Antimicrobial Susceptibillity Test-E Test™ and Disk Diffusion
Depicted above are two methods for determining antimicrobial susceptibility of GBS. It is especially important that the microbiologist determine the susceptibility of GBS to erythromycin and clindamycin as these two drugs are used as substitutes if the patient cannot be treated with penicillin.
The long strip on the left is called the E-testTM. This pictures shows the drug azithromycin (this is shown as an example: we recommend using an erythromycin or clindamycin E-testTM). In this test, the strip contains a gradation of antibiotic with the strongest being at the top of the strip and the weakest at the bottom. The minimum inhibitory concentration of antibiotic that will inhibit the bacteria is determined by where the growth of the bacteria starts, or the area of the growth where the ellipse growth meets the strip.