Iron and its import systems enhance copper accumulation in Streptococcus pneumoniae

ABSTRACT Metal ions strongly influence bacterial metabolism, where fluctuations in metal availability can shape enzyme function and cellular fitness. In Streptococcus pneumoniae (pneumococcus), iron and copper play important yet contrasting biochemical roles. Iron is essential for growth, serving as a cofactor for enzymes involved in DNA synthesis and energy metabolism. In contrast, copper is toxic, as excess intracellular copper promotes mismetallation and induces oxidative stress. While the pneumococcus has dedicated systems for exporting copper, how copper enters cells remains unclear. Here, we show that iron enhances copper accumulation in the pneumococcus, an effect not observed with other trace metals, such as zinc or manganese. Similar results were observed in a capsule-deficient strain. Metal-specific interactions were consistent with copper accessing the cell through iron uptake pathways. In support of this interpretation, the deletion of selected iron transport proteins significantly reduces copper association. Furthermore, RNA-seq analysis revealed upregulation of the copper export system, suggesting that copper triggers the detoxification pathways. Biochemically, we demonstrate that iron substrate-binding proteins can interact with both iron and copper, supporting the idea of mismetallation. Together, these findings highlight how overlapping in metal-binding chemistry can shape metal homeostasis and reveal how essential metal uptake pathways facilitate toxic metal accumulation. IMPORTANCE Metal homeostasis is essential for maintaining bacterial metabolism. Disruptions to these pathways can stress cells and reduce viability, thus offering potential avenues for antibacterial therapies. For bacteria at the host-pathogen interface, iron acquisition is well characterized and crucial for survival. In contrast, copper is intrinsically toxic in excess, as reflected by the strong, ubiquitous copper-export systems. Despite this toxicity, the mechanisms by which copper enters bacterial cytoplasmic space, specifically Streptococcus pneumoniae , remain poorly defined. Here, we identify a previously unexplored route for copper accumulation, in which copper exploits iron transport pathways to enter the cell. This finding reveals a potential vulnerability in metal homeostasis that may be leveraged to better understand bacterial physiology and the metallo-environment.