Pathological and molecular characteristics of a new infectious Streptococcus agalactiae from farmed Schizopygopsis pylzovi in southern China

Abstract

During the months of November and December 2022, a severe disease outbreak led to substantial mortality among Schizopygopsis pylzovi (S. pylzovi) in the Manwan reservoir of Lincang, China. The prominent clinical symptoms observed were exophthalmos with cataract, iris congestion, splenomegaly, and a pale liver. To elucidate the etiology, bacterial isolation was performed, followed by identification using Diff-Quik staining, 16s rDNA sequencing, and specific PCR. The results revealed that all three isolates were a non-hemolytic streptococcus with 93% identity to the 16S rDNA sequence of Streptococcus agalactiae (S. agalactiae), and further specific PCR demonstrated that it belonged to S. agalactiae. In addition, to further elucidate the molecular characteristics of the pathogen, a comprehensive analysis was conducted, encompassing pathological diagnosis, serotyping, multilocus sequence typing (MLST), physiological and biochemical identification, as well as the detection of related virulence genes. The findings showed that the isolates were S. agalactiae serotype Ib ST-891, exhibiting significant damage to the eye, spleen, and brain. Notably, its virulence gene profile and physiological and biochemical characteristics exhibited a significant degree of similarity to those from S. agalactiae serotype Ib ST261. Finally, experimental infection revealed that mortality rates were 100% and 75%, respectively, after exposure to 1 × 108 CFU/mL and 1 × 107 CFU/mL of this isolate suspension. It is the first report of natural infection with S. agalactiae serotype Ib ST-891 in fish, thus providing evidence for the molecular genetic diversity of fish-derived S. agalactiae.