Inactivation of the sodA gene of Streptococcus suis type 2 encoding superoxide dismutase leads to reduced virulence to mice.

Superoxide dismutase (SOD) is a virulence factor of certain pathogenic bacteria by diminishing the effect of oxidative burst of phagocytic cells. Earlier reports indicated the presence of manganese-cofactored SOD in Streptococcus suis type 2 (SS2). However, the biological role of SOD and its coding sequence in SS2 has not yet been characterized. The SSU1356-ORF of a clinical SS2 strain ZJ081101 encodes a protein of 201 amino acids with 81-88% identity to SodA of other Streptococcus spp. A sod deletion mutant (Δsod) from the clinical strain was constructed. SOD activity was absent in the cell extract from the Δsod mutant, but present in that from the wild-type or the sod-complemented (CΔsod) strain. The Δsod mutant was more susceptible to oxidative stresses induced by hydrogen peroxide or paraquat. Survival of the sod deletion mutant in RAW264.7 macrophages was only half of that of the wild-type strain. Deletion of sod significantly attenuated virulence of SS2 to mice. Effects of such genetic deletion were complementable using the strain CΔsod. The co-inoculation experiment in mice revealed that the Δsod mutant was far more easily cleared from the body than the wild-type strain as shown by about 3-log reduction of its infection potential in blood and tissues. In summary, we reveal an important role of SOD in pathogenesis of S. suis type 2, most probably by scavenging reactive oxygen species from macrophages.