Impact of Infection with Flavobacterium psychrophilum and Antimicrobial Treatment on the Intestinal Microbiota of Rainbow Trout

Oral administration of a new copper (I) complex with coumarin as ligand: modulation of the immune response and the composition of the intestinal microbiota in Onchorhynchus mykiss

[Cu(NN1)2]ClO4 is a copper (I) complex, where NN1 is an imine ligand 6-((quinolin-2-ylmethylene) amino)-2H-chromen-2-one obtained by derivatization of natural compound coumarin, developed for the treatment of infectious diseases that affect salmonids. In previous research, we showed that the Cu(I) coordination complex possesses antibacterial activity against Flavobacterium psychrophilum, providing protection against this pathogen in rainbow trout during challenge assays (with an RPS of 50%). In the present study, the effects of administering [Cu(NN1)2]ClO4 to Oncorhynchus mykiss over a 60-days period were evaluated with regard to systemic immune response and its potential to alter intestinal microbiota composition. In O. mykiss, an immunostimulatory effect was evident at days 30 and 45 after administration, resulting in an increment of transcript levels of IFN-γ, IL-12, TNF-α, lysozyme and perforin. To determine whether these immunomodulatory effects correlated with changes in the intestinal microbiota, we analyzed the metagenome diversity by V4 16S rRNA sequencing. In O. mykiss, both [Cu(NN1)2]ClO4 and commercial antibiotic florfenicol had comparable effects at the phylum level, resulting in a predominance of proteobacteria and firmicutes. Nonetheless, at the genus level, florfenicol and [Cu(NN1)2]ClO4 complex exhibited distinct effects on the intestinal microbiota of O. mykiss. In conclusion, our findings demonstrate that [Cu(NN1)2]ClO4 is capable of stimulating the immune system at a systemic level, while inducing alterations in the composition of the intestinal microbiota in O. mykiss.

[Cu(NN1)2]ClO4, a Copper (I) Complex as an Antimicrobial Agent for the Treatment of Piscirickettsiosis in Atlantic Salmon

Piscirickettsia salmonis is the pathogen that most affects the salmon industry in Chile. Large quantities of antibiotics have been used to control it. In search of alternatives, we have developed [Cu(NN1)2]ClO4 where NN1 = 6-((quinolin-2-ylmethylene)amino)-2H-chromen-2-one. The antibacterial capacity of [Cu(NN1)2]ClO4 was determined. Subsequently, the effect of the administration of [Cu(NN1)2]ClO4 on the growth of S. salar, modulation of the immune system and the intestinal microbiota was studied. Finally, the ability to protect against a challenge with P. salmonis was evaluated. The results obtained showed that the compound has an MIC between 15 and 33.9 μg/mL in four isolates. On the other hand, the compound did not affect the growth of the fish; however, an increase in the transcript levels of IFN-γ, IL-12, IL-1β, CD4, lysozyme and perforin was observed in fish treated with 40 μg/g of fish. Furthermore, modulation of the intestinal microbiota was observed, increasing the genera of beneficial bacteria such as Lactobacillus and Bacillus as well as potential pathogens such as Vibrio and Piscirickettsia. Finally, the treatment increased survival in fish challenged with P. salmonis by more than 60%. These results demonstrate that the compound is capable of protecting fish against P. salmonis, probably by modulating the immune system and the composition of the intestinal microbiota.

Effects of Erythromycin on Nereis succinea and the Intestinal Microbiome across Different Salinity Levels

The exposure of aquatic organisms to pollutants often occurs concomitantly with salinity fluctuations. Here, we reported the effects of erythromycin (0.250, 7.21, and 1030 μg/L) on marine invertebrate N. succinea and its intestinal microbiome under varying salinity levels (5‰, 15‰, and 30‰). The salinity elicited significant effects on the growth and intestinal microbiome of N. succinea. The susceptibility of the intestinal microbiome to erythromycin increased by 8.7- and 6.2-fold at salinities of 15‰ and 30‰, respectively, compared with that at 5‰ salinity. Erythromycin caused oxidative stress and histological changes in N. succinea intestines, and inhibited N. succinea growth in a concentration-dependent manner under 30‰ salinity with a maximum inhibition of 25%. At the intestinal microbial level, erythromycin enhanced the total cell counts at 5‰ salinity but reduced them at 15‰ salinity. Under all tested salinities, erythromycin diminished the antibiotic susceptibility of the intestinal microbiome. Two-way ANOVA revealed significant interactive effects (p < 0.05) between salinity and erythromycin on various parameters, including antibiotic susceptibility and intestinal microbial diversity. The present findings demonstrated the significant role of salinity in modulating the impacts of erythromycin, emphasizing the necessity to incorporate salinity fluctuations into environmental risk assessments.

Systemic immune response of rainbow trout exposed to Flavobacterium psychrophilum infection

Bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum is one of the most serious bacterial diseases leading to significant economic loss for rainbow trout (Oncorhynchus mykiss) aquaculture. However, little is known about the systemic immune response of rainbow trout against F. psychrophilum infection. This study investigated the immune response of rainbow trout to F. psychrophilum infection using multiple experiments, including bacterial load detection, phagocyte activity assessment, enzyme activity evaluation, and gene expression profiling. Results showed that the spleen index and intestinal pathogen load reached a peak at 3 days post-infection, with strong pro-inflammatory gene expression observed in rainbow trout. Leukocytes RBA and PKA were significantly elevated in the spleen, blood and intestine at 7 days post-infection. Heat map analysis demonstrated that the spleen had a more substantial pro-inflammatory response compared to the intestine post-infection and exhibited higher expression levels of immune-related genes, including IgM, il1β, il6, cd4, cd8a, cd8b, c1q, chathelicidin, inos, and lysozyme. Both Th1 and Th2 polarized responses in the spleen were activated, with Th2 (il4/13a, gata3) (FC > 4) being more intense than Th1 (tnfα, t-bet) (FC > 2). Tight junction proteins exhibited down-regulation followed by up-regulation post-infection. Collectively, the results of this study expand our current understanding of the immune response of rainbow trout post F. psychrophilum infection but also provide new avenues for investigation in salmonid aquaculture.