Antimicrobial agents (µg) Diffusion zone breakpoints (mm) Aminoglycosides Gentamycin ≤ 12 Cephalosporins Penicillin ≤ 13 Imipenem ≤ 13 Phenicols Chloramphenicol ≤ 12 Macrolides   Erythromycin ≤ 14 Tetracycline Tetracycline ≤ 14

Table 6: Standard range of antimicrobial susceptibility.