Pathological insights and molecular responses to Morganella morganii infection in largemouth bass (Micropterus salmoides).

Morganella morganii has emerged as a significant pathogen in aquaculture, yet its effects on largemouth bass (Micropterus salmoides) remain poorly understood. We isolated a pathogenic M. morganii strain (SH6) from diseased bass, caused over 76 % mortality in challenge tests with characteristic hemorrhaging, ascites, and hepatic lesions. Histopathological analysis revealed severe granulomatous inflammation and melanomacrophage centers in infected livers. Transcriptomic profiling identified 5799 differentially expressed genes, with significant enrichment in lipid metabolism pathways (fatty acid degradation) and immune responses (TLR signaling, cytokine-cytokine interactions). Notably, immune regulators (il10, irak4, myd88) were upregulated while drug metabolism enzymes (cytochrome P450) were suppressed. Antibiotic susceptibility testing indicated clinical potential for enrofloxacin and ciprofloxacin treatment, though resistance to azithromycin was observed. These findings demonstrate M. morganii's significant threat to bass aquaculture through metabolic disruption and immune function, meanwhile identifying potential treatment options.