Thiol Peroxidase Is an Important Component of Streptococcus pneumoniae in Oxygenated Environments

Streptococcus pneumoniae is an aerotolerant Gram-positive bacterium that causes an array of diseases, including pneumonia, otitis media, and meningitis. During aerobic growth, S. pneumoniae produces high levels of H2O2. Since S. pneumoniae lacks catalase, the question of how it controls H2O2 levels is of critical importance. The psa locus encodes an ABCMn 2-permease complex (psaBCA) and a putative thiol peroxidase, tpxD. This study shows that tpxD encodes a functional thiol peroxidase in-volved in the adjustment of H2O2 homeostasis in the cell. Kinetic experiments showed that recombinant TpxD removed H2O2 efficiently. However, in vivo experiments revealed that TpxD detoxifies only a fraction of the H2O2 generated by the pneumococ-cus. Mass spectrometry analysis demonstrated that TpxD Cys58 undergoes selective oxidation in vivo, under conditions where H2O2 is formed, confirming the thiol peroxidase activity. Levels of TpxD expression and synthesis in vitrowere significantly in-creased in cells grown under aerobic versus anaerobic conditions. The challenge with D39 and TIGR4 with H2O2 resulted in tpxD upregulation, while psaBCA expression was oppositely affected. However, the challenge oftpxDmutants with H2O2 did not affect psaBCA, implying that TpxD is involved in the regulation of the psa operon, in addition to its scavenging activity. Vir-ulence studies demonstrated a notable difference in the survival time of mice infected intranasally with D39 compared to that of mice infected intranasally with D39tpxD. However, when bacteria were administered directly into the blood, this difference disappeared. The findings of this study suggest that TpxD constitutes a component of the organism’s fundamental strategy t