Towards control of Streptococcus iniae

A divalent DNA vaccine based on Sia10 and OmpU induces cross protection against Streptococcus iniae and Vibrio anguillarum in Japanese flounder

Streptococcosis and vibriosis caused by Streptococcus iniae and Vibrio anguillarum respectively have affected fish culture industries around the world. Previous studies have indicated that the S. iniae antigen Sia10 and the V. anguillarum outer membrane protein OmpU, when used as DNA vaccines, induce protection in turbot (Scophthalmus maximus) and Asian seabass (Lates calcarifer) respectively. In this study, with an effort to develop effective vaccines against S. iniae and V. anguillarum, we constructed three DNA vaccines based on Sia10 and OmpU and examined their immune effects in a model of Japanese flounder (Paralichthys olivaceus), which in China is known to suffer from both streptococcosis and vibriosis. Of the three DNA vaccines constructed in this study, pIDSia10 and pIDOmpU express Sia10 and OmpU respectively, while pSiVa1 expresses Sia10 and OmpU as two individual proteins translated from a single bicistronic mRNA transcript. At 7 and 28 days post-vaccination, vaccine plasmids and expression of the vaccine-encoding genes were detected in the muscle, spleen, kidney, and liver of the vaccinated fish. Immunocolloidal gold electron microscopy detected production of Sia10 and OmpU proteins in pIDSia10- and pIDOmpU-vaccinated fish respectively, while both Sia10 and OmpU proteins were detected in pSiVa1-vaccinated fish. At one and two months post-vaccination, fish vaccinated with pIDSia10 and pSiVa1 exhibited comparable relative percent of survival (RPS) rates (80%-87%) following lethal S. iniae challenge. Similar protection rates were produced by fish vaccinated with pIDOmpU and pSiVa1 following lethal V. anguillarum challenge. Immunological analysis showed that (i) all vaccines induced specific serum antibody production which enhanced complement-mediated bactericidal activity, and (ii) pSiVa1 modulated the expression of a wide spectrum of immune relevant genes in a time-dependent manner. Together these results indicate that pSiVa1 is an effective bivalent vaccine that induces strong cross protection in flounder against S. iniae and V. anguillarum.

Epidemiology and pathogenicity of zoonotic streptococci

Zoonotic infections caused by Streptococcus spp. have been neglected in spite of the fact that frequency and severity of outbreaks increased dramatically in recent years. This may be due to non-identification since respective species are often not considered in human medical diagnostic procedures. On the other hand, an expanding human population concomitant with an increasing demand for food and the increased number of companion animals favour conditions for host species adaptation of animal streptococci. This review aims to give an overview on streptococcal zoonoses with focus on epidemiology and pathogenicity of four major zoonotic species, Streptococcus canis, Streptococcus equi sub. zooepidemicus, Streptococcus iniae and Streptococcus suis.

The Streptococcus iniae transcriptional regulator CpsY is required for protection from neutrophil-mediated killing and proper growth in vitro

The ability of a pathogen to metabolically adapt to the local environment for optimal expression of virulence determinants is a continued area of research. Orthologs of the Streptococcus iniae LysR family regulator CpsY have been shown to regulate methionine biosynthesis and uptake pathways but appear to influence expression of several virulence genes as well. An S. iniae mutant with an in-frame deletion of cpsY (ΔcpsY mutant) is highly attenuated in a zebrafish infection model. The ΔcpsY mutant displays a methionine-independent growth defect in serum, which differs from the methionine-dependent defect observed for orthologous mutants of Streptococcus mutans and Streptococcus agalactiae. On the contrary, the ΔcpsY mutant can grow in excess of the wild type (WT) when supplemented with proteose peptone, suggesting an inability to properly regulate growth. CpsY is critical for protection of S. iniae from clearance by neutrophils in whole blood but is dispensable for intracellular survival in macrophages. Susceptibility of the ΔcpsY mutant to killing in whole blood is not due to a growth defect, because inhibition of neutrophil phagocytosis rescues the mutant to WT levels. Thus, CpsY appears to have a pleiotropic regulatory role for S. iniae, integrating metabolism and virulence. Furthermore, S. iniae provides a unique model to investigate the paradigm of CpsY-dependent regulation during systemic streptococcal infection.