Human use of copper dating back to the Bronze Age has shaped the evolution of bacteria, leading to bugs that are highly resistant to the metal's antibacterial properties. Large amounts of copper are toxic to people and to most living cells. But our immune systems use some copper to fend off bacteria that could make us sick. More copper in the environment leads to more bacteria, including E. coli, that develop a genetic resistance. And that could pose an increased infection risk for people, said Jason Slot, who directed a new copper-resistance study and is assistant professor of plant pathology at The Ohio State University. Today, copper is widely used, including in animal feed and to make hospital equipment - areas that could be particularly conducive to bacteria developing even greater resistance, Slot said. Under the pressure of "copper stress," bacteria have traded DNA that enabled some to outlive the threat, said Slot, who specializes in fungal evolutionary genomics. And over