Identification and Pathogenicity of Emerging Fish Pathogen Acinetobacter johnsonii from a Disease Outbreak in Rainbow Trout (Oncorhynchus mykiss)

In November 2017, a group of farmed rainbow trout (Oncorhynchus mykiss) died in Yunnan Province, China, likely due to infection. These exhibited slow movement, no feeding, dark body color, exophthalmia, and occasional ulcers on the surface. Pathogens were isolated from diseased livers, head kidneys, spleens, eyes, strain was preliminarily identified as Acinetobacter johnsonii based morphology, biochemical tests, 16S rRNA rpoB sequences. The strains highly sensitive florfenicol, ciprofloxacin, oxolinic acid, norfloxacin but resistant ampicillin, doxycycline, sulfadiazine, thiamphenicol, sulfamethazine. selected isolate performed for experimental infection confirm its pathogenicity. Experimentally infected fish showed disease symptoms similar those observed naturally with these bacteria. Vacuolar degeneration prevalent liver spleen fish. Cytoplasm volume very high lymphocytes kidney. protein structure topology monomeric outer membrane A (OmpA), 34 (Omp34), nucleoside-specific transporter Tsx (OmpTsx) A. predicted by AlphaFold 2. local distance difference test (pLDDT) score used assessment prediction. Comprehensive analysis antigenic determinants OMP family member, OmpA, contains three determinants, which are 90% epitopes baumannii. OmpA is recombinant 35 kDa expressed Escherichia coli system. Based findings, regarded an emerging opportunistic pathogen trout. can be subunit vaccine candidate molecule johnsonii.