Host-Mediated Copper Stress Is Not Protective against Streptococcus pneumoniae D39 Infection

Abstract

Metal ions are required by all organisms for the chemical processes that support life. However, in excess they can also exert toxicity within biological systems. During infection, bacterial pathogens such as Streptococcus pneumoniae are exposed to host-imposed metal intoxication, where the toxic properties of metals, such as copper, are exploited to aid in microbial clearance. However, previous studies investigating the antimicrobial efficacy of copper in vivo have reported variable findings. Here, we use a highly copper-sensitive strain of S. pneumoniae, lacking both copper efflux and intracellular copper buffering by glutathione, to investigate how copper stress is managed and where it is encountered during infection. We show that this strain exhibits highly dysregulated copper homeostasis, leading to the attenuation of growth and hyperaccumulation of copper in vitro. In a murine infection model, whole-tissue copper quantitation and elemental bioimaging of the murine lung revealed that infection with S. pneumoniae resulted in increased copper abundance in specific tissues, with the formation of spatially discrete copper hot spots throughout the lung. While the increased copper was able to reduce the viability of the highly copper-sensitive strain in a pneumonia model, copper levels in professional phagocytes and in a bacteremic model were insufficient to prosecute bacterial clearance. Collectively, this study reveals that host copper is redistributed to sites of infection and can impact bacterial viability in a hypersusceptible strain. However, in wildtype S. pneumoniae, the concerted actions of the copper homeostatic mechanisms are sufficient to facilitate continued viability and virulence of the pathogen. IMPORTANCE Streptococcus pneumoniae (the pneumococcus) is one of the world’s foremost bacterial pathogens. Treatment of both localized and systemic pneumococcal infection is becoming complicated by increasing rates of multidrug resistance globally. Copper is a potent antimicrobial agent used by the mammalian immune system in the defense against bacterial pathogens. However, unlike other bacterial species, this copper stress is unable to prosecute pneumococcal clearance. This study determines how the mammalian host inflicts copper stress on S. pneumoniae and the bacterial copper tolerance mechanisms that contribute to maintenance of viability and virulence in vitro and in vivo. This work has provided insight into the chemical biology of the hostpneumococcal interaction and identified a potential avenue for novel antimicrobial development.