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8.3: Results

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    A. Disinfectant experiment

    Use the spaces below to sketch the appearance of your own TSA plate and your lab partner’s plate. Record your observations in the chart below.


    Disk # Name Observations

    Based on your results, which disinfectant(s) worked best against the bacteria you tested?

    Which disinfectant(s) were least effective?

    Compare your plate with that of your lab partner. Did you see the same pattern of zones of inhibition for both bacteria tested? Explain.

    B. Antibiotic susceptibility testing

    Observe the three Mueller-Hinton plates, and note any differences in their overall appearance. Which plate seems to have the most zones of inhibition?

    Which has the least?

    Do you see any small colonies within the zones of inhibition on any of your plates? What do these colonies represent?

    For each plate, measure zones of inhibition for 3-4 antibiotics as follows:

    1. With a metric ruler, measure the diameter of the zone of inhibition. Express the values in millimeters (mm). It is best to choose some small, medium-sized, and large zones for comparison.

    2. Use the charts provided by your instructor to look up the interpretation for each of the zones you measured. Record measurements and interpretations in the chart below.


    Figure 8.3.1: Measuring zones of inhibition

    Bacterial species Disk label Antibiotic/concentration Zone of inhibition (mm) Interpretation (R, I, S)
    E. coli
    S. aureus
    P. aeruginosa

    Based on your results, do all antibiotics work equally as well against all types of bacteria? Explain.

    Why do we need to look up values in the charts for each antibiotic?

    C. Antibiotic production in Streptomyces

    Examine the plates for evidence of antibiotic activity. Measure the zones of inhibition and record your observations in the table below.

    E. coli S. aureus M. smegmatis
    Streptomyces coelicolor
    Streptomyces violaceusniger

    Do you notice any differences in the ability of the Streptomyces strains to inhibit gram positive and gram negative bacteria?

    8.3: Results is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Joan Petersen & Susan McLaughlin.

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