Effectiveness of Different Household Products on the Bacteria Serratia Marcescens
by Pam Saenger

Introduction
    My question was which household product kills the bacteria S. marcescens most effectively, and how does the effect differ in the wet and dry forms? Among the collection of bleach, hydrogen peroxide, Windex, Lysol, and bread mold I hypothesized that bleach would be the most effective in killing the bacteria. I supported my hypothesis with the knowledge that we had used bleach to kill the bacteria before disposing of prior cultures in the garbage can. I also knew that bleach was harmful to animal tissue and exposure to it could cause irritation. Bleach, if ingested, is poisonous. I hypothesized that wet rather than dry discs would have a greater effect because the substance would be more potent because of the excess fluid surrounding the disc and the elimination of evaporation. I used bread mold because the powerful antibiotic penicillin is derived from mold.
Methods and Materials
    I decided to use paper discs to administer the substances. I would saturate them in the liquid for the wet discs and for the dry discs I would leave the wet discs overnight to dry. The paper discs are circular bits of filter paper made with a hole puncher. I put these discs on agar plates freshly streaked with S. marcescens. I used household bleach, Lysol, hydrogen peroxide, Windex, and bread mold from a loaf of bakery bread. In administering the bread mold, I just carved off a lump of mold the size of a disc of filter paper and put it mold side down on the agar dish. I used one disc per agar dish, and this was put in the middle. All dishes were put in the incubator set at 30† C.

    To measure the effectiveness of each product, I took a ruler and measure across the diameter of the circle of no bacterial growth a few times and took the average. My control was a dish of S. marcescens with a plain, dry paper disc on it. It was also put in the incubator at the same temperature.

Results
    The control had uniform growth across the dish although it was scanty like many of the other dishes. The bacteria grew right next to the bread mold and the only difference from the control was that the bread mold grew a bit, while the bacteria remained unimpeded. The wet bleach had a huge circle of effect around it, as big as 63 mm, and in this circle was not the read S. marcescens, but a whitish unidentified bacteria that flourished quite well, but only in the circle with no red bacteria. This happened in all wet bleach trials. The dry bleach did not impede the growth of the S. marcescens. The wet Lysol had two circles around it, one that was completely clear and one that had the traces of the original streaks but no further growth. The measurements were as big as 6mm small circle and 19 mm large circle. The dry Lysol did not impede the bacteria at all. The wet H2O2 did have a small circle of effect around it as big as 9 mm in diameter. The dry H2O2 had no ring of effect around it whatsoever. Both the wet and dry Windex had no effect on the bacteria at all.

    I made a culture of the white bacteria found in the wet bleach dishes. It flourished over one night in the incubator, covering the whole top of the agar with a thick, white bacteria. I put a wet bleach disc on the newly streaked white bacteria and found no circle of effect at all. The same occurred with dry bleach discs.

Discussion
    The wet bleach had the greatest effect on the bacteria because its circles of effect were so huge. The wet Lysol was second in effectiveness and had two circles, one that eradicated the bacteria completely and one that just halted any further growth. The wet H2O2 was next, halting the growth of the bacteria in a smaller area. The dry bleach, dry Lysol, dry H2O2, wet and dry Windex and bread mold all had no effect on S. marcescens whatsoever. They are not antibiotics for this particular bacteria.

    The white bacteria that grew alongside the red S. marcescens but only flourished where the red bacteria died turned out to be resistant to bleach in all forms. This means that the white bacteria, though submissive to the red bacteria, is hidden when the red bacteria is present. However, when the red bacteria is killed, the white bacteria can flourish, even near the damaging wet bleach. I knew the white bacteria was not contamination considering it showed up in all wet bleach trials. The white bacteria was present only in areas of little or no red S. marcescens, such as around its edges or in the circle of effect of wet bleach. The white bacteria seems to be a companion of the red bacteria, only growing on its limits, but when the S. marcescens is gone, it can flourish, but only in the case of the wet bleach.

    If possible, I would have wanted to make more trials, expand my selection of products, and kept my paper discs in a sterile environment. I would have preferred thriving cultures of S. marcescens to be my sources rather than the sickly, worn-out cultures I used. I would have like to do more experiments with the white bacteria.

Conclusion
    In order of descending bacteria-killing properties are bleach, Lysol, hydrogen peroxide, and then Windex and mold, both having no effect. In addition, I discovered that S. marcescens has a companion bacteria that only really shows on the edges of the red growth and in places where the red bacteria once was but then it was killed.
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