Science Buddies: "Ask an Expert"

I'm doing a science project on how does liquid soap affect the growth of bacteria. I'm going to have several jars with chicken broth,something, and liquid soap. But one jar will not have any liquid soap. For my project my independent variable is the liquid soap and the dependent variable is the bacteria. Thing is I have to be able to measure the bacteria cause its my dependent variable. And I have no idea how to measure it. So if you could please help me with how to measure it I'd be very grateful. Also I'm wondering if I mixed up the variables and liquid soap should be the dependent and bacteria the independent.

Hi,

Here is a Science Buddies Project Writeup that describes measuring bacterial content of various liquids:

https://www.sciencebuddies.org/science- ... 5&from=TSW

I think you could adapt this method to your project.

For help on doing the project you may want to consult the Science Buddies Project Guide:

https://www.sciencebuddies.org/science- ... ndex.shtml

For help on variables in particular, read the project guide page on variables:
https://www.sciencebuddies.org/science- ... bles.shtml

Good luck and ahave fun with the project!!!

Best regards,

Barrett L Tomlinson

Thank you. One more question though. Is it possible to put it in units? Because one of my teachers questions is "What data will you collect? (Units- think metric!)"

Hello Akiria
If you are referring to what unit is used in measuring bacteria the answer would most commonly be micrometers (µm).
If you have any more questions just ask and good luck!

Hi Akiria,

Science diplomat is correct; if you measure bacteria under a microscope, you would measure them in microns. However, it sounds like you are going to be counting the number of bacteria in various solutions, so you would be measuring number of bacteria per milliliter.

Did you read through all of the links that Barrett gave you? The information on the science buddies website is excellent. Since you will be measuring bacteria for the first time, I recommend that you do a trial experiment with one solution to develop your technique. If you will be measuring bacteria that have been growing for a while, you should know that the bacteria will be as high as 10,000,000 organisms per ml, so you will need to plan to do dilutions so you will be able to do accurate counts.

Please let us know if you have any more questions.


Donna Hardy

The standard units microbiologists use for what you're measuring is called CFU--colony forming units, measured per ml or whatever volume you're interested in. Each bacterium, presumably, will form one colony on an agar plate.

So I talked to my teacher and she suggested using perti dishes to measure the bacteria. I've looked around but I haven't been able to find out how to use the petri dish or how to put the bacteria on it.

Hi Akkira

Check out these instructions on how to use Petri dishes:

https://www.sciencebuddies.org/science- ... Agar.shtml

https://www.sciencebuddies.org/science- ... tion.shtml

https://www.sciencebuddies.org/science- ... ates.shtml

https://www.sciencebuddies.org/science- ... ques.shtml

Good luck and post again if you still have questions!

Amber