Closed-System Growth

A closed system-in this exercise, a broth culture in a flask-is one in which no nutrients are added beyond those present in the original medium and no wastes are removed. Bacteria grown in a closed system demonstrate four distinct growth phases: lag phase, exponential phase, stationary phase, and death phase.

The four phases together form a characteristic shape known as a microbial growth curve (Fig. 6.9). Lag phase, the first phase, constitutes what might be called an adjust- ment period. Initially, there is no cell division. It is believed that microorganisms use this time to repair damaged cellular components and synthesize enzymes to begin using the resources of the new environment. The duration of lag phase can be quite variable and depends on many factors. Actively reproducing cells transferred to a medium identical to the one from which they are removed will undergo a short lag period. Cells that are near death, have been refrigerated, or must adjust to a completely different medium will demonstrate a longer lag phase.

Eventually, as cells begin to use resources of the new environment and start reproducing, lag phase gradually gives way to the exponential (or log) phase. This phase is a time of maximum growth that is limited almost exclusively by the organism's reproductive potential. In a closed sys- tem, conditions usually are adjusted to be optimal for a specific organism; however, even under the best of condi- tions, the medium and other physical factors influence the growth rate slightly. Regardless, exponential growth repre- sents the maximum growth rate of that organism under the existing conditions. Exponential growth is characterized by cellular division and doubling of the population size at regular intervals, depending on the organism's generation time. Because not all cells are dividing at exactly the same time, exponential growth is marked by a smooth and dra- matic upward sweep of growth, as shown in Figure 6.9.

As nutrients in the medium decrease and toxic waste products increase, the growth rate declines to where the population's death rate is more or less equal to its reproductive rate. This leveling of growth is called the stationary phase. Notice that the transition into stationary phase is pretty sudden. This is because toward the end of exponential growth the population is still doubling, but the population size that is doubling is very large, which puts pressure on the system quickly. Stationary phase will last
until the nutrients are depleted or the medium becomes toxic to the organism.

Death phase, the final phase, is marked by the decline of the organism. As discussed in Exercise 6-6, microbial death is the reverse of microbial growth and is exponential. That is, a fixed proportion of the population will die in a given time (the time is specific to the organism), regardless of population size.

It is customary to plot growth curves on a semilogarithmic graph, where equal increments on the y-axis scale represent changes in the exponent. For instance, equal 2 increments on the scale might be 101, 10 , 103, and 104, rather than a more typical sequence of 5, 10, 15, and 20. This is done for two reasons. One, it allows plotting of data over a large numerical range, and two, a straight line results if the data are produced by a constant exponential function. Conversely (and usefully), seeing a straight line on a log graph tells the reader that the change is constant.

Several measurements are possible in a dosed-system growth experiment. These measurements include the duration of each phase, the mean growth rate constant, generation time, and the organism's minimum, maximum, and optimum (cardinal) temperatures.