Thousands of years of bacterial resistance.

• Position: Copper

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, indicates plant pathologist and evolutionary biologist Jason Slot in a study appearing in Genome Biology and Evolution.

Today, copper is used widely, including in animal feed and to make hospital equipment--areas that particularly could be conducive to bacteria developing even greater resistance. Under the pressure of "copper stress,"

bacteria have traded DNA that enabled some to outlive the threat, explains Slot, who specializes in fungal evolutionary genomics.

Over centuries, the genes that lead to copper resistance have bonded, forging an especially tough opponent for the heavy metal, a cluster scientists call the "copper homeostasis and silver resistance island," or CHASRI.

Slot and his colleagues created a molecular clock, using bacterial samples collected over time and evolutionary analysis to trace the history of copper resistance. The team studied changes in bacteria and compared those to human use of copper. Their work suggests there were repeated episodes of genetic diversification...