Since the beginning of this century it was widely known that when a population of bacteria is exposed to a toxic environment, some rare cells may acquire the ability to grow much better than most of the other cells in the population. It is easy to distinguish cells that have simply adapted to the toxic conditions from resistant mutants.
Adaption is a transient property that depends upon continued exposure to the toxic conditions.
Thus, if the cells are transferred from the toxic conditions and grown for many generations under favorable growth conditions they would de-adapt, so most of the cells would fail to thrive if subsequently transferred back into the toxic growth conditions. In contrast, distinguishing between induced vs spontaneous mutations is much more difficult. For many years, most microbiologists believed that mutations in bacteria were induced by exposure to a particular environment. They studied mutations that made E. coli resistant to phage T1. Phage T1 interacts with specific receptors on the surface of E. coli , enters the cell, and subsequently kills the cell.