The overlooked shadow killer: Elizabethkingia anophelis as a potent pathogen in American bullfrogs ( Aquarana catesbeiana)

Elizabethkingia anophelis has garnered significant attention due to its rising prevalence in clinical settings, yet its impact on animal infections remains underexplored. In this study, we isolated the strain AWYL-1 from infected American bullfrogs (Aquarana catesbeiana) and conducted comprehensive characterization of its biological properties. Morphologically and biochemically, AWYL-1 resembled the known bullfrog pathogen Elizabethkingia miricola, and exhibited multidrug resistance, being sensitive only to doxycycline. AWYL-1 demonstrated high virulence, with a 100 % mortality rate in bullfrogs at a concentration of 1 x 109 CFU/mL within 14 days, regardless of the infection route, including immersion and intramuscular injection. Gene expression analysis revealed significant upregulation of immune-related genes (e.g., P65, CXCL10, IFN gamma) and cell death-related genes (e.g., BNIP3, Caspase-8) in the kidney and spleen, particularly at 72 h post-infection. Furthermore, experimental infection in mice resulted in a 50 % mortality rate, confirming the cross-species infection potential of AWYL-1. Histopathological examination revealed multi-organ hyperemia, edema, and inflammation, with significant pathological changes in the liver, spleen, kidneys, and lungs. These findings suggest that E. anophelis poses a zoonotic risk, capable of infecting both amphibians and mammals. This study provides critical data for the prevention and control of E. anophelis infections in aquaculture, highlighting the need for ongoing vigilance and further research into its potential impact on public health.