Identification and pathogenicity of multidrug‐resistant Elizabethkingia miricola isolated from farmed American bullfrogs Rana catesbeiana in China with in vitro screening of herbal antimicrobial agents

Objective In 2021, an outbreak of an infectious disease characterized by torticollis, cataracts, and neurological disorders caused massive mortality in farmed American bullfrogs Rana catesbeiana in Hubei province, China. We identified the causal agent in this outbreak, characterized its pathogenicity, and screened candidate antimicrobial agents for future disease control. Methods Bacterium was isolated from the diseased American bullfrogs and identified based on biochemical tests, sequence analyses (16S ribosomal RNA; DNA gyrase subunit B), and experimental challenge. Furthermore, antibiotic sensitivity of the isolated strain was detected with Kirby–Bauer paper diffusion method, and the antibacterial activity of 60 traditional Chinese herbal extracts against the isolated strain was evaluated by agar disc diffusion and broth dilution assays. Result We identified Elizabathkingia miricola strain FB210601 as the causative agent of this disease. The isolated E. miricola strain FB210601 exhibited extensive antibiotic resistance to all tested quinolones, β‐lactam antibiotics, and aminoglycosides. Eight herbal extracts exhibited excellent antimicrobial activity against E. miricola FB210601, especially Caesalpinia sappan and Rhus chinensis, with minimal inhibitory concentrations less than 0.2 mg/mL. Additionally, the combined effects of two‐component herbal mixtures containing C. sappan or R. chinensis were greater than those of the individual extracts. Conclusion Our results provide a reference for understanding the pathogenesis of Elizabethkingia infection in frogs. Furthermore, this study will aid in the application of herbal extracts for protection against infections caused by multidrug‐resistant Elizabathkingia in the future. Impact statement This work studies the Elizabathkingia miricola isolated from diseased American bullfrogs in China, including bacterial identification, its sensitivity to different antibiotics and Chinese herbal extracts, which provides a scientific reference for understanding the pathogenesis of Elizabethkingia in frogs as well as for disease control and prevention.