RESUMO
In this study, V. gigantis strain C24 was isolated from cases of winter mortalities of hatchery-reared European seabass (Dicentrarchus labrax) broodstock in Türkiye. The first mortalities were reported in September 2016 and occurred annually in early autumn/late winter until the end of February 2019, when 15% of accumulated mortality was recorded. Diseased moribund fish exhibited general septicemic signs, including dermal ulcerations with hemorrhagic margins, distended abdomens, and hemorrhages below the pectorals, pelvic fins, and at the operculum. Postmortem findings showed congestion in several internal organs, hemorrhagic ascitic fluid, and congested prolapsed anal openings. The representative bacterial isolate V. gigantis strain C24 was characterized as Gram-negative, motile, nitrite-producing, and as vibrio static agent O/129-sensitive. The full-length 16S rRNA sequence (Accession No. ON778781) and gyrB gene sequence (Accession No. ON792326) of the C24 strain showed high similarity to V. gigantis strains. Moreover, the whole-genome average nucleotide identity (ANI) values (ANI > 97.7%) against four V. gigantis strains above the species demarcation limit unambiguously identified the C24 isolate as a member of this species. A preliminary virulence-gene analysis showed that the V. gigantis isolate C24 encoded at least three exotoxins, including two aerolysins and a thermolabile hemolysin. The experimental infection showed that the C24 isolate exhibited low to moderate virulence in experimentally infected European seabass juveniles. Interestingly, antimicrobial susceptibility testing revealed that the C24 isolate was susceptible to nalidixic acid, ciprofloxacin, and several other antibiotics but resistant to tilmicosin, kanamycin, streptomycin, and ampicillin. To our knowledge, this study is the first to report that V. gigantis could be considered an emerging bacterial pathogen in Türkiye, and it may threaten the international European seabass production.
RESUMO
Gill diseases may cause high mortalities in farmed Atlantic salmon. In seawater reared fish co-infections involving the epitheliocystis associated bacterium Ca. Branchiomonas cysticola, the microsporidian Desmozoon lepeophtherii, the causative agent of amoebic gill disease Paramoeba perurans and salmon gill poxvirus are common and histopathological lesions may be complex. Here, we report detection of these agents utilising multiplex real-time PCR and link the presence of agents to histopathologically visible gill lesions by in situ hybridisation (ISH) utilising RNAscope®. We show that Ca. Branchiomonas cysticola infections may remain undetected if diagnostic investigations are restricted to histopathology alone. Further, positive in situ labelling of Ca. Branchiomonas cysticola was observed within epitheliocysts, but also in small foci within areas of inflammation and necrosis in which histologically detectable epitheliocysts were not visible. In situ labelling of D. lepeophtherii corresponded well with tissue distribution patterns previously associated with this microsporidian. Salmon gill poxvirus was associated with apoptotic gill epithelial cells, while Ca. Piscichlamydia salmonis could not be associated with pathological changes. The multiplex real-time PCRs utilised were rapid and sensitive diagnostic tools and the results corresponded well with ISH. This study shows that the agents involved in complex gill disease can be linked to lesions using ISH and suggests that Ca. B. cysticola plays a crucial role in the development of gill disease in the farming of salmon in Norway.
RESUMO
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.
