Cell growth declining phase

Declining growth A growth phase in which the availability of food begins to limit cell growth. 

The growth of microbial populations is normally limited either by the exhaustion of available nutrients or by the accumulation of toxic products of metabolism. As a consequence, the rate of growth declines and growth eventually stops. At this point a culture is said to be in the stationary phase. The transition between the exponential phase and the stationary phase involves a period of unbalanced growth during which the various cellular components are synthesized at unequal rates. Consequently, cells in the stationary phase have a chemical composition different from that of cells in the exponential phase. 

At the end of the lag phase, growth begins and cell concentration increases exponentially. As the available nutrients are exhausted in the medium, the rate of growth declines and growth eventually stops. The stationary phase is usually followed by a death phase, in which the organisms in the population die due to the depletion of the cellular reserves of energy or due to the accumulation of toxic products. 

Finally, growth stops in the stationary phase. In some cases the rate of cell growth is limited by the supply of oxygen to the medium. When the stationary phase cells begin to die and destroy themselves (by lysis) in the declining phase, the result is a decrease in the cell concentration. 

Decline phase (1) The fourth of four major phases of the bacterial growth curve in which cells lose their abihty to divide (due to less supportive conditions in the medium) and thus die. (2) In the stages of a disease, the period during which the host defense finally overcome the pathogen and symptoms begin to subside. 

Both methods give the same results for bacterial suspensions in the growth phase. When they move into the stationary, and then the decline phase, the difference between the results increases. While counting by epifluorescence shows a slight decrease in the number of cells, there is a sharp drop in the number of colonies visible. This difference may be explained by the fact that part of the population of fluorescent cells is still biologically active but is incapable of the metabolic and physiological functions necessary for multiplication. They are described as viable, non-cultivable (VNC) cells. Table 6.3 shows the lactic bacteria count after sulfiting a wine. 

FIGURE 2-27 Cell density versus time in a batch culture. During the lag phase, the cells become acclimated. During the exponential phase, the number of cells increases exponentially as described by Monod growth kinetics, see Eq. [2-73]. During the stationary phase, cell density is constant for some time before the culture declines due to substrate depletion, waste accumulation, and/or excessive cell density. 

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