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

Abstract

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

Identification of Acinetobacter schindleri isolated from Chinese giant salamanders (Andrias davidianus)

“…This is the first report of A. schindleri infection in A. davidianus, and this strain has caused many deaths. The LD 50 of A. avidianus DN-03 was confirmed to be similar to that of A. baumannii, 25 A. johnsonii, 26 A. lwoffii, 27 etc., causing pathologic lesions in several organs and tissues and exhibiting strong pathogenicity. Acinetobacter species can be transmitted between humans and animals.…”

Identification of Acinetobacter schindleri isolated from Chinese Giant Salamanders (Andrias davidianus)

“…To the best of our knowledge, this is the rst report of A. schindleri infection in A. davidianus, and this strain has caused many deaths. It was con rmed by LD 50 of A. avidianus DN-03 to be similar to A. baumannii [22], A. johnsonii [23], A. lwo i [24] etc., showing strong pathogenicity in aquatic animals. Due to the ability of Acinetobacter members to be transmitted between humans and animals [17,25].…”

Biofilm formation and auto-aggregation abilities of novel targeted aqua-probiotics

“…Fish vibriosis is widespread in the seas and brackish waters, affecting both marine and coastal freshwater fish of various species, including salmon, cod, eel, herring, perch, and flounder [71]. Along with fish vibriosis, Aeromonas septicemia, edwardsiellosis, columnaris and streptococcus, as well as a number of other diseases, are largely responsible for economic losses in aquaculture production [72][73][74][75][76][77][78][79][80][81][82][83][84].…”