A population of bacteria can double every 20 minutes. Nitrobacter can double every 8 hours. From Aquatic Eco-Systems. Only certain strains can form spores or cysts when growing conditions are not maintained. Most strains, however, die when necessary conditions are not met.
Some bacteria form floc, the slippery material one feels on a tank wall.
However, many which are involved in water purification are free livers. These may get trapped in the floc, but they typically flow through the system. The bacteria on a biological filter are immobile and water movement is essential to assure that food is brought into contact with them. Watch a gold fish suck gravel into its mouth, then reject it. The fish has eaten bacterial floc off the gravel.