Streptococcus pneumoniae (the pneumococcus) is a bacterial pathogen of global significance and the primary aetiological agent of community acquired bacterial pneumonia (CABP). CABP is highly prevalent globally and exerts a significant disease burden, both through healthcare costs and escalating morbidity and mortality. Treatment of CABP requires antibiotic intervention to prevent systemic dissemination of S. pneumoniae, but this is complicated by antibiotic resistance. As a possible avenue for new therapeutic development, previous studies have identified that high concentrations of the metal ion, copper, are mobilised to sites of infection. However, the putative role of copper as an antimicrobial at the host-pathogen interface remained poorly understood.
Here, we sought to investigate the chemical biology of the lung in response to pneumococcal pneumonia. Using an invasive serotype 2 strain (D39) we induced pneumonia in a murine model and determined tissue copper abundance. Our data show that total copper concentrations in the lung increased only modestly. However, subsequent spatial mapping of lung copper distribution revealed accumulation in highly spatially discrete ‘hot-spots’, containing copper concentrations ~100-fold higher than naïve levels.
To investigate if this increased copper abundance could exert a direct antimicrobial effect on S. pneumoniae, mutant derivatives that lacked the primary copper resistance mechanisms were constructed (copper efflux; ∆copA, copper buffering; ∆gshT, or both; ∆copA∆gshT). In vitro growth kinetics revealed that the double mutant was highly susceptible to copper intoxication and accumulated significantly more cellular copper than the single mutants. A subsequent pneumonia infection model revealed that the ∆copA∆gshT strain was significantly perturbed for in vivo virulence compared to wild-type. However, this perturbation was not observed when lung infection was bypassed in a bacteraemia infection model, illustrating the antimicrobial potential of the copper hot-spots in the lungs.
Collectively, these data demonstrate that mobilisation of copper by the innate immune response has the potential to exert potent antimicrobial activity on S. pneumoniae. This suggests that enhancing copper toxicity within the lung through subversion of resistance mechanisms may provide a novel therapeutic strategy to address the waning efficacy of current antibiotics for treatment of CABP.