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Virulence and immune efficacy of the piscine Streptococcus
agalactiae ΔcrRNA and ΔcpsE strains

NIE Meng , DONG Yuhao , SUN Yaru , XU Ting , LIN Shiyu , LIU Yongjie *
(College of Veterinary Medicine, Nanjing Agricultural University, Nanjing 210095, China)

Abstract: Streptococcus agalactiae, also known as group B Streptococuus (GBS), is an opportunistic Gram-positive bacterium
with a broad host range that includes humans, terrestrial animals, and aquatic species. China has become the world's largest pro-
ducer and consumer of tilapia. However, S. agalactiae infections severely impede the development of the industry. To assess
the virulence and immune efficacy of two mutant strains (ΔcrRNA and ΔcpsE) derived from the piscine S. agalactiae strain
GD201008-001, and to develop novel strategies for preventing and controlling S. agalactiae infections in tilapia in China, we
first analyzed the epidemiological profiles of S. agalactiae isolates from tilapia in China. Based on molecular epidemiological
analysis of 49 publicly available S. agalactiae genomes from tilapia, we found that the dominant isolates in China belong to
serotype Ia and sequence typing (ST) 7, which clustered within a distinct phylogenetic clade. Safety analysis revealed that,
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compared to the wild-type strain GD201008-001 (LD 50 = 1.04×10 CFU), both the ΔcrRNA (LD 50 = 1.36×10 CFU) and ΔcpsE
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(LD 50 > 1.0×10 CFU) strains exhibited significantly attenuated virulence in Nile tilapia (Oreochromis niloticus), with no
observable histopathological lesions in brain or spleen tissues. Efficacy analysis demonstrated that the ΔcrRNA strain provided
higher RPS than ΔcpsE strain via both injection or immersion vaccination, with 100% RPS following intraperitoneal adminis-
tration and 78.57% RPS following immersion immunization within 14 dpi. Furthermore, ΔcrRNA strain induced a significant
upregulation of immune-related genes (MHC Ⅱβ, CD4-1, IFN-γ and IgM). This study systematically evaluates and compares
the immunological efficacy of the ΔcrRNA and ΔcpsE strains, providing a new vaccine candidate for the prevention and con-
trol of S. agalactiae infections in Oreochromis niloticus.
Key words: Oreochromis niloticus; Streptococcus agalactiae; live attenuated vaccine; capsular polysaccharide; crRNA; pro-
tective efficacy
Corresponding author: LIU Yongjie. E-mail: liuyongjie@njau.edu.cn

Funding projects: Joint Funds of the National Natural Science Foundation of China (U23A20254); Postgraduate Research &
Practice Innovation Program of Jiangsu Province Nature (SJCX23_0220)

https://www.china-fishery.cn 中国水产学会主办 sponsored by China Society of Fisheries
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