Isolation, identification and drug sensitivity of the pathogen causing hemorrhagic disease of Rana nigromaculata tadpole

To identify the pathogen causing hemorrhagic disease of Rana nigromaculata tadpoles and screen out sensitive drugs to control the disease, in this study, a strain KD-CXB-1 was isolated from diseased tadpoles, and healthy tadpoles showed hemorrhagic symptoms similar to those of naturally infected ones after being artificially infected with strain KD-CXB-1. Meanwhile, a dominant strain consistent with the morphological, physiological and biochemical characteristics of strain KD-CXB-1 was isolated from the dead tadpoles artificially infected. These results indicated that strain KD-CXB-1 was the pathogen causing hemorrhagic disease of R. nigromaculata tadpoles. Moreover, based on strain KD-CXB-1, several experiments were performed including morphological observation, gram staining, physiological and biochemical tests, 16S rDNA gene sequencing, construction of phylogenetic trees, and drug susceptibility. The morphological observation showed that the KD-CXB-1 was a rod-shaped Gram-negative bacterium with the length of about 1 μm and rough surface. Strain KD-CXB-1 was identified as Aeromonas hydrophila by 16S rDNA sequencing and physiological and biochemical tests. Drug susceptibility tests indicated that strain KD-CXB-1 was highly sensitive to 10 drugs such as florfenicol, Syzygium aromaticum and Caesalpinia sappan; moderately sensitive to Prunus mume, Rhus chinensis and Scutellaria baicalensis, and insensitive to 8 drugs such as amoxicillin, Rheum palmatum and penicillin. In conclusion, our study proved that A. hydrophila was the pathogen causing hemorrhagic disease of R. nigromaculata tadpoles, and the highly sensitive drugs screened out by drug susceptibility tests may provide theoretical reference and practical basis for the effective control of hemorrhagic disease of R. nigromaculata tadpoles.