Science Buddies: "Ask an Expert"

Hello,
We're trying to figure out how to measure the amount of dirt, bacteria, etc. on objects dropped on the floor. Any ideas?
Thanks!
S

I wish there was an easy way to test your idea because it is interesting. To determine numbers of bacteria, you need nutrient agar to grow bacteria on agar plates or use rapid tests with sophisticated lab equipment. It may make the most sense to not just determine the number of bacteria, but to distinguish between the good (neutral) guys and the bad guys, i.e. the potentially pathogenic bacteria (e.g. E. coli or Salmonella species). Such a differentiation of bugs requires further microbiological tests normally not available in the kitchen.

Hope this helps. There may be testing I am unaware of, but from my knowledge it may behoove you to further break down your question.

Hi Sahram,
In addition to the advice given by mg23, I would advise you to not lose hope! Although one would think dealing with bacteria requires expensive lab equipment, there is probably still a way to achieve what you would like to at your home (with safety precautions) but without counting specifically the number of bacteria present on the agar plates, rather the number of bacterial colonies, which is super easy! You could also try and identify the different types of microorganisms on your plates using the colors, shapes, and sizes observed. So let's get started .
First, take a look at this experiment: https://www.sciencebuddies.org/science- ... #procedure
Now, how do we make it yours? I would suggest using the petri dish coated with agar as the "object dropped on the floor". This would mean that you use the same procedure offered by the science project linked above, just with your own locations which could include the floor and your dinner table to gather data for comparing the amount of bacteria growing on objects on the floor to the amount of bacteria growing on objects on the table. Maybe you could tell us, based on your data, if it's really that bad to eat food off of the ground (considering that you want the least amount of bacteria from each location regardless of if the bacteria gathered have positive or negative effects on humans) --> We'd love to hear about your results! https://www.sciencebuddies.org/science- ... eityourown
. I hope our advice was helpful! Make sure to read the bacterial safety guide (provided in the procedure of the first link) thoroughly to ensure your safety and the safety of others around you!

Hi Sahram,

In addition to Roya's suggestions I would like to point out that the usual way scientists test for bacteria on a surface is by using a cotton-tipped swab moistened with sterile water. Swab the floor in several different locations and rub each swab onto nutrient agar on a separate Petri dish. Write the location on the bottom of the dish and attach the top to the bottom with a couple pieces of tape so it can't come off accidentally.

Here's a video that shows the correct way to make a swab on agar: https://www.youtube.com/watch?v=C4COfoS8Y6Y

Your project is asking just how dirty the floor is or can be. Obviously that's going to depend on how much traffic there is in the area, how often it is swept, vacuumed or mopped, whether you have pets, etc. Make a note of all these things for your report.

Also make a list of what you think may be in the dirt on the floor--bacteria, molds, yeasts, spores, parasites as well as sand, hair, plant and insect material. If you can, it would be a great idea to sweep up a bit of the floor dirt and take a photo of it under low power with a camera shooting through the eyepiece of a microscope. If you don't have access to a school microscope you can use a good digital camera (at least 10 megapixels resolution) to take a close-up photo of the dirt. You can enlarge this on the computer and print it so people can see what the dirt looks like at the microscopic level.

After you swab the plates and seal the lids, put them upside down in a warm place in the dark and check them after 24 hours. You should see some tiny colonies at this point but will have to let them grow for 24-48 hours longer to see well-developed colonies. Take photos through the lid at each 24 hour interval to make a record of the growth.

Remember that the dirt you swabbed on the agar may contain mold spores as well as bacteria, so you probably will see some fungal as well as bacterial growth. You can distinguish fungi from bacteria by the shapes and appearance of their colonies: https://www.sciencebuddies.org/science- ... ates.shtml


While you cannot identify a particular bacterium or fungus by colony appearance alone, you can make some pretty good guesses as to what might be in your dirt and how much: http://www.scienceprofonline.com/microb ... teria.html
http://www.bacteriainphotos.com/bacteri ... llery.html
http://www.microbiologyonline.org.uk/te ... petri-dish

Here's an intersting and funny article you should read about what your house dust can reveal: http://www.livescience.com/51988-what-h ... eveal.html

Good luck with your cultures and do let us know how they come out.

Sybee