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.

La furonculose, causée par la bactérie Aeromonas salmonicida, représente une des principales causes de mortalité chez les salmonidés d’élevage. L’antibiothérapie constitue l’approche la plus largement répandue pour contrer les effets néfastes de cette maladie. Cependant, le développement de bactéries résistantes aux antibiotiques représente un problème de plus en plus préoccupant. La présente recherche a visé à explorer une nouvelle option pour lutter contre la furonculose, soit la possibilité d’utiliser des bactériophages comme moyen de prévention pour contrôler les populations de A. salmonicida. La sensibilité de 19 souches de A. salmonicida, résistantes à aucun, un, deux ou trois antibiotiques, a été évaluée vis‑à‑vis de 12 bactériophages. Les résultats ont montré que les souches de A. salmonicida résistantes aux antibiotiques utilisés dans l’industrie piscicole canadienne sont aussi sensibles à de nombreux bactériophages, tout comme des souches sensibles aux antibiotiques. Il serait donc possible d’envisager un traitement préventif à base de bactériophages pour lutter contre la furonculose chez les salmonidés d’élevage.Aquaculture represents an increasingly important source of food fish worldwide. The aquaculture industry currently produces between 25 and 30% of all seafood for human consumption. In Canada, salmonids (salmon, rainbow trout, arctic char and brook trout) account for the majority of food fish production. Furonculosis involving the bacterium Aeromonas salmonicida is one of the most important infections observed in salmonid farms. An A. salmonicida infection results either in morbidity and mortality with few clinical signs, or in weakened fish with skin ulcers that make them unmarketable for human consumption. The A. salmonicida bacterium uses a number of mechanisms to counteract the natural barrier of the immune system. Bacterial growth is encouraged by an increase in the ambient temperature and in the concentration of organic matter in the water.During recent years, a relationship between therapeutic failures and the development of bacterial resistance to antibiotics has been reported in salmonid farms. This problem is complicated by the fact that only four antibiotics are authorized for the aquaculture industry in Canada. One consequence of this increasing resistance is a renewed interest in alternative therapies and prevention. Bacteriophages (bacterial viruses) may represent one such alternative. In recent decades, interest in bacteriophages as antibacterial agents has been growing in the Americas and in Asia. Some researchers have tried to exploit the potential of bacteriophages to reduce bacterial populations in infections affecting humans, while others have tried to identify uses in veterinary medicine.The overall objective of this research was to explore a new treatment against furonculosis infection based on the use of bacteriophages to inhibit growth of A. salmonicida cells. In this study, we looked at 19 strains of A. salmonicida, resistant to zero, one, two or three antibiotics, and e...