Molecular characterization and pathogenicity of Klebsiella pneumoniae species complex associated with mass mortality of Oncorhynchus mykiss in the Ladakh trans-himalayas

Bacterial diseases pose a significant threat to the entire aquaculture system, both in cold and warm-water environments. The Himalayan region has been widely utilised for trout farming. However, this region is presently under tremendous stress due to the outbreak of several diseases, which are responsible for a lot of mortality and have resulted in significant economic loss. In the present study, Klebsiella pneumoniae belonging to the K. pneumoniae species complex was isolated from diseased rainbow trout Oncorhynchus mykiss cultured in the trans-Himalayan region of Ladakh. Infected fish were identified based on exhibited lethargy, tail fin erosion, abdominal distension, abnormal swimming, and intraperitoneal haemorrhage. The pathogen was identified through morphological, biochemical, and molecular analyses, including partial 16 S rRNA gene sequencing. The phylogenetic analysis confirmed the close genetic relationship of the SLB4 isolate to K. pneumoniae (NR_036794.1), as supported by strong bootstrap values, though we acknowledge that 16 S rRNA sequencing has limitations in discriminating among members of the K. pneumoniae species complex. Antimicrobial susceptibility testing was also carried out, revealing multidrug resistance to ceftazidime, ampicillin, rifampicin, ceftriaxone, trimethoprim-sulfamethoxazole, cefixime, and cefepime, but sensitivity to several aminoglycosides, carbapenems, fluoroquinolones, and other agents. Histopathological examination of infected muscle, liver and kidney tissues revealed severe lesions, including melano-macrophage centres, cytoplasmic vacuolation, focal necrosis, and structural disruption of renal tubules and glomeruli. The present findings confirm a member of the K. pneumoniae species complex as a pathogenic agent in O. mykiss, underscore its capacity to cause systemic organ damage, and highlight its potential role in disseminating multidrug resistance in aquaculture environments. In addition, based on the above findings, active surveillance and antibiotic stewardship have been recommended to mitigate the impact of these infections in cultured rainbow trout.