Pseudomonas anguilliseptica, an emerging pathogen for cultured largemouth bass (Micropterus salmoides) in China

Pseudomonas anguilliseptica , the causative agent of winter disease in marine fish, has inflicted severe economic losses on the aquaculture industry. In recent years, farmed largemouth bass ( Micropterus salmoides ) in China have suffered outbreaks of a novel disease marked by exophthalmia, dermal hemorrhage and ulceration, and multiple organ injuries, yet its etiology remains undetermined. In this study, three Gram-negative bacterial strains (SICAU PA001, SICAU PA002, and SICAU PA003) were isolated from diseased largemouth bass collected in different regions of Sichuan Province, China. To identify the pathogen and characterize its pathogenicity, morphological observation, antibiotic susceptibility tests, experimental challenge, 16S rRNA gene analysis, multi-locus sequence analysis (MLSA), and complete genome sequencing analyses were conducted. The findings verified that the isolates were P. anguilliseptica and were classified as a novel sequence type (ST), ST8. Antibiotic susceptibility tests revealed that they were sensitive to florfenicol, levofloxacin, enrofloxacin, sulfisoxazole, co-trimoxazole, amoxicillin, and cefotaxime, but were remarkably resistant to aminoglycoside and lincoamide antibiotics. Experimental challenge demonstrated that the LD 50 to largemouth bass was 1.68 × 10 5 CFU/kg. SICAU PA001 had a complete genome sequence of 5.24 Mbp and contained 5249 complete coding regions. By annotating the sequencing genes using the virulence factor database (VFDB) and the comprehensive antibiotic resistance database (CARD), it was determined that the genome contained 237 virulence genes and two antibiotic resistance genes. This study reports the first confirmed case of P. anguilliseptica isolation and identification from diseased largemouth bass in China, highlighting its potential role as a previously unrecognized pathogen in other freshwater fish species. • Pseudomonas anguilliseptica (ST8) was identified as an emerging pathogen in farmed largemouth bass in China. • Infection features exophthalmia, dermal hemorrhage and ulceration, leading to high cumulative mortality (≥35 %). • The pathogen demonstrates strong virulence for largemouth bass, with a low LD 50 of 1.68 × 10⁵ CFU/kg. • Complete 5.24 Mbp genome contains 5249 coding regions, 237 virulence, and 2 antibiotic resistance genes.