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Fish aquaculture faces important losses as a result of bacterial resistance to antibiotics. Bacteriophages have proven a useful alternative therapy in other domains, but remain to be tested with fish. The interaction between Aeromonas salmonicida HER 1107, bacteriophage HER 110, and brook trout Salvelinus fontinalis was studied in 70‐L aquariums maintained at 9°C. Populations of A. salmonicida (108 colony‐forming units per milliliter) declined by six log units (base 10) in 3 d when bacteriophage HER 110 was introduced in a multiplicity of infection factor of 1. Concentrations of bacteriophages and bacteria in the open water of the aquariums were 2–3 log units lower than those in gravel interstitial water. However, the relative drop in bacterial populations with time was the same in both environments. Addition of the bacteriophage HER 110 delayed by 7 d the onset of furunculosis in brook trout. Further addition of A. salmonicida HER 1107 showed that bacteriophages remained in the aquariums. Mutants of the bacterium were isolated and used as targets for bacteriophage HER 110 and nine other phages. The tests revealed that more than one phage could infect A. salmonicida HER 1107 and that mutants resistant to bacteriophage HER 110 were sensitive to one or more phages. Bacteria resistant to bacteriophage HER 110 had a slower generation time than the original strain, and the success rate of replating in tryptic soy agar (TSA) was very low. More than 25% of the mutants seemed to revert to the original‐strain phenotype after a first replating in TSA. All mutants were sensitive to three or more phages. Finally, stock cultures of 109 plaque‐forming units per milliliter of bacteriophage HER 110 decreased by only one log unit in 80 d when held at 4°C in liquid brain–heart infusion broth culture medium. These results suggest that bacteriophage combinations could be successfully used in preventive programs on fish farms.

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