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    Comparing the Effects of Commercial Antibacterial Soaps on Bacteria

By Jen Rubel
February 24, 1997


    Because I spend so much time with children, who often can be bacteria magnets, I am very adamant about cleanliness. There are so many soaps available in markets today, that advertise as the "best anti-bacterial soap." I figured that as I was studying bacteria in class, I should test the claims of the soap-producing companies, and perform an experiment to test which soap was actually the most disinfectant. This was the bias of my experiment. I tested three popular anti-bacterial liquid soaps to find their effect on the bacteria commonly found on hands.


    The materials that I used in this experiment were--
  1. agar dishes
  2. incubator
  3. Bunsen burner
  4. filter paper, hole puncher
  5. tweezers & wire loop
  6. 3 types of antibacterial liquid soaps {Lever 2000, Safeguard, Dial}
  7. ruler

    There were many possible method that could have been chosen to do this experiment. I opted for what I saw as the most straight-forward method. To obtain the correct type of bacteria needed to perform the experiment, I rubbed my finger along the surface of a clean agar dish. I incubated the dish overnight for 24 hours. When I took it out of the incubator there were three different types of bacteria (DIG. 1). I chose one of the colonies and made a source culture from that by sterilizing the wire loop, swirling it through the colony, and smearing it on another clean agar dish, and I placed that dish in the incubator for 24 hours. When I removed that dish, I had a healthy source culture of finger bacteria, which also served as the control to my experiment. It displayed a healthy growing colony of "finger bacteria."

    I made three dishes using the same smearing technique, however, before I placed these dishes in the incubator, I infected the dish with antibacterial soap. To do this, I punched holes in the filter paper to make small paper disks of a uniform shape and size. then, using the tweezer, I dipped the disk into the liquid anti-bacterial soap until the paper was saturated. Then, I wiped the excess soap off of the disk, and placed the disk into the middle of the smeared agar dish. I then placed these dishes into the incubator. After 24 hours, I took the dishes out, and using a ruler, measured the diameter of the clear agar that surrounded the disk (DIG. 2). I later used the diameters as a judge of effectiveness when comparing the different soaps.

    At one point I had wanted to test bar soaps as well. To do this, I had to vary my procedure slightly. Rather than using saturated paper disks, I actually scraped slivers of bar soap, and punched holes out of the shards of soap directly.

    I observed the dishes for seven days, taking three trials of each soap. At the end of seven days, I cleaned out the dishes. To do this, I wore safety goggles and poured clorox bleach into the dish, on top of the agar. after waiting approximately a minute, I poured the liquid into the sink and rinsed off the top of the agar with lightly running water. Next, I took tweezers and removed the agar from the dish to throw it in the garbage. This was the last step of my experiment.


    [Not Available Yet]


    * During day one of each observation, the actual circle was cloudy, and hard to recognize (DIG. 3), therefore, during my first trial, I did not know how to correctly measure the diameter of the affected area. During the next two trials, however, I became more accustomed to searching for the ring and was able to measure more accurately.

    As I observed the dishes I noted that during the first three days, the diameter of the circle was changing. That was when I decided that I would continue the observations for an entire week (7 days). It seems as though the soaps all followed the same pattern: comparatively large diameter until the third or fourth day of observation, and then there was a rapid decrease in the diameter of the affected area. I also found that Lever 2000 was the most effective soap of the three. The effectiveness of the soaps remained constant throughout the three trials. The specific measurements did not vary much from the average for the day.


    The main difficulty that I had with this experiment was recording data on the weekends. It would have been wiser for me to trace the diameter through 4 or 5 days rather than 7, because then I would not have to take the data down on a Saturday or Sunday. As I performed the experiment, I had to do more than three trials of each. Fortunately, the trials were extremely compatible, so I could insert the data from the missing days into the table, and there was very little variation.

    I performed three side experiments over the course of my research. The first was an experiment to see if there really was a difference between anti-bacterial and regular non-antibacterial soaps. I prepared a dish the same was as I had with the other anti-bacterial soaps, except that I used palmolive liquid soap instead of an anti-bacterial soap. I only continued to watch the dish for three days, and found that the diameter was significantly smaller than the advertised anti-bacterial soaps.

    After I had found that the anti-bacterial soaps do inhibit bacteria growth, I wanted to test if they also killed surrounding bacteria. To do this I used one of the Lever 2000 dishes, because that was the most effective soap, as a source culture. I took bacteria from the edge of the circle (DIG 4) to make a smeared dish on clean agar. I also tried this experiment on bacteria from the Palmolive dish, to compare to the anti-bacterial soap, and I used the original, non-soap-tainted source culture as the control. The results showed that the bacteria is not killed, by even the strongest of the three anti-bacterial soaps, nor was it killed by the non-anti-bacterial soap. The cultures grew perfectly and matched the original source culture exactly.

    The third side experiment came in the middle of the testing process, when I came across a fourth type of soap that I wanted to test. The soap was a bar soap, and to test this I used the afore-mentioned technique, changing the original technique slightly. Ultimately, the bar soap melted in the incubator, and it was impossible for me to measure the affected area in a way that would be comparable to the other dishes. If I had had more time to continue the project, I may have tried other techniques such as saturating a disk in boiling water and lathering it on the surface of the soap, or I may have tried various methods of using the bar soap to make a highly-concentrated water/soap solution.

    If I had had time and more advanced equipment, I would also like to have tried to determine which ingredient was so affective in the best soap, this was I could compare to other soaps and find which ones are also good to use.


    I am happy to have done this experiment, because I feel that for the first time I have used science to help me solve a problem that is pertinent in my life. I have found that science is applicable to many of life's dilemmas. By making a make-shift anti-bacterial disk, I was able to determine the effectiveness of specific products, which are all commercially advertised as "the best." I know now that Lever 2000 has the strongest effect on bacteria, and the next time I take the kids on my little league team out to post-game ice cream, I will know which soap to hand them first.

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