Isolation, identification and characterization of Shewanella algae from reared tongue sole, Cynoglossus semilaevis Günther

Abstract

In this study, a Gram-negative bacterium was isolated from diseased Cynoglossus semilaevis (C. semilaevis) and tentatively named strain CSG-15. It was preliminary identified as Shewanella algae (S. algae) based on morphological, physiological and biochemical characteristics. The sequenced 16S rRNA gene of CSG-15 strain (Genbank accession no. KX455851) was 1509bp. And it exhibited 99.86%, 99.86% and 99.80% of identities compared with those of S. algae strains ATCC51192 (NR117771), YJ06114 (EF542799), and MAS2737 (GQ372875), respectively. Phylogenetic analysis grouped CSG-15 strain in the S. algae cluster, and showed the closest relation to S. algae ATCC51192 and YJ06114. Infection experiment showed that the median lethal dosage (LD50) for CSG-15 was calculated as 1.0×106CFU/g fish weight. The pathological observation on the intestine showed that CSG-15 strain caused noticeable histological lesions, such as inflammatory cell infiltrating and villi shedding. Antimicrobial susceptibility test showed that it was highly sensitive to most of aminoglycosides, β-lactams, tetracyclines, quinolones, macrolides aminocyclitol, sulphonamides, chloramphenicols antibiotic. Growing characteristics showed that CSG-15 could grow under the condition of temperature 26–34°C, pH5–9 and 20–50ppt, and had a better adaptability to environment. The growth curve of strain CSG-15 under the condition of 30°C, pH7.0 and 30ppt showed four distinct phases of bacterial growth. To the best of our knowledge, this is the first report of S. algae linked to diseased C. semilaevis. Statement of relevanceIn this study, a new Shewanella algae strain was isolated from diseased C. semilaevis for the first time. It was identified based on morphological, physiological, biochemical characteristics and 16S rRNA gene sequences analysis. Subsequently, antimicrobial susceptibility, pathogenicity and growing characteristics of S. algae were studied. This report can provide a scientific reference for characterization of Shewanella species, prevention and treatment of bacterial disease caused by S. algae in fish.