Quantitative methods that require measurement of zone diameters give the most precise estimates of antibiotic susceptibility. One such standardized single disc procedure has been recommended for use with discs to test susceptibility to erythromycin. Resistant organisms produce zones of 13 mm or less, indicating that other therapy should be selected. Organisms of intermediate susceptibility produce zones of 14 to 17 mm. If alternate drugs are not available, confirmation by dilution tests may be indicated.
Standardized procedures require the use of control organisms.
The 15-mcg erythromycin disc should give some diameter between 22 and 30 mm for S aureus ATCC 25923 control strain. Although the mechanism by which this form acts in reducing inflammatory lesions of acne vulgaris is unknown, it is presumably due to its antibiotic actions.