Co-isolated bacteria from septicemia of female Siberian sturgeon broodstock: Lactococcus petauri resisting Aeromonas hydrophila infection

The rapid development of sturgeon aquaculture has satisfied the market demand for sturgeon products and effectively avoided the dilemma of a sharp reduction of wild resources. However, the risk of infectious death after artificial propagation of female Siberian sturgeon (Acipenser baerii) has threatened sturgeon farming. To find out the pathogeny, bacteria were isolated from the female A. baerii broodstock infection, and their effect was researched. The results showed that the strain JLAH was identified as Aeromonas hydrophila by the phylogenetic tree of 16S rRNA gene with β-hemolytic activity and typical virulence genes of hemolysin (hlyA) and cytotoxic enterotoxin (alt). The other strain JLlg08PX was identified as Lactococcus petauri by genome sequencing analysis, with the ANI value of 98.55% and the dDDH value of 86.00%, which was the closest relative to L. petauri 159469T. In vivo, the cumulative mortality of A. hydrophila infection reached 88.89% with necrosis of hepatocytes and the damage of valvular intestinal as consistent with the spontaneous symptoms. Interestingly, co-infection with A. hydrophila and L. petauri could decrease the mortality to 33.33%, and the pathological damages were alleviated in the liver and valvular intestinal. No death or clinical symptoms were observed in L. petauri infection by intraperitoneal injection, oral gavage, or immersion. In vitro, the growth of A. hydrophila, A. veronii, and Yersinia ruckeri was inhibited by the cell-free supernatant (CFS) of L. petauri. Taken together, A. hydrophila is the etiological agent, while the co-isolated L. petauri could resist A. hydrophila infection. This study enriched the understanding of the parental sturgeon infection after artificial propagation and enlightened the prevention strategies of antagonistic strain in Aeromonas infection.