Antimicrobial susceptibility test. Image showing two methods for determining the antimicrobial susceptibility of group B streptococcus (GBS). It is important to determine the susceptibility of GBS to erythromycin and clindamycin as these two drugs are used as substitutes if a patient cannot be treated with penicillin. The long strip (left) is called the E-test and contains tetracycline. The two disks (right) contain erythromycin and tetracycline. The size of the zones of inhibition of growth (rings around strips) determine whether or not the bacteria is susceptible or resistant. Here, the bacterium is susceptible to erythromycin (the zone is large >26MBmm) and resistant to tetracycline (zone is small <12mm).