Isolation, identification, pathogenicity and drug resistance of Acinetobacter johnsonii ST2436 from Pelteobagrus fulvidraco

Pelteobagrus fulvidraco is widely distributed in China. Because of its delicious meat, few intermuscular thorns and rich nutrition, it is deeply loved by many people. P. fulvidraco in an aquaculture enterprise in Jingzhou City, Hubei Province had fin hemorrhage, green liver, ascites, and spleen hemorrhage, which were repeated after drug treatment. To identify the pathogen causing the ascites disease in P. fulvidraco and to screen out effective preventive and therapeutic drugs. The dominant strain was isolated from the diseased P. fulvidraco, and the pathogen was studied in terms of morphological observation, physiological and biochemical identification, motility test, 16S rRNA and rpoB sequencing identification, multi-locus sequence typing (MLST) analysis, regression infection experiment, drug sensitivity experiment, and sensitivity test of traditional Chinese medicine. The isolated strain is a Gram-negative bacterium with the "three-negative" characteristics of Acinetobacter. The 16S rRNA result shows that the isolated strain has the highest homology with Johnson Acinetobacter (NR_117624), reaching 98.41%. The rpoB gene sequencing result further determines that the isolated strain is Johnson Acinetobacter, named AJ2407. The MLST analysis result shows that the isolated strain is ST2436 type. Johnson Acinetobacter AJ2407 carries 6 virulence genes (OmpTSX, alt, act, lip, flgA, flaH), which cause ascites and internal organ hemorrhage in P. fulvidraco, similar to the clinical symptoms of naturally infected P. fulvidraco. The liver, spleen, kidneys and brain tissues of P. fulvidraco can be infected with Johnson Acinetobacter AJ2407, with a lethal concentration (LD₅₀) of 1.88×109 CFU/mL, among which the bacterial load in the spleen is the highest. The isolated strain carries 7 drug resistance genes (aadA1, strA, strB, Aph (3') -Ia, sull, int1, VR) and has moderate biofilm formation ability, it is resistant to 17 drugs such as penicillin, oxacillin, and piperacillin, and sensitive to Chinese medicines such as jujube, honeysuckle, and schisandra. This study provides theoretical basis and data support for the infection of Acinetobacter in yellow catfish and comprehensive prevention and control.