a good idea, but I have no idea how they work. I am also concerned because I wonder if the bacteria in pond water will decrease/increase if I leave it for a long period of time? I know in order to count the bacteria from agar plates, I have to let them grow first. If I store pond water in a large bottle, then take out a small amount and let the bacteria in it grow to measure the bacteria in pond water, then take some pond water from the stored bottle and put the testing substances in that water, I wonder if the pond water that was left in the bottle will have a change in the amount of bacteria during the period when I was growing the bacteria in the original pond water to figure out how much bacteria there was originally. Does that make any sense? I want to make the experiment as fair as possible.. If the last part was a bit confusing, it is okay. I just want to know how exactly I can measure the amount of bacteria in the water. All the information I have googled was very confusing to me.

Hi Nicole,

This sounds like an interesting investigation, but I would warn that exposing one-self to bacteria, some of which may be harmful to humans, can be dangerous.

Professional labs use autoclaves which can effectively kill bacteria. You can google autoclave to see how they work.

I don't have specific information on you question. In general, I would simply comment that bacteria often create a slime around themselves to protect themselves. The slime can be wiped away in order for chemicals to actually reach the bacteria, otherwise they might remain safe in their slime. Consider this as the reason why, when we clean the toilet, we need to use a brush to effective get at the bacteria....even simple treatment with a chemical cleaner might not effectively kill or remove them.

If I were in your situation I would make an appointment to visit the nearest university or college that has a good reputation for research in Biological topics. They can expand on some of the challenges and difficulties you might experience, even with a lab like they operate. They might be able to provide some guidance on a slightly altered experiment/investigation that would be safe for you to perform.

Please read the following warning, provided by NEWTON BBS:

NEWTON BBS does not recommend growing/culturing bacteria without the supervision of a microbiologist, and a properly equipped microbiology laboratory. Safety is our main concern! Growing dangerous bacteria species unknowingly is a real possibility and serious illness may occur without proper handling techniques. Furthermore, without proper bacterial disposal procedures such as an autoclave can guarantee, there is a danger to anyone who comes in contact after disposal.

Thank you for using NEWTON! Ric Rupnik

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