This semester we will carry out a multi-week project to design, perform, and analyze an experiment to investigate the efficacy of antibacterial soap.
Soaps are small, amphipathic molecules containing both hydrophilic and hydrophobic regions. The dual nature of soap allows it to interact closely with water and grease, oils, and organic molecules commonly involved in soiling. Soap molecules cluster together with the polar groups oriented outward toward water and the hydrophobic groups oriented inward. These soap clusters are called micelles. These properties of soap enable it to disinfect skin by both chemical and physical means. Soaps can kill bacteria by solublizing cell membranes and lysing the cells. The extent to which soaps are able to kill bacteria by this means probably depends critically on the exposure time. Soaps also perform an important cleansing function by increasing the attraction between skin oils (and microorganisms adhering to them) and water used in washing. They increase the ability of water to physically rinse the microorganisms from the skin.
Many popular liquid soaps contain an antibacterial agent such as triclosan as well. There has been controversy about whether the addition of antibacterials to soap provides any benefit over regular soap and whether their addition actually has negative effects, such as contributing to the development of resistant bacteria, increasing the prevalence of allergies in children by killing the normally-occurring skin bacteria, and unintended effects on non-target organisms in the environment. In our work, we will examine the relative efficiency of antibacterial and non-antibacterial soaps in eliminating a common bacterial species, Escherichia coli.
Please note that this is a relatively harmless laboratory strain of E. coli and not the pathogenic strain which has caused deaths in cases of food poisoning. Nevertheless, use reasonable caution when handling the bacteria. For example, do not handle food, gum, etc. during the experiment and wash your hands at the conclusion of the experiment and after examining the cultured plates. Do not let pipette tips that have been in contact with the bacterial suspensions come in contact with the bench top and use the methods that you learned previously to dispose of contaminated materials.