The well-established killing of bacteria by copper surfaces, also called contact killing, is currently believed to be a combined effect of bacterial contact with the copper surface and the dissolution of copper, resulting in lethal bacterial damage. Iron can similarly be released in ionic form from iron surfaces and would thus be expected to also exhibit contact killing, although essentially no contact killing is observed by iron surfaces. However, we show here that the exposure of bacteria to iron surfaces in the presence of copper ions results in efficient contact killing. The process involves reduction of Cu~(2+) to Cu~(+) by iron; Cu~(+) has been shown to be considerably more toxic to cells than Cu~(2+). The specific Cu~(+) chelator, bicinchoninic acid, suppresses contact killing by chelating the Cu~(+) ions. These findings underline the importance of Cu~(+) ions in the contact killing process and infer that iron-based alloys containing copper could provide novel antimicrobial materials.
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