Fluoroquinolones display rapid bactericidal activity and low mutation frequencies against Aeromonas salmonicida

Oxolinic acid and five new fluoroquinolones, sarafloxacin, enrofloxacin, PD127, 391, PD117, 596 and C1934, were evaluated for in vitro activity against Aeromonas salmonicida. Sarafloxacin, enrofloxacin, PD127, 391 and PD117, 596 were significantly more active than cither oxolinic acid or C1934 in terms of their bactericidal activity against both oxolinic‐acid‐resistant and sensitive strains of A. salmonicida, killing 99% of the bacteria after 1h exposure. Resistant mutants developed at lower frequency to the fluoroquinolones than to oxolinic acid or oxytctracycline at 5, 10 and 20 times the MIC. However, mutants selected with one quinolone were cross‐resistant to all the other quinolones tested.