The effect of NaCl on measures of the antimicrobial agent susceptibility of Aeromonas salmonicida in standard disc diffusion assays

The Clinical and Laboratory Standards Institute protocol (M42-P) was used to investigate the influence of NaCl on zone sizes obtained in disc diffusion assays. Replicate assays of the susceptibility of Aeromonas salmonicida (NCIMB 1102) to eight antimicrobial agents were made on Mueller-Hinton agar with and without the addition of 2% (w v) NaCl. The zones obtained for ampicillin, erythromycin, enrofloxacin, florfenicol, oxolinic acid and oxytetracycline, discs were, on average, 10 mm larger on NaCl supplemented media. For gentamicin and trimethoprim sulfmethoxazole discs the increases in zone sizes were smaller but still significant.