Laboratory Investigations in Microbiology

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Chapter 25: Hand washing


Doesn't everyone always tell you to wash your hands? Sure. And for a number of good reasons. Your hands are the most likely body surface to pick up germs, and thus serve as one of the most important means of transmission of pathogens. Viruses, such as those that cause the common cold, and bacteria (e.g. E. coli) are easily transmitted this way. And yet these microbes are not normally found living on your hands & fingers. They are considered to be transient microbes. Your skin secretes fats, oils, and other substances which coat your skin, especially on your fingertips. These can then trap microorganisms from the environment. Good hand-washing protocols are designed to remove these fats and oils, and the accompanying transient microbes which are sometimes associated with disease. Hand-washing does not, however, get rid of the normal resident microbiota found on your skin.

A good hand-washing protocol includes using hot water, soap, and vigorous rubbing (friction) to dislodge microbes and dissolve the oily film that traps transient microbes. You use hot water and soap for the same reason that you'd use these to wash greasy dishes!

Recently, many antimicrobial soap products have also come on the market. The foam & alcohol-based soap now used on the Plattsburgh campus in one example. We will use this type of soap, as well as standard liquid/solid soap, and compare their relative effectiveness in doing the job. To evaluate the effectiveness of hand-washing, we will make "before" and "after" fingerprint impressions on agar and count the number and variety of microbial colonies that grow after incubation.

Materials & Methods



  1. Important: Do NOT wash your hands at the start of today's lab!
  2. You will be assigned one of 4 hand washing protocols
  3. Label your TSA plates "before" and "after"
  4. Place a fingerprint of all 5 fingers onto the agar surface of the 'before' plate.  
  5. Follow the hand-washing protocol assigned to you
  6. Place a fingerprint of all 5 fingers onto the agar surface of the 'after' plate.  
  7. Bring both plates up front to be incubated
After 24 - 48 hours of incubation
  1. Count the number of colonies on each agar plate. Add up the colonies from all 5 fingers. There will likely be a lot of colonies to count! Put a small dot onto the back of the agar plate as you count a colony - this will help you not lose track of which colonies you have already counted.
  2. Next, determine how many different kinds of colonies you have on your agar plates. Note down the physical characteristics of the colonies in your notebook. For example, you may note that a colony is white and small.
  3. Record your data on your data sheet and on-line. Copy other students' data on your data sheet as well
  4. After all lab sections have uploaded their data, download the class data. Copy it into an excel spreadsheet. Sort the data by protocol. Calculate the average for each category. Calculate the average CHANGE in each category.


© 2003 - 2015 Josť de Ondarza, Ph.D.