How do bacteria become resistant to high levels of metals that would otherwise be toxic to them?

The high concentration of a metal becomes a selective pressure on the bacteria. Most of the bacteria die or reproduce very slowly. Some individuals have mutations or something in their genetic makeup that allows them to survive in spite of the metal that would normally be toxic. Whatever gene or genes are responsibility for this survival become more prevalent in successive generations. These are the basic steps of evolution, and evolution happens very quickly in bacteria, because (under ideal conditions) they can have generation periods as short as 20 minutes.

How do bacteria become resistant to high levels of metals that would otherwise be toxic to them?
The same way they are resistant to antibiotics. The environment (in this case heavy metals) selects for the small portion of the population that is naturally resistant. Those resistant cells survive and multiply, while all others are killed. So if the metals persist, you will be left with a population of metal-resistant bacteria.





Short answer: Natural selection.