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

In 2021, an outbreak of an infectious disease characterized by torticollis, cataracts, and neurological disorders caused massive mortality in farmed American bullfrogs Rana catesbeianain Hubei province, China. We identified the causal agent in this outbreak, characterized its pathogenicity, and screened candidate antimicrobial agents for future disease control. 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. We identified Elizabathkingia miricolastrain FB210601 as the causative agent of this disease. The isolated E. miricolastrain FB210601 exhibited extensive antibiotic resistance to all tested quinolones, β‐lactam antibiotics, and aminoglycosides. Eight herbal extracts exhibited excellent antimicrobial activity against E. miricolaFB210601, especially Caesalpinia sappanand Rhus chinensis, with minimal inhibitory concentrations less than 0.2 mg/mL. Additionally, the combined effects of two‐component herbal mixtures containing C. sappanor R. chinensiswere greater than those of the individual extracts. Our results provide a reference for understanding the pathogenesis of Elizabethkingiainfection in frogs. Furthermore, this study will aid in the application of herbal extracts for protection against infections caused by multidrug‐resistant Elizabathkingiain the future. Impact statementThis work studies the Elizabathkingia miricolaisolated 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 Elizabethkingiain frogs as well as for disease control and prevention. This work studies the Elizabathkingia miricolaisolated 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 Elizabethkingiain frogs as well as for disease control and prevention.