Bacteria replicate at an enormously fast rate when they are well fed, about once every 15 to 20 minutes, depending on the species. The bacteria will replicate as long as the pollutants in the system can feed the population. As the pollution level decreases, the bacteria die off and reproduce less often. In this way, the population naturally tailors itself to the pollution level. By the time the job is done, the bacteria have died back to a normal population. Some will go into dormancy, and reactivate if the pollution level begins to rise again.