Killing of Bacteria by Copper Surfaces Involves Dissolved Copper

Bacteria are rapidly killed on copper surfaces. However, the mechanism of this process remains unclear. Using Enterococcus hirae, the effect of inactivation of copper homeostatic genes and of medium compositions on survival and copper dissolution was tested. The results support a role for dissolved copper ions in killing.The rapid killing of bacteria by solid copper surfaces is receiving rapidly growing attention. In laboratory experiments, it has been shown that many bacterial species, such as Escherichia coli O157, Staphylococcus aureus, Salmonella enterica, Campylobacter jejuni, Clostridium difficile, and Mycobacterium tuberculosis, are efficiently killed on copper or copper alloy surfaces (1, 3, 4, 6, 7, 12-14). In contrast, on stainless steel, living cells could be recovered even after 28 days. The antimicrobial activity of copper and copper alloys is now well established, and copper has recently been registered at the U.S. Environmental Protection Agency as the first solid antimicrobial material. A key focus is the use of copper in health care facilities, food processing plants, and other areas where clean or aseptic working procedures are required (2). In this connection, it has become important to understand the mechanism of bacterial killing, as it may bear on the possibility of the emergence of resistant organisms, cleaning procedures, and material and object engineering. We here used wild-type and mutant strains of Enterococcus hirae to investigate the influence of copper resistance genes on killing rates. We also evaluated copper dissolution by various media and its relation to killing efficiency. Our findings provide support for a prominent role for dissolved copper in the killing process.