The disk diffusion method is often used to test the ability of compounds to inhibit microbial growth.
Basically, bacteria are spread across an agar plate. Then, filter paper discs, impregnated with various
compounds, are placed on the plates. The cultures are incubated overnight creating a lawn of bacterial
growth on the agar plates. As the compounds diffuse from the discs inhibition of growth may occur
forming a clear zone around the disc. We refer to the clear area or lack of growth around the disc as a
zone of inhibition. In general, the larger the zone of inhibition the more effective the compound is at
inhibiting bacterial growth.
1. List the disinfectants that appear to inhibit S. aureus. What about E. coli? What about P.
aeruginosa? Which organism most resistant? What is the gram reaction of all three organisms?
Is there a trend related to their gram reactions? Would S. aureus “behave” differently if it
produced endospores? Why?
2. P. aeruginosa is a common cause of nosocomial infections. If O-phenylphenol was the active
ingredient in your hospital disinfectant would this be a good cleaner to use for cleaning surfaces
in patient rooms? Why or why not.
3. In looking at the results above, what would be the best disinfectant to use when dealing with all
three organisms? Can you describe a common commercially available disinfectant that contains
a chlorine based oxidizing agent that would be effective against all three organisms? What is the
working concentration of this product and is it safe to use?
One micrometer is 10-6 meters. And, one nanometer is 10-9 meters. How many nanometers are “in”
one micrometer? In the image above, how long, in nanometers is E. coli? How many micrometers long
is the E coli cell? What is the diameter of the RBC (in nanometers and micrometers)? How large is the
adenovirus (in nanometers and micrometers)? What about poliovirus? So, you have a heat sensitive
medication that needs to be filter sterilized. You decided you will use a 0.2 micron (micrometer) filter to
trap contaminants. What size are the filter pores (in micrometers and nanometers)? If the medication
contains adenovirus, poliovirus, E coli and RBCs and is applied to the 0.2 micron filter will the filtered
medication be sterile? Will any cells or viruses be present in the filtered medication? If so, which ones?
If not, why not?