Stationary and Death Phase

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. 

The stationary phase is usually followed by a death phase in which the organisms in the population die. Death occurs either because of the depletion of the cellular reserves of energy, or the accumulation of toxic products. Like growth, death is an exponential function. In some cases, the organisms not only die but also disintegrate, a process called lysis. 

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Cell lysis generally becomes important when viability is low. Examples include continuous culture (with or without cell retention) at low dilution rates and the late stationary and death phases of batch culture. Cell lysis is also important in stirred reactors with very high agitation rates. In order to quantify cell growth parameters under these conditions, the lysed cells must be accounted for. One way to do this is to measure the amount of the cytosolic enzyme lactate dehydrogenase (LDH) released into the medium (see Chapter 2, section 2.5). A procedure for measuring LDH activity is described in Chapter 4, section 4.7. 

Cells growth is described as an autocatalytic reaction. All growth requires an initial seed or inoculum of cells. As the cells are given nutrients, they metabolize this food to make more cells. The nutrient requirements vary considerably with cell type, but all cells undergo the growth phases shown in Fig. 1—stationary, exponential, deceleration, stationary, and death phases. 

While the simple Monod-type function is valid only in the exponential and decelerating growth phases, extensions can be introduced to extend the range of vahdity to the lag, stationary, and death phases. 

For each run, calculate and plot the cell biomass concentration, glucose concentration, ethanol concentration, and pH as a function of time. Identify the major phases in batch fermentation lag, exponential, stationary and death. 

Microbial growth is generally described in terms of cell numbers, although an increase in the mass of the cell population also usually occurs. In laboratory culture, bacteria exhibit a growth curve that can be divided into four main phases lag exponential stationary and death (Fig. 5.45). 

Fonr phases are observed during a batch cnltnre lag, exponential, stationary, and death (Eigure... 

The growth of bacteria passes through several distinct phases these include a lag phase, log phase, stationary phase and death phase (Figure 1.7). The lag... 

During the stationary phase, the growth rate is zero as a result of the depletion of nutrients and essential metabolites. Several important fermentation produets (ineluding most antibioties) are produeed in the stationary phase. The stationary phase is followed by a phase where eells die or sporulate. During the death phase, there is a deerease in live eell eoneentration, whieh results from the toxie byproduets eoupled with the depletion of the nutrient. The number of viable eells usually follows an exponential deeay eurve during this period. 

In a well mixed bioreactor a homogeneous suspension exists and typical growth kinetics an be observed as illustrated in Figure 5.17. Six phases can be distinguished the lag phase acceleration phase the exponential growth phase the deceleration phase the stationary phase and the phase at which death/decline occurs. 

The stationary phase can be represented by X = const, whereas the death phase can be modeled according to Eq. (8.7) with kd as the death rate constant lime 1 or upon integration based on Xma.. as the cell density after the stationary phase [Eqs. (8.8a) and (8.8b)]. 

Cell growth phases comprise lag phase, exponential or log growth phase, stationary or plateau phase, and senescence or death phase, as shown in Figure 2.5. Cell growth can be mathematically represented by the following general equation ... 

The curve is ordinarily divided into different phases. Lag phase corresponds to no growth or even a decline in number. After the lag period, the growth rate continuously increases and the bacteria are in the exponential or logarithmic phase in which the multiplication rate is most rapid and can be represented by a straight line on a log-linear plot. Then, the bacteria reach the maximum cell number and the multiplication rate decreases. The number of bacteria remains constant and this phase is called the stationary phase. The death phase occurs when the cell number decreases at a faster rate than the rate new cells are formed. 

Figure 7.5 EHfferent stages of cell growth in batch processes (1) lag phase, (2) exponential growth phase, (3) stationary phase, and (4) death phase.

Consider again a closed volume with uniform composition and temperature — in practice a batch reactor, in which only changes with time occur. From its initial composition the medium evolves after seeding by an inoculum, which is a small amount of the living cell culture with optimized composition, through respectively a lag phase, an exponential growth of the number of cells, a stationary phase during which the increase in number of cells is compensated for by their destruction and a death phase with an (exponential) decrease in the number of cells. 

The model does not include the death phase, but industrially the amount of nutrient is chosen to terminate the microbial growth before the onset of the death phase, anyway. The cell growth is initiated by inoculation with cells from a stationary nutrient exhausted culture, i.e., when Ca = Cr = G and M = D. Numerical integration of (15.2-5 to 15.2-8) leads to results for a batch culture given in Fig. 1.5.2-1. The model shows a lag phase, an exponential growth phase, a change in the cell composition and a stationary phase with a relatively small number of cells. 

The growth cycle has three principal phases a limited growth phase (2-5 days) increases the population to between 10 and 10 cells/ml a quasi-stationary phase follows and lasts about 8 days finally, the death phase progressively reduces the viable population to 10 cells/ml. The final phase can last for several weeks. 

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