Metals ions are essential for bacterial growth, serving structural and catalytic roles in the cell. However, in excess, they also exert significant toxicity, necessitating the strict control of their acquisition. Metalloproteomic analyses indicate that at least 6% of all bacterial proteins require the transition metal ion, zinc [Zn(II)]. Due to the importance of Zn(II) for bacteria, the human immune system manipulates the bioavailability of the metal ion to limit or prevent bacterial infections. To overcome the reduced Zn(II) bioavailability, bacteria utilise high affinity membrane transporters to satisfy their metal ion requirement. My research focuses on the Zn(II) acquisition pathways employed by the human pathogens Pseudomonas aeruginosa and Streptococcus pneumoniae.

1. Zinc acquisition in Streptococcus pneumoniae
Streptococcus pneumoniae (the pneumococcus) is a globally significant human pathogen, responsible for a broad range of diseases including pneumonia, meningitis, bacteremia and otitis media. Within the pneumococcus, Zn(II) is the second most abundant transition metal ion. To acquire Zn(II), the bacterium utilises an ATP-binding cassette importer, AdcBC, and two high affinity Zn(II)-specific solute-binding proteins, AdcA and AdcAII, for transport of the metal ion across the membrane. In response to Zn(II) limitation, adcAII is significantly up-regulated, together with its co-transcribed gene, phtD. PhtD is one of four highly conserved polyhistidine triad (Pht) proteins (PhtA, PhtB, PhtD and PhtE), which are present in the cell wall of the pneumococcus. Each protein features 5 or 6 histidine triad motifs (H_XXH_XH) that are predicted to bind Zn(II) ions. Zinc acquisition by AdcAII under Zn(II)-limiting conditions has been shown to be dependent on the Pht proteins, with the four Pht proteins sharing overlapping functionality. Further research is required to better understand the role of each of these proteins in pneumococcal Zn(II) acquisition.

2. Characterisation of novel zinc acquisition mechanisms in Pseudomonas aeruginosa
Pseudomonas aeruginosa infections are the leading cause of death and disease in children with cystic fibrosis (CF). Within the CF lung, much of the Zn(II) is bound by the human protein calprotectin to prevent bacterial colonisation. To overcome this, P. aeruginosa is predicted to utilise multiple import mechanisms to scavenge Zn(II). My recent study characterising the ZnuABC importer from P. aeruginosa revealed it to hold a major role in Zn(II) acquisition, with a mutant strain lacking the znuA gene exhibiting an ~60% reduction in cellular Zn(II) content. RNA sequencing analysis of this strain identified a number of additional, previously uncharacterised acquisition systems which likely contribute to Zn(II) acquisition by the bacterium. Together my research is seeking to better understand the role and relative contribution of these putative transport systems to P. aeruginosa Zn(II) acquisition.