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How can we test the effectiveness of rubbing alcohol in killing bacteria?
Question Date: 2011-07-07
Answer 1:

I am sorry this has taken awhile to get back to you. Here are some instructions for an experiment that might take a full day (lots of waiting time) to complete.

You will need to be very careful about how you measure the volume and make sure everything you use is as clean as possible (including cleaning the sink before you clean your equipment).

This experiment is not how scientist testing lysol and other cleaning products would test the effectiveness of killing bacteria, but it is significantly cheaper and safer. Remember one reason why we care about killing bacteria is because it can make us sick, and to really test how well a product kills bacteria you have to have grow it and count it which requires handling it (along with $600 worth of volumetric equipment and microscopes and several years practice recognizing what a particular strain of bacteria look like under the microscope.)

So, this will not be a test where someone sneezes on a counter and then someone wipes it off with alcohol and you "see" what germs are left like the TV adds. But it will be a good way to measure how many bacteria you can kill with a certain volume of alcohol or other cleaning product. What you need first is a reliable source of bacteria that you know what that bacterium is. The best choice is fast acting yeast from the store. This may sound silly but many scientists do initial tests on cleaning products with yeast, so it is actually pretty valid. Yeast is also a good choice because you do not need a microscope to count how many yeast you have killed or have to worry too much about bacteria from other sources are growing instead and ruining your results. The reason why yeast is easier to measure is because when yeast eat (sugar or carbohydrates) they burp a lot CO2 as a waste product, whereas, other bacteria dont give off nearly as much. This is actually why we put yeast in bread dough and make it rise. If you capture the CO2 and measure the volume of gas given off, it is a good measure of how many yeast bacteria you have or have killed off. To do this in a measurable way you need to make sure that there is the same number of yeast in a container that has alcohol or other cleaning product as a similar container that doesn't. This allows you to compare the volume of gas produced in each one to tell how many died from the alcohol. This test will not give you numbers like "1 teaspoon of alcohol killed 3500 yeast" but will give data like "1 teaspoon of alcohol decreased the volume of CO2 produced by 5%, 2 tea spoons reduced it by 10%..."

Now that you have a good bacteria source you need to decided what type of efficiency you are going to test. Is it the volume of alcohol to kill all of the yeast? Is it going to be a test between different cleaning products to see which one kills more yeast ( ie. 2 table spoons of alcohol killed 25% of yeast and 2 table spoons of lysol killed 50%). What ever type you do you will need to have at least three tests where every thing is the same to get an average since one test can always go wrong and give you funny results. (As an example, three with no alcohol, three with 1 table spoon, three with 2 table spoons.)

Here is what I would do to test how much alcohol is needed to kill a certain percentage of yeast.

-12 soda or beer glass bottles (all the same kind, preferably the same brand and flavor of soda, but definitely not a mix of brown and clear or green bottles. The yeast may grow faster in one color and throw off your results. Clean them very well with very hot water and soap, rinse them well.
- 12 medium parry balloons. (These will trap the CO2 given off by the yeast. Small ones might not hold enough gas and the long skinny ones that they make animals with need too much pressure to fill up.
-12 good rubber bands that can wrap around the neck of the bottles several times. (Will hold the balloons in place and will help you seal the balloons for testing.)
- A plastic soda or water bottle with a cap that can hold around 2 cups of liquid.
- Measuring cups and spoons (again very clean and dry)
- Tin foil or wax paper rolled into a cone to help add the yeast to the bottles, again clean and dry well.
- 1 clean bowl or pot to make the sugar syrup. This will be the food for your yeast.
- 2 Table spoons of sugar per bottle
-3 packs active dry yeast (or 1/4 tea spoon per bottle)
-3 cups warm water (or cup per bottle) (chap store purified bottled water would be good for this but not essential)
- 1 tub big enough to hold all of your bottles and sides tall enough to hold 3 inches of water
- 1 bottle alcohol
- 1 bucket big enough to completely hold a fully blown up balloon filled to the top with water.
- 1 tub that the bucket can sit in AND hold a balloon full of water without spilling over the sides. A sink wash tub should do.
- 1 observation note book.
-1 clock of watch f

Answer 2:

Hm, I guess the best question is what you are going to compare it to. I would suggest plating out bacteria on a Petri dish, divide it into quadrants, let the bacteria grow for a week or two, and smear rubbing alcohol over one of the quadrants, and then some kind of placebo (like water) over a different quadrant. Does the alcohol stop the bacteria from continuing to grow?

You could also take something that you know rots, a tomato or something like that, and put one in water and another in alcohol, leave them for a while, and see which one falls apart first.

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