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BIO2073: Microbiology Lab

Antibiotic Susceptibility Testing: Kirby Bauer Disk Diffusion Method

1. What does the term MIC stand for? Does a lower MIC indicate that a microbe is more or less resistant to an antibiotic?

2. Standardization is critical to obtaining accurate, reliable results with this testing method. Predict what effects each of the following changes might have on the results of the test. (Your lab text has critical information that will help you answer this question.)

a. The agar is only 2 mm deep, instead of the standardized 4 mm:

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b. There is visible moisture on the plate: _____________________________________

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c. The turbidity of your culture was much higher than the 0.5 McFarland standard:

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5. Normally, cultures that have been diluted to match the 0.5 McFarland standard must be used within 30 minutes. Why do you think this is important?

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6. How does the antibiotic get from the disk into the agar? (I’m looking for a chemical concept, this can actually be summed up in a single word.) _____________________________

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7. After incubation, does the antibiotic extend into the agar beyond the zone of inhibition? How does the MIC relate to the zone of inhibition?

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8. Suppose you do this test on a hypothetical Staphylococcus species with the antibiotics penicillin (P 10) and chloramphenicol (C 30). You record zone diameters of 25 mm for both the chloramphenicol and penicillin disks. Which antibiotic would be more effective against this organism? What does this tell you about comparing zone diameters to each other and the importance of the zone diameter interpretive table?

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9. Bonus! Name one other type of test for antimicrobial susceptibility. (1 pt) Explain how this test differs from the Kirby-Bauer test. (1 pt)