Defining the links between thiol metabolism and the toxicity of copper and silver ions in staphylococcus aureus

The rise of antibiotic-resistant infections such as methicillin-resistant S. aureus (MRSA) highlights the need for new therapeutic strategies. One emerging strategy is the antibacterial activity of copper (Cu), a clinically and physiologically relevant antimicrobial. A transposon library was cultured in the presence and absence of Cu(II) and a transposon sequencing approach was used to identify factors involved in Cu tolerance. Analyzing this data lead us to investigate links between cysteine homeostasis and Cu toxicity. Mutations that increase activity of the cystine transporter TcyP, through disruption of its repressor CymR or loss of the alternative importer TcyABC, heighten Cu sensitivity, suggesting that TcyP mediates Cu import. The Cu sensitivity of a cymR mutant is suppressed by loss of TcyP, supporting this model. However, a tcyABC tcyP double mutant shows extreme Cu sensitivity, indicating our model was incomplete. Further investigation linked thiosulfate utilization to increased Cu sensitivity. The unexpected Cu sensitivity of a tcyABC tcyP mutant, led to the hypothesis that cysteine synthase (CysM), which is solely responsible for the cell’s cysteine needs in these conditions, is inhibited by Cu. While relieving metabolic pressure on CysM confers Cu resistance, we found no evidence of direct or indirect Cu-mediated inhibition, suggesting additional links between cysteine metabolism and Cu toxicity.