TESTING ANTIBIOTIC-SENSITIVITY OF BACTERIA
WARNING: If you are allergic to any antibiotics, do not handle any antibiotic discs!
In this era when infections are often caused by bacteria that are multiply-drug-resistant, bacteria causing such infections are often taken from the patient, cultivated in the laboratory, and tested against a variety of antibiotics in vitro (i.e., in the Petri dish). This test, known variously as a “culture and sensitivity assay” or “Kirby-Bauer assay”, can provide information as to which antibiotics could be used to possibly kill those bacteria in vivo (i.e. in the patient).
The Kirby-Bauer assay is performed by streaking the entire surface of a large Mueller-Hinton agar plate with a bacterial culture. Small paper discs containing antibiotics are placed on the agar’s surface. During incubation, drug molecules diffuse from the disc into the surrounding agar. If the drug molecules kill or interfere with the growth of the bacteria, a clear zone of inhibition appears around the drug disc. After incubation, zone diameters are compared to sizes published on a standard chart (“Zone Diameter Interpretive Chart”). If a drug disc’s zone is too small (according to the interpretive chart), the bacterium is considered resistant to the drug; because a too-small zone indicates that the concentration of drug molecules required to interfere with the bacteria is too great to achieve in the patient’s body. If a drug’s disc zone is large enough (according to the interpretive chart), the bacterium is considered susceptible to the drug. A large-enough zone (according to the interpretive chart) indicates that the concentration of drug molecules required to interfere with the bacteria is low enough to achieve in the patient’s body.
Begin your online Kirby-Bauer experience by watching the video in this module titled “Antibiotic Susceptibility Testing”. Pay particular attention to the first 1:45 of the video, which shows you how to set up a Kirby-Bauer plate, and does a very good job of explaining the theory behind the Kirby-Bauer test. You should also focus on the test results, which start at about 5:35 in the video.
Next, watch the video in this module titled “Interpreting Kirby Bauer Results”. This video shows how to measure and interpret a finished Kirby-Bauer plate.
Now you are ready for the simulation, put together by some very talented folks at Michigan State University (I am so grateful they did this on put it online for free! J). Click on the simulation link in this document, read the instructions, and do the exercise (the exercise is also called a “module” on the Michigan State webpage). This exercise shouldn’t take more than 30 minutes and will probably not even require that much time. Record your results, as you will need them to fill out the lab report that’s included in this module.
This is the link for the lab simulation:
Finally, fill out the lab report included in this module. I have posted the lab report both as a Word document and as a PDF. You fill either of these out, depending on your software compatibility, and upload them to the Dropbox in the Kirby-Bauer module by the due date.
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