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.