Science Buddies: "Ask an Expert"

Hello, I am performing a science experiment testing the antibacterial properties of certain metals - those being copper, silver, and zinc. I have come up with a procedure but am unsure if it is flawed or if I am forgetting a step. My procedure involves using the kirby-bauer disc diffusion test and measuring the inhibition zone of the bacteria to determine antibacterial properties. I will be using coffee filters and hole punching them to a size of about 7mm to use as my paper discs. I will be using colloidal silver (500ppm), copper (30ppm) and zinc (30ppm) in my experiment and will be applying a certain amount (i'm not too sure how much of the colloidal I should add, any advice would be appreciated) to the filters and then leave them outside to dry for about 3 hours. I will then create a bacterial lawn of E. Coli (K-12) on my petri dishes using a sterile swab and will streak it across the plate in a zig-zag manner. I will split the petri dish into 4 zones, including the control (no solution on the filter paper), the copper, the zinc, and the silver colloidal solution. I will carefully place the filter paper in the center of the quadrants and incubate for about 24 hours as 37 degrees Celsius. I will then measure the inhibition zones on the back of the plate using a ruler.

Any information regarding how I can improve my procedure or if it is a viable bleep would be greatly appreciated. Thank you.



[Administrator note: This project may be helpful: https://www.sciencebuddies.org/science- ... inhibition ]

Follow up-question: Do I need to let my filter paper dry for 3 hours, or can I directly place the paper on the petri dish? Thanks.

Hi,

It's a really interesting idea. I have some suggestions that may help you with the procedure and answer your follow-up question:

• Drying Time: Allowing the filter paper to dry fully is crucial to prevent excess liquid from spreading on the agar, which could skew the results. Three hours should suffice, but ensure the papers feel completely dry to the touch before placing them on the bacterial lawn.

• Colloidal Solution Volume: Use a consistent volume for accuracy, such as 10 µL per disc. Use a micropipette if available to ensure precision.

• Bacterial Lawn Streaking: Instead of zig-zag streaking, use a sterile swab to streak the plate in three directions (lawn technique) for even coverage.

• Filter Paper Size: Ensure the punched discs are uniform in size and thickness for fair comparisons.

• Control Disc: Include a control disc with no treatment (dry, plain filter paper) to account for baseline bacterial growth and ensure the results are due to the metals.

• Sterility: Maintain strict sterility while handling the filter papers, colloidal solutions, and plates to avoid contamination.

I hope this helps! Let me know if you have any other questions.