Abstract

Outbreaks of diseases in farmed fish remain a recurring problem despite the development of vaccines and improved hygiene standards on aquaculture farms. One commonly observed bacterial disease in tropical aquaculture of the South-East Asian region is tenacibaculosis, which is attributed to members of the Bacteroidetes genus Tenacibaculum, most notably T. maritimum. The impact of tenacibaculosis on fish microbiota remains poorly understood. In this study, we analysed the microbiota of different tissue types of commercially reared Asian seabass (Lates calcarifer) that showed symptoms of tenacibaculosis and compared the microbial communities to those of healthy and experimentally infected fish that were exposed to diseased farm fish. The microbiota of diseased farm fish was dominated by Proteobacteria (relative abundance{+/-}standard deviation, 74.5%{+/-}22.8%) and Bacteroidetes (18.07%{+/-}21.7%), the latter mainly comprised by a high abundance of Tenacibaculum species (17.6%{+/-}20.7%). In healthy seabass Proteobacteria had also highest relative abundance (48.04%{+/-}0.02%), but Firmicutes (34.2%{+/-}0.02%) and Fusobacteria (12.0%{+/-}0.03%) were the next two major constituents. Experimentally infected fish developed lesions characteristic for tenacibaculosis, but the microbiota was primarily dominated by Proteobacteria (90.4%{+/-}0.2%) and Firmicutes (6.2%{+/-}0.1%). The relative abundance of Tenacibaculum species in experimentally infected fish was significantly lower than in the commercially reared diseased fish and revealed a higher prevalence of different Tenacibaculum species. One strain was isolated and is described here as sp. nov. Tenacibaculum singaporense TLL-A1T (=DSM 106434T, KCTC 62393T). The genome of T. singaporense was sequenced and compared to those of T. maritimum DSM 17995T and the newly sequenced T. mesophilum DSM 13764T.\n\nImportanceFish production from aquaculture facility has become a major source of protein for human consumption and is expected to further grow to meet the growing demands. Devastating fish diseases, such as tenacibaculosis, can eradicate entire stocks of aquaculture fish in a short time and pose a serious threat to individual fish farmers and overall fish production. Understanding the disease processes and the individual microbial players involved has the potential to develop methods to prevent or mitigate infections on aquaculture farms. This study provides important insights into the microbial ecology of tenacibaculosis from an aquaculture facility in Singapore and highlights the complexity of this fish disease at two different disease stages. Furthermore, the isolation of a novel Tenacibaculum species and comparative genome analysis of three different Tenacibaculum species enhance our view of this economically and environmentally important bacterial genus.