Exercise 21: Antiseptics and Disinfectants


There are a wide variety of chemical agents available that are supposed to control microbial growth. Two examples of antimicrobial agents are disinfectants and antiseptics. In this exercise, you will test the effectiveness of a variety of antiseptics and disinfectants on a Gram-positive bacterium and a Gram-negative bacterium.



Pseudomonas aeruginosa

Staphylococcus aureus


1 TSA plate


Sterile swabs

Sterile paper disks (1/4 inch)

Pasteur pipettes

Miscellaneous antiseptics and disinfectants


Day 1 (inoculation)

1. Dip a sterile swab into the culture of Pseudomonas aeruginosa

2. Swab a TSA plate over the entire surface by going in one direction; then turn the plate and swab the entire plate going in the other direction.

3. Repeat swabbing the entire plate, swabbing in three directions (by turning the plate).

4. Choose two agents form the basket of miscellaneous antiseptics and disinfectants, or your own from home.

5. Using flamed forceps, remove a sterile 1/4-inch disk from the vial and place it on the inside of the cover of the TSA plate.

6. Use a Pasteur pipette to add a drop of a liquid agent to the disk or squeeze a sample out of the container onto the disk.

7. Use the flamed forceps to transfer the saturated disk onto one-half of the TSA plate. Gently tap the disk so that it adheres to the agar.

8. Repeat the same procedure with another agent, and place the disk on the other half of the TSA plate.

9. Other students will follow the same procedure above with Staphylococcus aureus.

10. Invert all the plates and incubate at 35 degrees Celsius until the next lab period.

Day 2 (Measurement of Disks)

1. Use a millimeter ruler to measure any zone around the disks on each of the plates.

2. Record the results in the Evaluation of Results section.

3. Compare your results with the other students in the class.


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