CorA2-mediated magnesium transport is essential for stress adaptation and virulence of Streptococcus agalactiae

Magnesium (Mg²⁺) homeostasis is critical for bacterial survival and virulence. In Streptococcus agalactiae, a major piscine and mammalian pathogen, the CorA2 protein functions as a key Mg²⁺ transporter. To define its role in pathogenesis, we constructed a corA2 deletion mutant (ΔcorA2), and a complemented strain (CΔcorA2). Functional assays confirmed that CorA2 is a Mg²⁺ importer, and that the GGGG and E206K mutations confer gain- and loss-of- function phenotypes, respectively. Importantly, we found that Mg²⁺ availability was vital for serum resistance, with bacterial survival significantly higher under Mg²⁺ sufficient conditions. The ΔcorA2 mutant exhibited impaired growth under Mg²⁺ limitation and increased susceptibility to oxidative and metal stresses. Furthermore, the mutant showed a severe defect in resisting phagocytosis and intracellular survival within macrophages. Despite displaying enhanced adhesion and invasion capabilities in vitro, the overall virulence of the ΔcorA2 strain was significantly attenuated. This was confirmed in a tilapia infection model, where the mutant demonstrated a marked reduction in bacterial loads and virulence. Our findings establish that CorA2-mediated magnesium homeostasis is essential for S. agalactiae to overcome host immune defenses and establish a systemic infection, highlighting CorA2 as a potential target for developing novel anti-infective strategies against this pathogen.