Along with the main elements of carbon, oxygen, nitrogen, magnesium and sulphur, organic organisms also require trace amounts of certain other elements, including some metals. The most useful thing about the metals required by the body is that their outer electron orbitals are very close together, making it easy for them to both accept and loose a few electrons here and there. By holding and releasing electrons they can take place in redox reactions, which are used to produce energy for the cell.
Copper can cycle between two different ionic forms: Cu+ and Cu2+. Its most important use is as an electron carrier for the process of creating the energy-rich molecule ATP. Although it is vital to the cell in small amounts, in large quantities it can become toxic. By 'large quantities' we are talking about greater than one atom per cell, so anyone suggesting that consuming or being close to large quantities of copper is beneficial for your health is probably wrong.
Large amounts of copper are toxic for pretty much all living cells, which can be exploited by the human immune system to fight off invading bacteria. When macrophages (the white blood cells that surround and break down invading bacteria) are activated and engulf a bacteria they start to accumulate copper ions inside the cell, in particular in the part of the cell that has just engulfed the bacteria. As a response, many bacteria have increased resistance to copper ions, and those that have lost this resistance are more susceptible to being broken down by the macrophages. While large amounts of copper are indeed bad for your cells, inadequate copper levels can compromise the immune system and make people more likely to suffer from bacterial infection.
As bacterial cells are much smaller and more compact than human cells, they tend to keep all their copper-containing enzymes very close to the cell membrane, to prevent them causing unwanted redox reactions inside the cell. When presented with elevated copper levels, the bacteria can turn on a group of genes which produce proteins capable of shuttling copper out of the cell. These are the copper resistance genes. As a large number of pathogenic bacteria contain these copper-resistant genes, and as there are very few copper-containing enzymes in bacteria, it is likely that an excess rather than a deficit of copper is a problem for bacteria inside the human body.
The use of copper as a strategy against bacteria may have important clinical implications. While using copper as a therapeutic strategy would be potentially dangerous for the host, targeting bacterial copper resistance may provide help for the macrophages and immune system cells to clear the bacteria naturally. Developing a way to compromise the bacteria by reducing their copper resistance could also be used along with antibiotic therapy in order to increase the effectiveness of the antibiotic.
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Reference: Festa RA, Thiele DJ (2012) Copper at the Front Line of the Host-Pathogen Battle. PLoS Pathog 8(9): e1002887. doi:10.1371/journal.ppat.1002887