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The genetic heterogeneity of the close relatives Vibrio anguillarum and Vibrio ordalii, both serious pathogens of fish causing extensive losses in aquaculture, was studied. Eight housekeeping genes, i.e., atpA, ftsZ, gapA, gyrB, mreB, rpoA, topA, and pyrH, were partially sequenced in 116 isolates from diverse fish species and geographical areas. The eight genes appear to be under purifying selection, and the genetic diversity in the total data set was estimated to be 0.767 ؎ 0.026. Our multilocus sequence analysis (MLSA) scheme identified several widespread clonal complexes and resolved the isolates, for the most part, according to serotype. Serotype O2b isolates from diseased cod in Norway, Ireland, and Scotland were found to be extremely homogeneous.Horizontal gene transfer appears to be fairly common within and between clonal complexes. Taken together, MLSA and in silico DNA-DNA hybridization (DDH) calculations suggest that some isolates previously characterized as V. ordalii, i.e., 12B09, FF93, FS144, and FS238, are in fact V. anguillarum isolates. The precise taxonomic situation for two isolates from Atlantic cod that display several traits consistent with V. ordalii, i.e., NVI 5286 and NVI 5918, and a single environmental strain that was previously considered to represent V. ordalii, i.e., FF167, is less clear. IMPORTANCEIt is still being debated whether V. anguillarum and V. ordalii represent separate bacterial species. Our study addresses this issue and elucidates the degree of genetic variability within this group of closely related bacteria, based on a substantial number of isolates. Our results clearly illustrate the existence of different populations among putative V. ordalii isolates. On the basis of additional full-length genomic analysis, we conclude that most environmental isolates previously identified as V. ordalii lie firmly within the species V. anguillarum. While bona fide fish-pathogenic V. ordalii isolates display a very close genetic relationship with V. anguillarum, they combine a clearly divergent evolutionary pattern with clear phenotypic differences. The study also highlights the need for further characterization of fish-pathogenic isolates from the northern Atlantic region that share phenotypic characteristics with V. ordalii but are genetically closer to V. anguillarum. The retention of taxonomic distinctions between the phenotypically different groups of bacteria is of practical advantage to microbial ecologists and veterinarians. V ibriosis, i.e., systemic infection with either Vibrio anguillarum or Vibrio ordalii (1), probably constitutes the most significant bacterial disease of wild and farmed fish in temperate marine environments worldwide. While 23 different serotypes of V. anguillarum have been identified (2), by far the majority of fish-pathogenic isolates belong to serotypes O1 and O2. In addition, strains not corresponding to known serotypes are commonly identified, mainly from environmental sources (3). Despite considerable phenotypic differences, V. ordalii and ...

Live-feed is indispensable to commercial fish larviculture. However, high bacterial loads in rotifers could pose a biosecurity risk. While this may be true, live-feed associated bacteria could also be beneficial to fish larvae through improved feed utilization or pathogen inhibition following host microbiota modification. The study objective was to elucidate the largely unexplored microbiota of rotifers propagated on five different diets through bacterial community profiling by 16S rRNA gene amplicon sequencing. Investigated rotifer samples had a median observed alpha-diversity of 338 ± 87 bacterial species. Alpha- and Gamma-Proteobacteria dominated the rotifer microbiota followed by members of classes Flavobacteriia, Cytophagia, Mollicutes, Phycisphaerae and Bacteroidia. Different diets significantly altered the bacterial communities associated with rotifers according to PERMANOVA test results and beta dispersion calculations. A common core rotifer microbiome included 31 bacterial species present in relative abundances over 0.01%. We discuss the functional role of some microbiome members. Our data suggested the presence of several known fish pathogens in stock rotifers. However, we found no evidence for increased loads of these presumptive taxa in propagated live-feed rotifers during this field trial.

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