Handwashing lab
Background:
Our bodies are homes for many different kinds of microbes, including many different bacteria. The
microbes in your skin are divided into two general categories:
Resident microbes are permanent inhabitants of the skin of most people and are found on the
superficial skin surface. However, many of these resident microbes are also found in deeper layers
of the skin and in skin crevices, where skin oils and hardened skin make their removal difficult.
Therefore, complete sterilization of the skin is impossible (and not really desirable either). [Note:
for more on this topic, see the article How Clean is Too Clean? at
http://www.cdc.gov/ncidod/eid/vol7no2/larson.htm]
Resident microbes are “friendly” microbes that protect us!
• In our skin and intestines: normal resident microbes prevent the growth of potentially harmful
bacteria by:
- competing for attachment sites and nutrients,
- producing substances that kill or inhibit the growth of nonresident bacteria,
- maintaining proper pH
• On our skin, resident microbes help absorb UV radiation and help destroy free radicals.
• In our intestines, normal resident microbes:
- manufacture vitamins that we absorb as nutrients, such as vitamins K, A, many
B vitamins, and several essential fatty acids.
- help break down carbohydrates, fats, and proteins.
- stimulate the production of immune cells.
- help change fecal matter into a form that can be eliminated.
Transient bacteria as the name implies, are not permanent inhabitants of the skin. Transient
bacteria are mostly found on the superficial skin surface (unless the skin is abraded or otherwise
damaged). Almost all disease-causing bacteria belong to this category. Transient microbes may take
advantage of some disturbance in the populations of normal resident microbes to gain a foothold
and cause infections and illness. Transient bacteria are deposited on the skin through direct contact
or by aerosol.
Experimental questions:
We will divide into 5 groups. Each group will receive 3 agar plates with which to carry out
experiments that address questions related to handwashing. . The experimental questions for which
you will develop hypotheses deal with the effect of handwashing on the growth of microbes. For
example:
1) how does the length of time of handwashing affect growth of microbes?
2) how does handwashing with soap affect the growth of microbes compared with handwashing
with water alone?
3) what is the effect of drying your hands with paper towels after handwashing on the growth of
microbes?
4) How does mechanical force (i.e. scrubbing while washing your hands) affect the growth of
microbes?
Experimental Design:
Each group will need at least two plates for your experiment. Your group also has a third plate that
you can use for additional tests or controls, or to sample bacteria in the environment as described in
Activity #1 in your lab book (pp. 171-173).
For each of the two agar plates in the handwashing experiment:
• divide the plates into four quadrants by marking on the bottom of the plate as
shown in this drawing (your instructor will also demonstrate this technique).
• Label the quadrants of each plate 1 through 4 as shown.
1
2
3
4
Each group should address question #1 regarding time of handwashing and one (or more) other
question(s). Develop your hypothesis and decide upon an experimental design and check these with
your instructor before proceeding with your experiments. The time of handwashing question will
be addressed by performing the handwashing for increasing lengths of time before sampling your
microbes (touching the plate). A sample procedure would be as follows:
1. Go get your hands “dirty” by going around touching doorknobs, faucets in the bathroom,
whatever you might normally touch around the building—but don’t wash your hands yet.
2. Touch quadrant 1 with your fingers by gently wiping across the quadrant—do not push down and
puncture the agar.
3. Wash your hands for 30 seconds (under whatever conditions you’ve designed in your experiment
(i.e. with just water, with water + soap, drying by shaking off water vs. drying with a paper towel,
etc.). Then touch quadrant 2 in the same manner as you did for quadrant 1.
4. Repeat the handwashing in the same way as in the previous step for an additional 30 seconds and
touch quadrant 3.
5. Repeat the handwashing once more and touch quadrant 4.
We will let our microbes grow in an incubator until the next class period when we will then look at
what kind of microbes and how many microbes grew in each quadrant.
Which questions are you addressing, and what are your hypotheses for each question?
Question # _____. Hypothesis: _____________________________________________________
Question # _____. Hypothesis: _____________________________________________________
Describe your experimental design in enough detail that someone else could repeat this
experiment.
Analysis:
Record how many colonies of microbes appear in each of your quadrants. Also, describe the size,
color, and other noticeable characteristics about the microbes that grew on your plate.
What conclusions can you make regarding our experimental questions about handwashing? We
will go over our group results together as a class.