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Procedures - Day Two:
- Observe the plates for zones of inhibition. Remember that the zone of inhibition is the clear area around the paper disk in which bacteria have not grown.
- Draw your results in the Data section. In the Descriptive Column, note whether the zone of inhibition is completely clear of bacterial growth or not.
- Use a metric ruler to measure the zone of inhibition in millimeters. Measure across the zone and the antibiotic disk to get the diameter. Record the measurements in the Data section.
- Use your observations and measurements to determine whether the bacteria are resistant or sensitive to each antibiotic. The bacteria are sensitive if there is a clear zone of inhibition around the disk
where no bacteria have grown. The bacteria are resistant if there is no clear zone of inhibition around the disk or the bacteria have grown all around it.
- Dispose of the agar plates as directed by your teacher. Wash the lab table and your hands.
Data Collection:
S.aureus E.coli
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Antibiotic Disk-Numbe r and Name
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S.aureus Descriptive
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S.aureus Zone in mm
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E.coli Decscriptive
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E.coli Zone in mm
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Conclusion: Please write a concluding paragraph based on your hypothesis, what you tested and your results.
Summary Questions:
- Did you see any differences between S. aureus and E.coli in their resistance to any of the antibiotics? If yes, please state the differences.
- Can you tell by using this test whether the sensitive bacteria were killed or just not able to grow?
- Did you observe any zones with a few colonies of bacteria within the zone of inhibition? If yes, what is a possible explanation for this occurrence.
- List some factors that a physician would need to consider other than zone of inhibition size before prescribing an antibiotic.
ADAPTED FROM: Hudson, Barbara K. Microbiology in Today’s World Second Edition. Kendall / Hunt Publishing Company, 1998 and Harley, John P.
and Lansing M. Prescott. Laboratory Exercises in Microbiology Fourth Edition. WCB McGraw-Hill, 1999.
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