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The Mechanical Effect of Hand Washing
While much of the focus of fighting infection has traditionally been on the type of soap used when washing hands and the frequency with which people wash their hands, one of the most important factors in the removal of potentially dangerous microbes is the physical act of hand washing. Rubbing the hands together while washing, whether with antibacterial or non-antibacterial soap, removes many dead skin cells, which actually may reveal greater numbers of culturable bacteria to be present on the skin after washing than before. In several experiments, there was no significant difference in the results from plain soap and antibacterial soap.

Another possible explanation for the apparent lack of effectiveness of hand soap is that most people do not spend the recommended 30-60 seconds washing their hands, regardless of the type of soap they use. However, even those who wash their hands thoroughly have been found to have more bacteria on their skin after washing, perhaps because washing with soap and water removes the visible or invisible layer of soil from the environment that may be covering the bacteria on the hands, and exposes bacteria that are normally found on the skin which have no negative effects on health. Since the studies that have been done do not distinguish between these “native” bacteria and possible pathogens, it is unsurprising that the amount of bacteria that can be found on the hands may not change, or may even increase slightly, after one wash.

Activity Against Spore-Forming Bacteria
Clostridium difficile is a common spore-forming bacterium found in hospitals and other public institutions that has been the subject of tests comparing the effectiveness of hand washing with plain and antibacterial soap at different water temperatures and disinfecting hands with alcohol-based bactericidal rubs. The number of colonies of Clostridium difficile that survived was smallest for those that washed their hands with warm water and plain (non-antibacterial) soap. Overall, washing with any kind of soap and water kills more bacteria than simply using an alcohol rub. In this situation, as compared to those discussed above, the colonies being tested for were specifically the bacterium Clostridium difficile, so it was possible to determine the reduction in colony counts by each soap. The presence of other bacteria was not taken into consideration.

The relative ineffectiveness of the alcohol rub is likely due to alcohol’s inability to destroy the spores produced by the bacterium. Spores, tough, dormant versions of their parent bacteria, are by their nature highly resistant to harsh environmental factors that interfere with the normal life cycle of a bacterium. In the case of C. difficile, the more vigorous act of washing the hands, especially accompanied by the flow of water during the process, promotes the physical removal of both active C. difficile and many spores left behind by already-lysed bacteria. Further evidence of this is that the use of antiseptic wipes, which also involves more friction against the surface of the skin, is also more efficient at decreasing the number of bacterial colonies recovered from the hands after use, though it is still not as effective as washing with soap and water.

Effect of Dilution on Antibacterial Activity
The efficacy of any antibacterial agent, such as the commonly-used triclosan, is reduced significantly as it is diluted when incorporated into a soap. The United States Food and Drug Administration sets standards for the soaps used in the healthcare and food industries, requiring a reduction factor (a ratio describing the number of bacterial colonies that can be isolated before and after use) of more than two for antibacterial soaps. However, commercially-sold antibacterial soaps such as Softsoap and Provon do not meet this requirement in their activity against most common bacteria, since they contain only 0.15% and 0.30% triclosan, respectively. As the concentration of the antimicrobial agent decreases, its ability to kill bacteria also decreases; and since these soaps are further diluted in the water used when washing, the activity of the antibacterial is reduced even further. Both antibacterial and regular soaps begin to lose their bactericidal properties in concentrations of less than 1% in water.

Development of New Antibacterial Soaps
Two new soaps created in the laboratory showed greater activity against bacterial survival without an increase in the concentration of triclosan typically found in antibacterial soaps (0.13% - 0.30%) when they also contained either polyhexamethylene biguanide (PHB) and benzethonium chloride (BZT) or PHB, BZT, and farnesol. PHB, BZT, and farnesol are chemical compounds that are also known to have bactericidal properties.

A common concern about antibacterial soap is that its use may lead to the development of a very resistant “superbug” that will be all but impossible to kill. These new formulas were tested for this effect by repeatedly exposing bacteria to low, non-lethal dilutions of triclosan, PHB, and BZT. This exposure did not increase the resistance of the bacteria to the original formulas of either soap.

Further study of these soaps and the compounds they contain could potentially lead to the development of a truly safe and highly effective antibacterial soap that will aid in the prevention of the spread of infectious diseases, both in public and private settings.

References