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The antibacterial effects of synthetic cecropin B and cecropin P1 were tested against the fish‐pathogenic bacteria Vibrio anguillarum, Vibrio salmonicida, Aeromonas salmonicida, Edwardsiella ictaluri and Yersinia ruckeri. Both cecropins were active against all bacteria tested, but the effect was strongly influenced by the growth media used. In brain heart infusion medium, the minimum inhibitory concentrations of cecropin B ranged from 0.3 to 1.3 μm and from 0.3 to 1.0 μm for cecropin P1, except for E. ictaluri, which was noticeably less sensitive to cecropin P1 (61 μm). The present authors have compared the bactericidal activity of these two peptides, showing that the killing rate for the selected bacteria was higher for cecropin B than for cecropin P1. V. anguillarum was the most sensitive to the cecropins, and in the present study, no colony forming units were detected after 4 and 8 min of treatment with cecropin B and P1, respectively. Electron microscopy was performed to document the effect of cecropin on the bacterial surface.

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