Aims: The aim of this study was to understand the microbial community of intestinal contents and mucosal layer in the intestine of rainbow trout by means of culture-dependent conventional and independent molecular techniques. Methods and Results: Forty-one culturable microbial phylotypes, and 39 sequences from 16S rRNA and two from 18S rRNA genes, were retrieved. Aeromonadaceae, Enterobacteriaceae and Pseudomonadaceae representatives were the dominant cultured bacteria. Genomic DNA isolated from intestinal contents and mucus was used to generate 104 random clones, which were grouped into 32 phylotypes at 99% minimum similarity, most of which were affiliated with Proteobacteria (>70% of the total). However, unlike library C (intestinal contents), the phyla Bacteroidetes and Fusobacteria were not found in intestinal mucus (library M), indicating that the microbiota in the gut mucus was different from that of the intestinal contents. Twelve sequences were retrieved from denaturing gradient gel electrophoresis analysis, and dominant bands were mostly related to Clostridium. Conclusions: Many novel sequences that have not been previously recognized as part of the intestinal flora of rainbow trout were retrieved. Significance and Impact of the Study: The fish gut harbours a larger bacterial diversity than previously recognized, and the diversity of gut mucus is different from that of intestinal contents.

Aim: To develop a probiotic with effectiveness against Aeromonas sp., which was pathogenic to rainbow trout. Methods and Results: When Bacillus subtilis AB1, which was obtained from fish intestine, was administered for 14 days to rainbow trout in feed at a concentration of 10⁷ cells per gram either as viable, formalized or sonicated cells or as cell-free supernatant, the fish survived challenge with the pathogen. AB1 stimulated immune parameters, specifically stimulating respiratory burst, serum and gut lysozyme, peroxidase, phagocytic killing, total and α1-antiprotease and lymphocyte populations. Conclusions: Bacillus subtilis AB1 was effective as a probiotic at controlling infections by a fish-pathogenic Aeromonas sp. in rainbow trout. Significance and Impact of the Study: Disease control in fish is possible by means of the oral application of live and inactivated cells and their subcellular components with the mode of action reflecting stimulation of the innate immune response.

The deadly neurotoxins of Clostridium botulinum (BoNTs) comprise eight serotypes (A–G; X). The neurotoxin gene cluster encoding BoNT and its accessory proteins includes an operon containing an ntnh gene upstream of the boNT gene. Another operon contains either ha (haemagglutinin) or orfX genes (of unknown function). Here we describe a novel boNT gene cluster from Enterococcus sp. 3G1_DIV0629, with a typical ntnh gene and an uncommon orfX arrangement. The neurotoxin (designated putative eBoNT/J) contains a metallopeptidase zinc‐binding site, a translocation domain and a target cell attachment domain. Structural properties of the latter suggest a novel targeting mechanism with consequent implications for application by the pharmaceutical industry. This is the first complete boNT gene cluster identified in a non‐clostridial genome.