Do all antibacterial soaps kill germs equally well? Store shelves are full of brands that claim to be the best. This project puts three popular liquid soaps to the test on real bacteria.
You grow a colony of bacteria from your own finger on an agar plate. Then you soak small paper disks in each of the three soaps. Each disk goes onto a fresh bacteria-covered plate. After a day in the incubator you measure the clear zone around each disk.
A larger clear zone means the soap killed more bacteria. You track the zones over seven days to see whether the effect lasts or fades.
Hypothesis
The hypothesis is that Lever 2000 will be the most effective soap at killing bacteria.
You soak paper disks in three soaps and place each one on a bacteria-covered plate. After a day in the incubator, the clear ring around each disk — where no germs could grow — tells you how well each soap performed. Tracking those rings over seven days shows whether the effect holds steady or fades with time.
One strength of the Kirby-Bauer approach is that it compares multiple products side by side under identical conditions. You grow a colony of bacteria from your own finger on an agar plate, then soak small paper disks in three different antibacterial soaps and place each disk onto a fresh bacteria-covered plate. After a day in the incubator, the clear zone around each disk shows how far that soap stopped bacterial growth. A larger clear zone means more bacteria killed. Tracking the zones over seven days reveals whether that germ-killing power lasts or fades.
Method & Materials
You will rub your finger along the surface of a clean agar dish, incubate it overnight, and then smear it on another clean agar dish. You will then punch holes in filter paper to make small paper disks, dip them into the liquid antibacterial soap, and place them into the smeared agar dish. You will then place the dishes into the incubator and measure the diameter of the clear agar that surrounds the disk after 24 hours.
You will need agar dishes, an incubator, a Bunsen burner, filter paper, a hole puncher, tweezers and a wire loop, three types of antibacterial liquid soaps, and a ruler.
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After observing the dishes for seven days, it was found that Lever 2000 was the most effective soap of the three. The effectiveness of the soaps remained constant throughout the three trials.
Why do this project?
This science project is interesting because it allows us to compare the effects of different commercially advertised antibacterial soaps on bacteria.
Also Consider
Experiment variations to consider include testing bar soaps instead of liquid soaps, and testing different types of bacteria.
Full project details
Additional information and source material for this project are available below.
