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Five second Rule problems.

Postby Staceymullins » Thu Jan 10, 2019 6:24 am

I am doing an experiment to see if the 5 sec rule is true. I am having trouble with my control though. I am swabbing my food before I drop it on the floor as my control. I actually grow more bacteria on my control than on the food that is dropped on the floor. What am I doing wrong?

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Re: Five second Rule problems.

Postby SciB » Thu Jan 10, 2019 12:29 pm

Hi Stacey,

I think you need to change the way you do the control. The independent variable is dropping the food on the floor so the control would be NOT dropping the food on the floor. For statistical reasons you need to do at least three independent measurements for each test, so you need to have six identical pieces of food--or as nearly identical as you can make them.

The three control pieces will each be swabbed and the swabs rubbed on the agar surface (carefully so as not to break the agar). The other three pieces will be removed from the floor after five seconds and they will also be swabbed and the swabs applied to the agar.

I would suggest doing two times for the floor drop--five seconds and thirty seconds. This way you will have three time points: 0, 5 and 30 seconds that you can plot on a graph.

The biggest problem with this experiment is measuring the dependent variable-- the growth of microbes on the agar. I think a more scientific approach would be to put each swab into a tube with a measured amount of saline solution, shake it for about thirty seconds to separate the microbes from the swab and then spread one drop of this solution onto the agar surface using a bacterial spreader (I can give you directions how to make one). If you just smear the swab over the surface all you will see after the microbes grow in a couple of days is a smear. How can you compare two smears?

The scientific way to do this would be to shake the swabs in saline as I said and then make a dilution of the saline to get fewer microbes per drop and then spread the dilution onto the agar. If you make the dilutions correctly you will see individual bacterial and fungal colonies on the agar that you can count. Once you have accurate numbers, then you can do statistical comparisons and be able to PROVE that one sample has more bacteria and fungi than another.

Hope this helps you. Please ask more questions. There is a lot of info online about making dilutions, spreading bacteria on agar and other microbiological techniques. All you have to do is go to Youtube and type in what you want to learn about and you will get lots of videos showing how to do this and explaining what is going on.

Good luck!


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