Introduction
Saprolegniosis is a widespread freshwater fish disease caused by several species of the genus Saprolegnia, predominantly Saprolegnia parasitica and S. diclina. The disease affect all developmental stages, from egg to adult, and is most often opportunistic developing in stressed, injured, or coinfected individuals. Preventive health management is paramount in mitigating the risk of outbreaks and curative treatments Microbialbased strategies, both in-feed or by immersion, have the potential to enhance fish health and robustness but remain under looked in their capacity to mitigate Saprolegnia dissemination and the risk of outbreaks.
Material and Methods
In this study, two complementary approaches were used. First, three strains of Saprolegnia (S. diclina, S. parasitica and S. ferax) were tested in vitro to screen, amongst many, bacteria candidate best able to inhibit the growth of saprolegnia oomycetes. To do so, bacteria and saprolegnia strains were co-cultured on plate and on flask and the growth of the oomycete was measured after 1 week. Second, an in vivo trial was performed testing a yeast-based functional ingredient alone or in combination with one of three live-bacteria selected from in vitro screening and applied by immersion. To do so, freshwater Atlantic salmons (BW = 20 g) were fed to apparent satiation a control diet or the same diet supplemented with a yeast-based functional ingredient selected from prior knowledge. After 5 weeks of nutritional preparation, fish were randomly distributed into 24 tanks (12 tanks / diet group). Fish from each dietary origin were then exposed to one of four biocontrol treatment (Blank Control, Biocontrol A, B or C) applied in triplicate by 1h static immersion under aeration. Immediately after biocontrol exposure, fish were exposed to a controlled S. parasitica bath challenge and mortality was recorded daily during eighteen days. Skin and gill tissues were sampled at intervals for microbiota, histology and gene expression analyses.
Results
In-vitro screening allowed to select two Bacillus velezensis strains with a high growth inhibition capacity against three Saprolegnia strains. One of the B. velezensis strain was selected for in vivo testing in combination with Pediococcus acidilactici (Biocontrol C). Following S. parasitica challenge, mortality dynamics differed markedly between groups. When fed the control diet, mortality started earliest in the absence of biocontrol exposure and was clearly mitigated when exposed to Biocontrol C while Biocontrol B and A had intermediary and no benefits respectively. In contrast, in-feed supplementation with the yeast-derivatives slightly improved survival compared to the non-supplemented diet and overall reduced the apparent benefit of biocontrol exposure. Nonetheless under this regime, Biocontrol C followed by Biocontrol B reduced mortality hazard from day 8 post-challenge.
These outcomes reveal a strong diet-dependent interaction shaping host resilience and disease progression. Ongoing tissue analyses will be instrumental in elucidating the mechanisms underpinning the health benefits associated with Biocontrol C.
Conclusion
The trial demonstrates that both diet and biocontrol treatment significantly mitigate mortality following Saprolegnia exposure. Supplementation with the yeast-based ingredient provided the greatest single survival benefit, while Biocontrol C delivered the most consistent protective effect across dietary conditions by modulating the timing and intensity of mortality risk. Overall, the findings highlight the potential of microbial-based biocontrol strategies to mitigate saprolegniosis in freshwater salmonids. The consistent modulation of mortality dynamics by Biocontrol C, coupled with its dependence on dietary background, emphasizes the importance of integrating functional nutrition with targeted microbial interventions to enhance fish robustness.