Microorganisms which become incapable of growth under conditions that are normally suitable for growth must be either dead or dormant. The essential difference between the two states is that dormant cells can subsequently regenerate the normal growing state, whereas dead cells cannot. Generally, the production of dormant forms is an ordered process with a duration longer than the minimum doubling time of the vegetative form. Bacteria can die as a result of:
In many cases cell death is accompanied by cell lysis, a process in which the cell disintegrates and the contents of the cell enter the growth medium. Many of the components of which the cell consisted are frequently utilized for further cell growth. Growth on such lysis products is described as cryptic growth.
Cryptic growth is much more significant in mixed cultures than in pure cultures, in biotreaters, cryptic growth and protozoal predation are both important mechanisms for the reduction of the overall biomass yield coefficient and optimization for the conversion of the carbonaceous pollutants into carbon dioxide.