Streptococcus pneumoniae (the pneumococcus) is the world’s foremost bacterial pathogen, responsible for more than one million deaths each year. As a host-adapted pathogen, S. pneumoniae is able to colonise multiple anatomical niches, all of which have distinct extracellular environments. Adaptation to fluctuating metal-ion concentrations at the host-pathogen interface is essential for successful colonisation and presents a major challenge for invasive bacterial pathogens. S. pneumoniae addresses this by using a combination of metal homeostatic mechanisms to tightly regulate intracellular metal concentrations. However, the molecular details of how this is achieved remains poorly understood.
Recently we showed that selective metal-ion uptake was dependent upon essential protein-metal interactions that were dictated by a complex interplay of metal-ion coordination chemistry at the metal-binding site (Couñago, Ween, Begg et al 2014. Nature Chemical Biology). However, our work also revealed that the intrinsic flexibility within protein structures permitted non-physiological ions, such as cadmium, to subvert these selectivity determinants, resulting in non-cognate interactions (mismetallation). Here we show that although PsaA, the manganese-binding protein of S. pneumoniae, has evolved to optimally bind and release manganese, imperfect steric selection of metal-ions also make it permissive for the acquisition of cadmium. Here, we combined transcriptional analyses, whole cell metal-ion determination, scanning electron microscopy, and high-resolution structural data, to show that the inability of the pneumococcus to regulate cadmium uptake resulted in a dramatic decrease in the cellular accumulation of manganese and zinc ions. This was due to mismetallation of the proteins involved in metal-ion uptake and efflux, as well as other metal-responsive transcriptional regulators. This resulted in a global dysregulation of pneumococcal metal homeostasis (Begg et al 2015. Nature Communications). Collectively, our work provides new insights into fundamental aspects of bacterial metal-ion homeostasis and also reveals how chemically similar metal-ions can disturb the molecular determinants responsible for metal-ion selection.