Effect of a multi-citrus extract-based feed additive on the survival of rainbow trout ( Oncorhynchus mykiss ) following challenge with Lactococcus garvieae

Growing global concerns about antibiotic resistance have generated a considerable interest in the search for alternative environmental-friendly approaches. This study was aimed to assess the antimicrobial activity of a multi-citrus extract-based feed additive (Biocitro ® ) against some fish pathogens, as well as evaluate its capacity to protect rainbow trout ( Oncorhynchus mykiss ) to lactococcosis. A broth dilution method was used to determine the minimum inhibitory concentration (MIC) of Biocitro ® , and the results showed a strong antibacterial activity against Aeromonas salmonicida , Lactococcus garvieae and Yersinia ruckeri with MIC values of 2.0 µg/mL. Afterwards, rainbow trout juveniles were fed a Biocitro ® -enriched diet (750 mg/kg feed) at a daily rate of 1.5% body weight for 4 weeks, then they were challenged with L. garvieae by the cohabitation method. At the end of the experimental period, fish treated with Biocitro ® showed significantly (P < 0.001) improved protection against L. garvieae compared to control fish. Although further studies are needed to understand how Biocitro ® increases rainbow trout resistance to L. garvieae , this feed additive could be considered as a useful alternative to chemotherapeutic treatment in aquaculture.