Capsular polysaccharide of Streptococcus agalactiae is an essential virulence factor for infection in Nile tilapia (Oreochromis niloticus Linn.)

Abstract

Streptococcus agalactiae (Group B Streptococcus, GBS) is associated with diverse diseases in aquatic animals. The capsule polysaccharide (CPS) encoded by the cps gene cluster is the major virulence factor of S. agalactiae; however, limited information is available regarding the pathogenic role of the CPS of serotype Ia piscine GBS strains in fish. Here, a non‐encapsulated mutant (Δcps) was constructed by insertional mutagenesis of the cps gene cluster. Mutant pathogenicity was evaluated in vitro based on the killing of whole blood from tilapia, in vivo infections, measuring mutant survival in tilapia spleen tissues and pathological analysis. Compared to wild‐type (WT) GBS strain, the Δcps mutant had lower resistance to fresh tilapia whole blood in vitro (p < 0.01), and more easily cleared in tilapia spleen tissue, and was highly attenuated in tilapia and zebrafish. Additionally, compared to the Δcps mutant, numerous GBS strains and severe tissue necrosis were observed in the tilapia spleen tissue infected with WT strains. These results indicated that the CPS is essential for GBS pathogenicity and may serve as a target for attenuation in vaccine development. Gaining a better understanding of the role, the GBS pathogenicity in fish will provide insight into related pathogenesis and host–pathogen interactions.

Trial Evaluation of Protection and Immunogenicity of Piscine Bivalent Streptococcal Vaccine: From the Lab to the Farms

“…Based on our research on S. agalactiae characterization, we could isolate 2 major serotypes, serotype Ia and serotype III, that caused disease in tilapia, which was similar to the surveillance in other countries in SE Asia such as Malaysia [ 17 , 21 , 22 , 37 , 38 , 39 ]. Based on our findings, both serotypes showed similar distribution throughout Thailand during 2012–2014; however, it was found that serotype III exhibited higher virulence than serotype Ia [ 21 ].…”

Construction of Streptococcus agalactiae sialic acid mutant and evaluation of its potential as a live attenuated vaccine in Nile tilapia (Oreochromis niloticus)

“…Although many reports stress the importance of CPS in the survival of GBS in hosts (Hammerschmidt et al, 2005; Lowe et al, 2007; Millard et al, 2012; Zeng et al, 2016; Zhang et al, 2019), studies on GBS Sia are limited. In the present study, we successfully constructed the ΔSia mutant by neuA‐D gene cluster deletion from GBS WC1535 by homologous recombination method to explore the role of GBS Sia in host–pathogen interaction and evaluate the potential of the ΔSia mutant as a live attenuated vaccine.…”

“…A previously established approach was used to observe the WT and ΔSia mutant WC1535 using transmission electron microscopy (TEM) (Zhang et al, 2019). Bacteria in the logarithmic growth phase were collected and fixed in phosphate buffer containing 2.5% glutaraldehyde.…”

Research Advances on Tilapia Streptococcosis

“…Similarly, using tilapia challenge assays, CPS-deficient GBS showed attenuated pathogenesis. And this mutant GBS was also cleared more easily from tilapia spleen tissue compared to the wild type GBS strain examined [ 34 ]. It is now understood that CPS can suppress the aggregation of the complement factor C3b to inhibit phagocytotic killing by host cells [ 35 , 36 ].…”