Streptococcus pneumoniae requires manganese for colonization of the human host, but the underlying molecular basis for this requirement has not been elucidated. Recently, it was shown that zinc could compromise manganese uptake and that zinc levels increased during infection by S. pneumoniae in all the niches that it colonized. Here we show, by quantitative means, that extracellular zinc acts in a dose dependent manner to competitively inhibit manganese uptake by S. pneumoniae, with an EC₅₀ of 30.2 µM for zinc in cation-defined media. By exploiting the ability to directly manipulate S. pneumoniae accumulation of manganese, we analyzed the connection between manganese and superoxide dismutase (SodA), a primary source of protection for S. pneumoniae against oxidative stress. We show that manganese starvation led to a decrease in sodA transcription indicating that expression of sodA was regulated through an unknown manganese responsive pathway. Intriguingly, examination of recombinant SodA revealed that the enzyme was potentially a cambialistic superoxide dismutase with an iron/manganese cofactor. SodA was also shown to provide the majority of protection against oxidative stress as a S. pneumoniae ΔsodA mutant strain was found to be hypersensitive to oxidative stress, despite having wild-type manganese levels, indicating that the metal ion alone was not sufficiently protective. Collectively, these results provide a quantitative assessment of the competitive effect of zinc upon manganese uptake and provide a molecular basis for how extracellular zinc exerts a ‘toxic’ effect on bacterial pathogens, such as S. pneumoniae.

Pseudomonas aeruginosa is a ubiquitous environmental bacterium and a clinically significant opportunistic human pathogen. Central to the ability of P. aeruginosa to colonise both environmental and host niches is the acquisition of zinc. Here we show that P. aeruginosa PAO1 acquires zinc via an ATP-binding cassette (ABC) permease in which ZnuA is the high affinity, zinc-specific binding protein. Zinc uptake in Gram-negative organisms predominantly occurs via an ABC permease and consistent with this expectation a P. aeruginosa ΔznuA mutant strain showed an ~60% reduction in cellular zinc accumulation, while other metal ions were essentially unaffected. Despite the major reduction in zinc accumulation, minimal phenotypic differences were observed between the wild-type and ΔznuA mutant strains. However, the effect of zinc limitation on the transcriptome of P. aeruginosa PAO1 revealed significant changes in gene expression that enable adaptation to low-zinc conditions. Genes significantly up-regulated included non-zinc-requiring paralogs of zinc-dependent proteins and a number of novel import pathways associated with zinc acquisition. Collectively, this study provides new insight into the acquisition of zinc by P. aeruginosa PAO1, revealing a hitherto unrecognized complexity in zinc homeostasis that enables the bacterium to survive under zinc limitation.