Antimicrobial Susceptibility

There are several factors to consider in choosing an appropriate antibiotic for treatment of a bacterial infection. The most basic of these is whether the causative organism is susceptible to a particular antibiotic, and how much of that antibiotic will be necessary to inhibit or kill the organism. While we do know a great deal about what antibiotics are effective against which microbes in general, the widespread problem of antibiotic resistance makes it essential to determine the susceptibility of a given bacterial isolate to multiple antibiotics in a short time to determine the most effective treatment.

Several standard methods have been developed to measure susceptibility of bacterial isolates to antibiotics, and the Kirby-Bauer disc diffusion method is one that is commonly used in clinical microbiology laboratories and has been standardized in the US by National Committee for Clinical Laboratory Standards. The Kirby-Bauer method is convenient in that the susceptibility of bacterial pathogens to multiple antimicrobial agents can be determined in a single assay. The results do not give actual MIC (minimum inhibitory concentration) values, but are interpreted as either susceptible, intermediate, or resistant based on comparison of zones of inhibition to a standardized table. In the virtual Kirby-Bauer exercise, one of three randomized Kirby-Bauer assay plates are presented, containing Staphylococcus aureus, Escherichia coli, or Pseudomonas aeruginosa that have been tested for susceptibility to 13 different antibiotics.

Using the virtual ruler (the mouse), the diameter of each zone of inhibition can be measured. These values are then compared to a standard chart of interpretive standards to the right of the plate, which provides a result of resistant, susceptible, or intermediate for each antibiotic. The box corresponding to each result is live and can be clicked on using the mouse. A correct result will turn the box green, and the box will remain green throughout the exercise (ie. measuring all zones of inhibition). Clicking on the incorrect result will cause a red X to appear in the box, which will disappear after a second or so. A different box can then be clicked until the correct answer is identified.

- Click to open the module
- Module instructions Kirby-Bauer method