Flavobacterium mekongense sp. nov., isolated from the Mekong River in Thailand

Two Gram-stain-negative, aerobic, non-motile, non-gliding, rod-shaped bacterial strains, designated as TBRC 19031^T and TBRC 19032, were isolated from water samples collected from the Mekong River, Thailand. Strain TBRC 19031^T was obtained from Chiang Saen in the upstream section near the borders with China and Myanmar, while TBRC 19032 originated from Khong Chiam, in the downstream section where the river exits Thailand. Colonies of both strains were circular, smooth and deep yellow on Reasoner's 2A agar and did not produce flexirubin-type pigments. Phylogenetic analysis with 16S rRNA gene sequences placed both strains within the genus <i>Flavobacterium</i>, showing the highest sequence similarity to <i>Flavobacterium cheonhonense</i> ARSA-15^T (98.29% for TBRC 19031^T and 98.22% for TBRC 19032). However, whole-genome comparisons between the strains and <i>F. cheonhonense</i> ARSA-15^T revealed average nt identity (89.39% and 89.29%), average aa identity (92.84% and 92.95%) and digital DNA-DNA hybridization (35.00% and 34.70%). The predominant fatty acids were iso-C_15:1, iso-C_15:0 and iso-C_15:0 3-OH, and menaquinone MK-6 was the major respiratory quinone. The major polar lipids of both strains included phosphatidylethanolamine, steryl ester and diacylglycerol. The genome sizes were 3.02 and 3.04 Mbp, with G+C contents of 38.3% and 38.2% for TBRC 19031^T and TBRC 19032, respectively. Comparative genomic analyses revealed the absence of genes involved in sulphate reduction and denitrification pathways and the presence of a gene encoding phosphatidylinositol synthase, distinguishing them from other <i>Flavobacterium</i> within the clade. Ecological profiling using public metagenomic datasets showed that both strains were associated with lotic freshwater environments. This study not only introduces <i>Flavobacterium mekongense</i> sp. nov. as a new species but also provides broader insights into the ecology, metabolism and environmental distribution of freshwater <i>Flavobacterium</i>. The genomic features identified here offer promising leads for future studies in microbial ecology, comparative genomics and functional gene mining in aquatic ecosystems. The type strain is TBRC 19031^T (TBRC 19031^T=NBRC 117006^T).