Abstract
The Siberian sturgeon (Acipenser baerii) is a long-lived and late-maturing fish; its natural populations are considered endangered and listed in the first class of protected animals in Kazakhstan. To date, attempts have been made to increase the number of sturgeons through artificial reproduction in aquaculture and a release of the resultant juveniles into the Ural River, which flows into the Caspian Sea. This species faces an increased risk of Aeromonas infections. Aeromonas hydrophila is the most important sturgeon pathogen in Kazakhstan, but studies on A. hydrophila infection in Kazakhstan are still inconclusive. In the present work, our purpose was to isolate and characterize dominant bacteria in diseased A. baerii. This isolate, tentatively named AB005, was identified as A. hydrophila in an analysis of its morphological, physiological, and biochemical features and 16S ribosomal-RNA and gyrB gene sequences. A pathogenicity test was carried out for the isolate on healthy Oreochromis niloticus and Acipenser ruthenus via intraperitoneal injection along the caudal peduncle of the fish. Half-lethal doses (LD50) of isolate AB005 for O. niloticus and A. ruthenus were determined: 8.37 × 105 and 2.89 × 106 colony-forming units per milliliter, respectively. Virulence gene profiling revealed the presence of seven virulence genes related to pathogenicity (acyltransferase, phospholipase A, serine protease, heat-stable cytotonic enterotoxin, nuclease, and aerolysins A and B) in this A. hydrophila isolate. Drug sensitivity testing showed that the isolate is sensitive to quinolones, aminoglycosides, nitrofurans, amphenicols, and tetracyclines. The present findings will lay the foundation for future research on this pathogen in Siberian-sturgeon aquaculture.