Influence of probiotic and prebiotic supplementation on intestinal microbiota and resistance to Edwardsiella ictaluri infection in channel catfish (Ictalurus punctatus) following florfenicol administration.

Enteric septicemia of catfish (ESC), caused by the gram-negative enteric bacteria Edwardsiella ictaluri, is a significant threat to catfish aquaculture in the southeastern United States. Antibiotic intervention can reduce mortality; however, antibiotic use results in an imbalance, or dysbiosis, of the gut microbiota, which may increase susceptibility of otherwise healthy fish to enteric infections. Herein, recovery of the intestinal microbiota and survivability of channel catfish in response to ESC challenge was evaluated following a 10-day course of florfenicol and subsequent probiotic or prebiotic supplementation. Following completion of florfenicol therapy, fish were transitioned to a basal diet or diets supplemented with a probiotic or prebiotic for the remainder of the study. Digesta was collected on Days 0, 4, 8 and 12, beginning on the first day after cessation of antibiotic treatment, and gut microbiota was characterized by Illumina sequencing of the 16S rRNA gene (V4 region). Remaining fish were challenged with E. ictaluri and monitored for 32 days post-challenge. Florfenicol administration resulted in dysbiosis characterized by inflated microbial diversity, which began to recover in terms of diversity and composition 4 days after cessation of florfenicol administration. Fish fed the probiotic diet had higher survival in response to ESC challenge than the prebiotic (p = .019) and negative control (p = .029) groups.

Association of cocaine- and amphetamine-regulated transcript (CART) messenger RNA level, food intake, and growth in channel catfish.

Cocaine-and Amphetamine-Regulated Transcript (CART) is a potent hypothalamic anorectic peptide in mammals and fish. We hypothesized that increased food intake is associated with changes in expression of CART mRNA within the brain of channel catfish. Objectives were to clone the CART gene, examine tissue CART mRNA distribution, and changes in the amount of CART mRNA in relation to changes in food intake in channel catfish. Our results showed that channel catfish CART was highly similar to those of other fish species, particularly in the biologically active portion of the peptide. Expression of CART mRNA was detected in the brain and testis but not in other somatic tissues. Thirty days of fasting decreased (P<0.05) the amount of CART mRNA within the brain of channel catfish, while refeeding for 15 days restored its amount to a level similar to the fed control. In a separate 7 week feeding study, CART mRNA expression was lower in fish that consumed more food and gained more weight (P<0.05). These results suggest that CART is involved in regulation of food intake in channel catfish, similarly as it has been reported in other fish and mammals.

Isolation and characterization of channel catfish natural resistance associated macrophage protein gene.

Natural resistance associated macrophage protein 1 (Nramp1) affects the ability of macrophages to kill pathogens. We cloned Nramp cDNA of channel catfish to identify potential molecular markers for disease resistance. Three different Nramp transcripts were identified: NrampCa-2912 nucleotides (nt), NrampCb-3245 nt, and NrampCc-3721 nt. At the 5' end, the transcripts have a common 2263 nt sequence containing the open reading frame. The differences are in the 3' untranslated region resulting from alternative splicing and polyadenylation. NrampCc is the predominant form expressed. The deduced 550 amino acid sequence of the channel catfish Nramp (NrampC) has high homology to Nramp from other vertebrates and a predicted conserved structure. The NrampC contains the 12 transmembrane domains, and the consensus transport motif. Post-transcriptional processing is also conserved. Phylogenetic analysis grouped NrampC with other fish Nramps and closer to Nramp2 than to Nramp1 of mammals. However, the catfish transcript does not contain an iron-responsive regulatory-protein binding site, a characteristic of Nramp2, and, like Nramp1, NrampC expression is induced in macrophage-rich tissues after exposure to lipopolysaccharide and in a macrophage cell line when stimulated. Thus NrampC is structurally closer to mammalian Nramp2 but may function similar to Nramp1.