Bioleaching uses bacterial microorganisms to extract precious metals, such as gold, from ore in which it is embedded. As an alternative to smelting or roasting, miners use bioleaching when there are lower concentrations of metal in ore and they need an efficient, environmentally responsible method. The bacteria feeds on nutrients in minerals, thereby separating the metal that leaves the organism's system; then the metal can be collected in a solution.

Bioleaching works because of how special microorganisms act on mineral deposits. They are a catalyst to speed up natural processes inside ore. The bacteria uses a chemical reaction called oxidation to turn metal sulphide crystals into sulfates and pure metals. These constituent parts of ore are separated into valuable metal and leftover sulphur and other acidic chemicals. Eventually, enough material builds up in the waste solution to filter and concentrate it into metal.

For some types of metal, such as copper, bioleaching is not always economically feasible or fast enough, even with its low cost. However, in certain areas of the world or with other metals, this simple, effective, and low cost method offers a smart choice. For example, developing countries often do not have the infrastructure or capital investment to begin smelting, yet their land contains enough ore that its extraction can significantly improve their national economy. One day we may use bioleaching to mine other metals, such as zinc and nickel, on the Moon.

After gaining popularity, about 20% of the extracted copper in the world currently comes from bioleaching. Mining companies must be careful of pollution that might result from solutions reaching a groundwater source. Yet overall, bioleaching produces less air pollution and little damage to geological formations, since the bacteria occurs there naturally. An ideal metal deposit must allow a certain amount of water into the rock to carry the bacteria. However, it should be surrounded by rock that is impermeable to water to make sure no ground water gets polluted with sulphur.