Science Buddies: "Ask an Expert"

We are conducting a bacterial experiment on the handle and seat of a shopping cart to see which one has more bacteria...we think our independent variables are the seat and handle but are not sure on the dependent or control variables.

Thank you for any help!

Given that independent variables are those that you will change or attempt to control to determine the effect on the dependant variables, I agree that your sample source (in your case, the seat vs the handle of the shopping cart) is an independent variable. If feasable, I would also add environment conditions at time of collection and during culture (i.e. temperature, humidity, etc...), as well as time. As for dependent variables, I would consider bacteria type (species) and population size as your primary indicators of change based on the independent variables.

I assume you will be growing the bacteria in a culture, it's important to obtain several samples from different shopping carts to get a good average level of contamination.

I'm not a microbiologist, but I hope this helps. Good luck.

That does help, thank you. Is there any guidance on how to define bacteria types (she is in 5th grade)?

Thank you

At her age, just some statements about observed differences in colonies may be enough. With a microscope or at least a decent magnifying glass, your sample bacteria's characteristics should be observable. Things like shape of the colonies (growth pattern), elevation (cross section), texture and color should provide clues as to what bacteria you have. At 5th grade level, it may be enough to identify if there is one or more types present and if those differ greatly between surfaces tested without getting down to specific scientific names.

Also, if I didn't mention before, be sure to note growth rate over time. This can easily be done by marking the outer edge of a colony with a pen on the bottom of the agar plate at some time (T0) and then again at specified intervals and noting % changed. This is an easy way to obtain a valuable data point to relate across samples.

Sounds like fun, hope you're not to grossed out when the results are in! Let us know if you have other questions.

Cheers.

That was incredibly helpful...I never thought to measure the growth rate and to mark with a marker on the bottom!!! This is a great way to measure and easy enough she will be able to do this.

Yes, i will report back (I have a feeling we will find all kinds of interesting looking things on the shopping care).

Thank you again for your help!