Streptococcus iniae: A Growing Threat and Causative Agent of Disease Outbreak in Farmed Chinese Sturgeon (Acipenser sinensis)

Streptococcus iniae infection of cultured fish species is in an alarming trend, due to the possible emergence of antibiotic resistant strains. The bacteria was responsible for the disease outbreak that caused massive mortality of Chinese sturgeon (Acipenser sinensis) in 2016. We isolated the pathogenic bacteria (HNM-1) from the infected A. sinensis and identified its identity by conventional physiological, biochemical, and molecular techniques. Virulence and pathogenesis of the disease were determined by intraperitoneal injection of the etiological agent to the healthy A. sinensis. Physiological and 16s rRNA molecular analysis identified Streptococcus iniae as the causative agent of the disease outbreak. The bacteria has a unique biochemical profile compared with most of the previously isolated strains. Antimicrobial susceptibility tests revealed that S. iniae HNM-1 is resistant to most of the important antibiotics, including Kanamycin, Amoxicillin, gentamycin, Penicillin G and spectinomycin. The bacteria is pathogenic to Chinese sturgeon via intraperitoneal injection. It instigated pathological changes in vital organs that lead to mortality of the infected A. sinensis with a cumulative mortality of 40% to 100%. S.iniae HNM-1 ability to evade the innate immune system was determined by whole blood killing, exposure to hydrogen peroxide, and biofilm assays. This study represents the first Report of S. iniae infection of Chinese sturgeon (Acipenser sinensis).