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Antimicrobial Susceptibility Test
Different antimicrobial agents have different effects on different organisms. Some organisms may be completely resistant to a specific antimicrobial while others are highly susceptible. The Kirby-Bauer or disk diffusion test is used to determine if an organism is susceptible or resistant to a selection of antimicrobial agents. When the test is run in a very specific manner, it can even be used to determine how susceptible an organism is to a specific antimicrobic. This is a very useful procedure when trying to determine a therapeutic course against a particular infection. It can also be used the test the efficacy of a new antimicrobic.
In a clinical setting, the Kirby-Bauer test would be strictly standardized. Mueller-Hinton agar plates would be poured to a depth of 4 millimeters. After solidifying, the plates would be streaked for confluent growth. The inoculum would come from a broth culture that has been diluted with sterile saline to match a 0.5 McFarland turbidity standard.
In our lab, we may use either Mueller Hinton or nutrient agar plates for this test. Some instructors will have you dilute your broth culture with sterile saline to match the 0.5 McFarland standard while others will have you use the undiluted broth culture. Although the results are more striking when the test is run properly, you will still see positive results.
After the plates have been inoculated, a variety of paper disks will be added. These disks have been infused with a specific amount of a microbial agent. The plates are then incubated. After incubation, there will be “bacteria-free” circles of varying sizes around some of the disks. These are called zones of inhibition. They indicate that the organism was susceptible to the antimicrobial agent. The larger the zone of inhibition surrounding an antimicrobial agent is, the more susceptible the organism is to the antimicrobic.
In a professional setting, the diameter of the zone of inhibition would be measured in millimeters. The resulting number would be compared to the data listed on a table of zone diameters, like the one on page 71 of your lab manual. If the zone of inhibition was acceptably large, the antimicrobial would be used to treat the infection; if it was too small, another agent would be used.