Identification of an Acinetobacter lwoffii strain isolated from diseased hybrid sturgeon (Acipenser baerii9x Acipenser schrenckii♂)

Members of the genus Acinetobacter are important opportunistic pathogens responsible for various nosocomial infections. However, Acinetobacter spp. are rarely isolated from fish. In this study, we isolated a dominant bacterial strain named I-702 from diseased hybrid sturgeons (Acipenser baerii9 x Acipenser schrenckii male) bred at a farm in Yichang City, Hubei Province, China. We identified the strain as Acinetobacter lwoffii based on morphological, physiological and biochemical, and 16S rRNA gene sequence analyses. Regression infection experiments using strain I-702 showed that the symptoms of the artificially infected fish were the same as those of natural diseased sturgeons, such as abdominal congestion and redness and swelling of cloaca. We calculated that the median lethal dose (LD50) of strain I-702 for hybrid sturgeons was 1.22 x 102 +/- 0.11 colony forming units/g fish. We also compared the proportion of different leukocytes between healthy and diseased fish; the proportion of neutrophils (30.89%) and monocytes (36.68%) in infected fish increased significantly, whereas that of lymphocytes (30.17%) decreased. Histopathological examination revealed that strain I-702 caused damage to the liver, and intestine tissues of sturgeons. Gut microbial analysis revealed that I-702 infection caused dysbiosis in sturgeons. The relative abundance of Acinetobacter in the diseased group (54.4%) was significantly higher than that in the healthy group (10.7%), whereas the relative abundance of Cetobacterium decreased significantly (healthy sturgeon: 25.6%, diseased sturgeon: 1.5%). Strain I-702 was susceptible to aminoglycosides (neomycin sulfate, amikacin, and gentamycin) and tetracyclines (minocycline and doxycycline). These results suggest that A. lwoffii is an important pathogen of sturgeons. Moreover, the study provides a scientific basis for the control and prevention of A. lwoffii infection in fish.