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Updated: Sep 26, 2026
Non-invasive Imaging of the Innate Immune Response in a Zebrafish Larval Model of Streptococcus iniae Infection
Published on: April 21, 2015
A Streptococcus iniae-associated outbreak in hybrid sturgeon (Acipenser baerii ♀ × Acipenser schrenckii ♂):
Luyun Ni1, Hangyu Lin1,2, Shuzhen Wang1
1Fisheries Research Institute, Sichuan Academy of Agricultural Sciences (Sichuan Fisheries Research Institute), Chengdu, China.
Introduction:
Streptococcus iniae is an important bacterial pathogen of cultured fish, but its pathological effects and the host responses it induces in hybrid sturgeon remain poorly understood. This study investigated an acute disease outbreak in 4-month-old hybrid sturgeon (Acipenser baerii ♀ × Acipenser schrenckii ♂) during a high-temperature period and examined the transcriptional responses of the liver and spleen using a separate controlled challenge experiment.
Methods:
A dominant β-hemolytic bacterial isolate was recovered from multiple internal organs of naturally diseased fish and identified using phenotypic characterization, 16S rRNA gene sequencing and phylogenetic analysis, and a species-specific PCR assay targeting the lctO gene. Histopathology and transmission electron microscopy were used to characterize tissue and subcellular alterations. In the controlled challenge, liver and spleen samples were collected at 48 h post-injection and analyzed using PacBio single-molecule real-time and Illumina RNA sequencing.
Results:
Affected fish exhibited abnormal swimming, a swollen and hyperemic anus, and pronounced gas and fluid accumulation in the spiral valve intestine and swim bladder. The isolate was identified as S. iniae. Histopathological examination revealed hepatic congestion and necrosis, splenic white pulp atrophy, intestinal mucosal exfoliation, myocarditis, and gill injury. Transmission electron microscopy showed coccoid bacterial structures in several tissues, together with mitochondrial swelling, cristae disruption, microvillar damage, and autophagosome-like structures. PacBio sequencing generated 120,215 non-redundant transcripts, with 94.4% completeness according to BUSCO analysis, providing a full-length transcript reference for short-read expression analysis. Gene set enrichment analysis showed negative enrichment of the fat digestion and absorption pathway and positive enrichment of the apoptosis pathway in the liver. In the spleen, the Toll and Imd and IL-17 signaling pathways were positively enriched. RT-qPCR analysis of eight selected transcripts showed expression directions consistent with the RNA-seq results.
Discussion:
These findings characterize the pathological features of S. iniae-associated disease in hybrid sturgeon and provide a transcript-level view of the liver and spleen responses following controlled experimental challenge.
