Science Buddies: "Ask an Expert"

Hi,

So I havn't decided on my exact variable yet, however I do know the concept I am going after.

I want to put crude oil in water and then add bacteria to to the water to study the oil degradation.(I'm still working on the variable)

So my question is this:

How do I measure the amount of bacteria that I am putting in the water?

-My 7th grade teacher suggested that I suspend the bacteria in fluid and then measure the density and do it that way. If this is a good way to do it can someone explain this method to me?

Thanks
A

Hi,

The project that you are working on seems incredibly useful! I think you want to choose your variables wisely, though I would always suggest having concentration (of bacteria in this case) as one variable.

I think your teacher is referring to a optical density (OD) measurement. Basically, you have to start with a liquid culture of bacteria and pipette just a small fraction into a cuvette and then measure the absorbance of the sample at a wavelength of 686 nm in a spectrophotometer. It's a quite simple but handy technique.

This (http://a-s.clayton.edu/furlong/BIOL3250 ... tion08.pdf) website lists some other techniques and provides additional detail.

Hope this helps!
Aaron Lin

Thanks for the advice!

I was also thinking about doing the varibles of: 2 or 3 Different types of bacteria(of any onbination there-of) and the addition of dispersants into the equation.

So(not including the concentration of bacteria) my test groups may look something like this:
Key:
D=Dispersants
A=Bacteria 1
B=Bacteria 2

Control Group
Group A
Group B
Group AB
Group DA
Group DB
Group DAB

Thanks and ill will definately be keepin yall updated

Just to quickly comment on your groups: You should probably also include a group with just dispersant (D). That way you will be able to look at the effects of the dispersant independently of all the rest.