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10: Antimicrobial Chemicals

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    Learning Objectives

    • Compare the antimicrobial capabilities of different antiseptic and disinfectant chemicals.
    • Identify which categories of chemicals are most effective.
    • Identify antimicrobial properties of some plants.

    Many herbs and spices MAY have (some documented, some just hearsay) antimicrobial activity: thyme, cinnamon, garlic, ginger, chamomile, oregano, sage, echinacea, wasabi, etc. We are going to try some out in this lab: fresh or dried, leaf or powder. In addition, you will be running various antimicrobial chemicals against your bacteria. To save on time, for the identification of antibacterial spices, each group will extract a different plant tissue and prepare enough herb/spice extract for each group. Each table will run 2 bacterial organisms with the 4 disinfectants and antiseptics along with the spice extract for their table plus 1 from another table. Therefore, each table will be running 6 chemicals against 2 bacterial organisms.

    MATERIALS NEEDED: per table

    • broth cultures of Staphylococcus and E. coli or other bacteria
    • TSA plates
    • sterile cotton swabs
    • sterile paper discs
    • an herb or spice extract (1 per table)
    • antiseptic and disinfectant chemicals (at least 4 per table)
    • small beakers for antiseptics and disinfectants
    • forceps and beakers of ethanol
    • 1ml pipettes+ pi-pump


    Note: Prior to the Lab

    Pick an antimicrobial chemical (NOT an antibiotic) that you have at home or work, or bring a chemical from the store.

    Bring the following information on your chemical:

    • the brand name of the antiseptic/disinfectant
    • the active chemical ingredient (NOT the inert chemicals such as glucose or water)

    The herb extracts

    1. The herb extracts have already been made for you, using about a half-dozen kinds of herbs. Your table will receive 1 or 2 of these extracts to use along with your antimicrobial chemicals.
    2. The herbal extracts will be added to small cotton discs which will be placed on the placed containing the bacterial. The discs may already be saturated with the herbal extract, or you may have to do that in lab. You will be directed by your instructor.

    Using the spice/herb extracts and antimicrobial chemicals

    1. Inoculate a TSA plate with 1 bacterium, using a sterile cotton swab to pick up an inoculum from the broth and then covering the agar plate thoroughly. Use a tight zigzag procedure to cover the ENTIRE AGAR SURFACE with the inoculum.
    2. Your table can choose 4-5 antimicrobial chemicals plus 2 spices/herbs to test each organism against. That will be 6 discs on each agar plate.
    3. Pour a small amount of each of your antiseptics and disinfectants, plus 1-2 spice/herb extracts into each of the beakers.
    4. Using alcohol-flamed forceps, pick up a sterile paper disc and dip it halfway into one of the antimicrobial solutions, then place the disc on the inoculated media.
    5. Repeat the procedure with the other chemicals. Both organisms will be tested with the same chemicals.
    6. Incubate the plates at room temperature, 25oC or 37oC, depending on what your instructor says.
    7. AFTER INCUBATION, measure the zone of inhibition for each chemical in millimeters . If there is no zone around the disk, call it 0.
    8. Record the results for the entire class in the table on this handout.


    Record the results for the various chemicals and spices used in this exercise.

    Chemical (brand name) or Spice Active Ingredient



    INHIBITION (in mm)

    E. coli


    1. The space created around the chemical disc is called the ________________.
    2. The most effective active ingredients found in the store-bought chemicals was _________________.

    Contributors and Attributions

    This page titled 10: Antimicrobial Chemicals is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Jackie Reynolds.

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