My lab partners and I conducted an experiment to determine which of the following are the best bacterial inhibitors: Lysol spray, hand soap, or Purell hand sanitizer. We used 6 Agar plates and...

My lab partners and I conducted an experiment to determine which of the following are the best bacterial inhibitors: Lysol spray, hand soap, or Purell hand sanitizer.

We used 6 Agar plates and divided them into 4 quadrants. We placed a control in quadrant 1 and then placed a each inhibitor in their assigned quadrants. 3 of the 6 Agar plates were diluted with water to each of the agents. (Lysol, hand soap, hand sanitizer) After a week sitting, our results showed that the hand soap had the largest Zone of Inhibition than the others for the undiluted solutions. 

Our question is that in the diluted solutions, the hand sanitizer had the largest Zone of Inhibition once water was added. Why?

Asked on by user427597

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ashley227 | College Teacher | (Level 2) Adjunct Educator

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Alcohols, contained in both Lysol and hand sanitizers, are effective as antimicrobial antiseptics and disinfectants.  Antiseptics are used on living tissue (hand sanitizer) and disinfectants are used in inanimate objects such as kitchen counters (Lysol).  

When alcohol is applied to a surface with bacteria, the alcohol interacts with lipids and proteins.  However, the depth of alcohol penetration into the cell depends on its concentration.  Concentrated or undiluted alcohol penetrates the outer cell wall of a bacterium, but quickly leads to coagulation of proteins within the inner region of the cell wall.  This coagulation actually seals off the cell and prevents any more alcohol form getting into the cell to kill it by disrupting the cell membrane.  Thus, many cells survive concentrated alcohol products.  When alcohol is diluted, the coagulation of proteins on the inner surface of the cell wall still occurs, however, this occurs much more slowly.  The decreased rate of coagulation allows the alcohol to penetrate beyond the cell wall to disrupt cell function and kill the cell.  This is why microbiologists typically use 70% ethanol (alcohol) rather than 100% ethanol to sterilize their benchtops prior to running experiments.

Because zones of inhibition are areas where there is a lack of bacterial growth (because the cells that were plated have been killed by the chemical) around a disk coated with an antimicrobial chemical, these zones will be larger when disks with diluted alcohol-containing chemicals have been applied compared to when concentrated alcohol products are used.  

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