Endogenously produced H2O2 is intimately involved in iron metabolism in Streptococcus pneumoniae

Abstract

ABSTRACT In the presence of molecular oxygen, the human pathogen Streptococcus pneumoniae produces and secretes large amounts of hydrogen peroxide (H2O2), which can readily interact with free and heme-bound iron. Here, we investigated the role of the endogenously produced H2O2 in iron acquisition. The data revealed that S. pneumoniae uses H2O2 to liberate iron from met-hemoglobin (Hb-Fe3+) extracellularly, allowing the bacterium to import and grow on free iron even when cultivated on met-hemoglobin as the only iron source. The loss of H2O2 production leads to a dramatic pneumococcal intake of heme and is associated with a robust upregulation of most iron uptake machinery (indicating an iron starvation signal). These and other data reveal a close and previously unexplored relation between H2O2 production and iron metabolism in S. pneumoniae. The data also show that, in addition to extracellular degradation, pneumococci are armed with H2O2-independent mechanisms for intracellular heme catabolism. IMPORTANCE Heme degradation provides pathogens with growth essential iron, leveraging on the host heme reservoir. Bacteria typically import and degrade heme enzymatically, and here, we demonstrated a significant deviation from this dogma. We found that Streptococcus pneumoniae liberates iron from met-hemoglobin extracellularly, in a hydrogen peroxide (H2O2)- and cell-dependent manner; this activity serves as a major iron acquisition mechanism for S. pneumoniae. Inhabiting oxygen-rich environments is a major part of pneumococcal biology, and hence, H2O2-mediated heme degradation likely supplies iron during infection. Moreover, H2O2 reaction with ferrous hemoglobin but not with met-hemoglobin is known to result in heme breakdown. Therefore, the ability of pneumococci to degrade heme from met-hemoglobin is a new paradigm. Lastly, this study will inform other research as it demonstrates that extracellular degradation must be considered in the interpretations of experiments in which H2O2-producing bacteria are given heme or hemoproteins as an iron source.

Keywords

• Streptococcus pneumoniae
• iron acquisition
• heme degradation
• hydrogen peroxide
• bacteria physiology
• iron metabolism