Figure 10-2 Zones of clearing for various antibiotics

Antibiotic (and disc identifier) Disk potency Inhibition zone diameter to nearest mm
Resistant Intermediate Susceptible
Ampicillin (AM10)
Gram-negative rods and enterococci
10 µg 11 12-13 14
Ampicillin (AM10)
Staphylococci and highly penicillin-sensitive organisms
10 µg 20 21-28 29
Bacitracin (B) 10 units 8 or less 9 to 12 13 or more
Chloramphenicol (C) 30 µg 12 or less 13 to 17 18 or more
Ciprofloxacin (CIP) 5 µg 15 or less 16 to 20 21 or more
Colistin (CL) 10 µg 8 or less 09 to 10 11 or more
Doxycycline (D) 30 µg 12 13-15 16
Erythromycin (E) 15 µg 13 or less 14 to 17 18 or more
Gentimycin (G) 10 µg 12 13-14 18
Kanamycin (K30) 30 µg 13 14-17 18
Methicillin (ME5) 5 µg 9 10-13 14
Nalidixic Acid (NA) 30 µg 13 or less 14 to 18 19 or more
Oxacillin 1 µg 17 18-24 25
Penicillin G (P) 10 units
staphylococci 28 or less - 29 or more
most others 11 or less 12 to 21 22 or more
Polymyxin B (PB) 300 units 8 or less 9 to 11 12 or more
Streptomycin (S) 10 µg 11 or less 12 to 14 15 or more
Sulfadiazine (SD) * 300 µg 12 or less 13 to 16 17 or more
Sulfaoxizole (G300) 300 µg 12 or less 13-16 17 or more
Sulfisoxazole (G25) * 25 µg 12 or less 13 to 16 17 or more
Tetracycline (TE) 30 µg 14 or less 15 to 18 19 or more

The zone of clearing varies depending upon the antibiotic and its chemical properties. The medical effectiveness of antibiotics in patients has been correleated with a certain zone size and this is used to decide whether an antibiotic will be useful for treating a test pathogen.
*For this agent, the zone is measured at the margin of heavy growth. Sulfonamides may not inhibit the tested culture until after several generations. Therefore, a band of slight, hazy growth just inside the margin of the heavy growth may be ignored.