Science Buddies: "Ask an Expert"

We have completed the science buddies experiment testing how food preservatives (we are using salt) effect the growth of microbials using prepared chicken broth with varying levels of salt and then culturing in nutrient agar plates. Everything went as expected (more colonies in lower does of salt and less in higher concentrations) except that the control petri dish never grew anything. The control had chicken broth with no salt added. (It was low sodium broth as well.) We did three trials but never had any growth in the controls. Why?

It's unusual that your controls had no bacterial growth in multiple trials, so the question that comes to mind for me is whether you treated your controls exactly identically as your experimental samples in everything except adding the salt. For example, if you used store bought chicken broth, when you added the salt I assume that you poured out some of the chicken stock, added salt, maybe heated it to dissolve the salt, stirred it, etc. Did you do the same thing for your control or did you just plate it directly from the can/box? The reason I ask is that I wouldn't expect there to be much (any?) bacteria in store bought stock since this stuff is usually sterilized during canning (so you can store it in your cupboard for a long time). Thus, the bacteria you saw on your plates was probably introduced when you added salt, mixed with a spoon, from the pot you used, etc.

Anyway, this is my best guess, and it provides a good lesson on why your controls need to be treated EXACTLY like your experimental samples if at all possible. If you did follow everything precisely, or if you used homemade stock, then I'm not sure what the explanation could be. Any other experts want to chime in?

JMP

Hello there,

Very interesting project idea. Just to be clear, would you mind providing more details about how you did your experiment so we can try to give you the best advice as possible? From what I understand, you mixed chicken broth with varying concentrations of salt and then plated those samples on nutrient agar plates to assess how many bugs grew. I'm slightly confused as to why you thought bacteria would grow from the chicken broth and the salt, since those two products should theoretically be sterile when you buy them from the stores. As JMP suggested, the bacteria was probably introduced into your samples from your spoon or in the container where you stirred and made your samples. When you mixed in the salt, did you use the same spoon for each round of mixing, but washed the spoon in between? For your control, I'd probably stir it with the spoon without adding any salt in just so it gets the exact same treatment as your other samples.

If you let us know more about your experimental procedures, we can better help you figure out what happened with your control!

Best,
Connie