Cetobacterium sinipercae sp. nov., isolated from Mandarin fish (Siniperca chuatsi)

A novel Gram-stain-negative, anaerobic, non-motile, rod-shaped bacterium, designated strain SF1 ^T , was isolated from the intestinal contents of a healthy adult mandarin fish ( Siniperca chuatsi ) collected from a commercial aquaculture farm in Hubei Province, China. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain SF1 ^T belonged to the genus Cetobacterium , with the highest sequence similarity to Cetobacterium ceti NCFB 3026 ^T (98.13%) and Cetobacterium somerae WAL 14325 ^T (95.05%). The genome of strain SF1 ^T was 2.55 Mb in size, with a DNA G + C content of 28.93 mol%. The genomic relatedness analysis showed that strain SF1 ^T had average nucleotide identity (ANI) values of 78.83% and 74.16%, and digital DNA-DNA hybridization (dDDH) values of 37.00% and 20.10% with C. ceti NCFB 3026 ^T and C. somerae WAL 14325 ^T , respectively. The major cellular fatty acids of strain SF1 ^T were C _14:0 , C _16:0 , and C _17:0 cyclopropane, which differed from those of its closely related species. The polar lipids included phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylmethylethanolamine, and several unidentified lipids, aminophospholipids, and phospholipids. Strain SF1 ^T grew at 20–45 ^° C (optimum 45 ^° C), pH 5.0–9.0 (optimum 7.0), and 0–2% (w/v) NaCl (optimum 1.5%), and exhibited excellent bile salt tolerance. Notably, the cell-free supernatant of strain SF1ᵀ exhibited strong DPPH radical scavenging activity (93.74 ± 3.65%), while superoxide anion and hydroxyl radical scavenging rates were 51.92 ± 1.47% and 12.48 ± 1.53%, respectively, indicating that the antioxidant capacity primarily originates from extracellular secretory components. Genomic annotation revealed multiple copies of antioxidant enzyme genes, including seven sod2 (Mn/Fe-SOD), six trxA , five trxB , one tpx (alkylhydroperoxide reductase), one dps , and two msrA , suggesting an intracellular oxidative defense network that may support extracellular antioxidant metabolite secretion. Based on phenotypic, phylogenetic and chemotaxonomic characteristics, strain SF1 ^T represents a novel species of the genus Cetobacterium , for which the name Cetobacterium sinipercae sp. nov. is proposed. The type strain is SF1 ^T (= GDMCC 1.5528 ^T  = KCTC 25974 ^T ).