The phase of prolonged decline is marked by a very gradual decrease in the number of viable cells in the population, lasting for months to years. Superficially, it might seem like a gradual march towards death of the population, but dynamic changes are actually occurring. Many members of the population are dying and releasing their nutrients, while a few "fitter" cells that are more able to cope with the deteriorating environmental conditions are actively multiplying. This dynamic process generates successive waves of slightly modified populations, each more fit to survive than the previous ones (figure 4.19). Thus, the statement "survival of the fittest" even holds true for closed cultures of bacteria.
Growth of a bacterial colony on a solid medium involves many of the same features as bacteria growing in liquid, but it is marked by some important differences. After a lag phase, cells multiply exponentially and eventually compete with one another for available nutrients and become very crowded. Unlike a liquid culture, the position of a single cell within a colony markedly determines its environment. Cells multiplying on the edge of the colony face relatively little competition and can use
Figure 4.19 Dynamic Population Changes in the Phase of Prolonged Decline Many members of the population are dying and releasing their nutrients, while a few "fitted" cells are actively multiplying.
O2 in the air and obtain nutrients from the agar medium. In contrast, in the center of the colony the high density of cells rapidly depletes available O2 and nutrients. Toxic metabolic wastes such as acids accumulate. As a consequence, cells at the edge of the colony may be growing exponentially, whereas those in the center may be in the death phase. Cells in locations between these two extremes may be in stationary phase.
Bacteria can be maintained in a state of continuous exponential growth by using a chemostat. This device continually drips fresh medium into a liquid culture contained in a growth chamber. With each drop that enters, an equivalent volume, containing cells, wastes, and spent medium, leaves through an outlet. By manipulating the concentration of nutrients in the medium and the rate at which it enters the growth chamber, a constant cell density and generation time of log phase cells can be maintained. This makes it possible to study a uniform population of cells over a long period of time. The effect of adding various supplements to the medium or altering the cellular environment on long-term cell growth can be determined.
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