A comparison was made of the in vitro activities of five quinolone antibacterials (nalidixic acid, piromidic acid, oxolinic acid, flumiquine and miloxacin) currently used in aquaculture in Japan and five newer quinolones (ofloxacin, norfloxacin, enoxacin, ciprofloxacin and tosufloxacin), against selected fish bacterial pathogens. Of the earlier quinolones, flumiquine and oxolinic acid showed the highest bacteriostatic and bactericidal activities against Gram-negatives. The newer quinolones, however, showed considerably higher activities, with ciprofloxacin and tosufloxacin most active. The newer quinolones were also active against strains resistant to the earlier quinolones. Grampositive bacteria were resistant to all the earlier quinolones, but some strains were sensitive to the newer quinolones, particularly to ofloxacin, ciprofloxacin and tosufloxacin. The activities of the newer quinolones were found to be less affected by serum, pH or the addition of magnesium, than the earlier quinolones. The newer quinolones were considered to have potential for the treatment of Gram-negative, and possibly Gram-positive, bacterial fish pathogens.
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