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

Abstract Magnesium (Mg 2+ ) homeostasis is critical for bacterial survival and virulence. Streptococcus agalactiae is a major piscine and mammalian pathogen, yet the molecular mechanisms governing its Mg 2+ acquisition remain undefined. Here, we identify and characterize CorA 2 as an essential Mg 2+ transporter in a hypervirulent S. agalactiae strain HN016. Through biochemical and genetic assays including a corA 2 deletion mutant (Δ corA 2 ), a complemented strain, and structure-guided point mutants (GGGG gain of function and D206K loss of function), we demonstrate that CorA 2 is essential for Mg 2+ uptake. CorA 2 deficiency severely impaired bacterial growth under Mg 2+ limitation and heightened susceptibility to host-mimicking stresses, including oxidative stress, acid stress, nitrosative stress, and metal ion toxicity. The Δ corA 2 mutant was markedly compromised in resisting phagocytosis and surviving within macrophages. Intriguingly, while the mutant exhibited enhanced adhesion to and invasion of host cells, its capacity to inflict damage was drastically reduced. In a tilapia infection model, the Δ corA 2 strain showed severe attenuation, with significantly reduced mortality (53.3% versus 93.3% in WT) and systemic bacterial burden. Our findings establish that CorA 2 -mediated magnesium homeostasis is essential for S. agalactiae to overcome host immune defenses and establish a systemic infection, highlighting CorA 2 as a potential target for developing novel anti-infective strategies against this pathogen.