(left) Under normal circumstances E. coli bacteria maintain a stable size. When a bacterial cell has lengthened by a fixed amount, it divides into two.
(middle) When stressed (e.g. when exposed to high temperatures or antibiotics), the bacteria continue growing but stop dividing. These elongated cells contain multiple copies of their DNA, as well as division rings that change position as the bacterium grows.
(right) When the period of stress has passed, the filamentous E. coli bacteria start dividing again, producing smaller cells. The location of the division sites is entirely dependent on the total length of the bacterium.