Kirby-Bauer disk diffusion susceptibility test on coagulase-negative Staphylococcus aureus and E.coli grown on Mueller-Hinton agar with tetracycline (30 µg), cephalothin (30 µg), erythromycin (15 µg), chloramphenicol (30 µg), vancomycin (30 µg), penicillin (10 µg), streptomycin (10 µg), and novobiocin (30 µg). Staphylococcus aureus and E.coli were chosen to show the differences in Gram - and + organisms reactions to different antibiotics.
Zones are measured in millimeters by either measuring:
•(A) Radius: Measure half the distance of the zone then multiply by 2. Use this method when part of the zone is not clear or has grown into another zone.
•(B) Diameter: Measure the entire length of the zone.