Science Buddies: "Ask an Expert"

I have a student doing this science fair project and we're having difficulties gathering any results. We've tried different agar mediums and different ways of putting down the jewelry. We're not sure what we could be doing wrong. Any suggestions?

https://www.sciencebuddies.org/science- ... welry-real

Hello,
This is a really interesting project! Could you provide more details about what is going wrong? Does the control plate have a lawn of bacteria, or is there no bacterial growth at all? If there is no growth, it could be that the incubation temperature is too low or too high; the ideal temperature is 37 degrees Celsius for E. coli. Too much exposure to air could also affect its growth; I would try adding more culture to each plate and using the clamshell method to minimize contamination.
Carissa

The weird thing is we weren’t seeing any growth at all. This last trial we did we finally have growth but it doesn’t seem we have any areas of inhibition. The jewelry we used has these little pearls on it. Do you think that would matter?

It appears that gold being an inert agent, requires contact with moisture to release ions that eventually create the oligodynamic effect. The science buddies guide on the project mentions some key steps that may help create the effect:
1. Washing the jewelry in mild dish detergent with warm water.
2. The pearls could be having any effect as it decreases the amount of surface area jewelry is in contact with the agar.
Is it possible that inhibition is only visible directly under the jewelry?
Check the Incubation temperatures the agar plates were kept at. Like Carissa noted ideal temperature would be 37 C.
Also, like Carissa noted too much exposure to air could be an issue. The Science Buddies project guide mentions that you may want to keep them in a plastic bag next to a heating vent or the clothes dryer. The incubation time will be longer than in an incubator. Start checking your control plates after 72–96 hours of growing time.