3.6: Cell growth

Bacterial and archaeal cells grow by binary fission, in which each individual cell divides into two cells. The two cells are identical unless there is some genetic mutation that occurs when the cell’s genome is replicated. To carry out the process, the cells elongate, increase abundance of all cellular components, and then form a partition via inward growth of the cell membrane (Fig. \(3.7\)).

Growth rate refers to the change in number cells or cell mass over time. When bacterial cells are introduced to a culture, the following sequence of phases can result (Fig. \(3.8\)):

1. Lag phase. Initially, little growth may occur. During this period, cells may be adapting to conditions, gathering materials, and healing.
2. Exponential growth phase. During this phase, cells are in their healthiest state and the rate of increase in cell abundance is proportional to the size of the population. This increase in cell count reflects growth by doubling. One cell becomes two (2¹), which then become four (2²), then 8 (2³), then 16 (2⁴), and so forth. We can represent this relationship as \[N = N_{0} 2^{n}\]where \(N\) is the population size, \(N_{0}\) is the initial population size, and \(n\) is the number of generations that have occurred during the period of exponential growth (Madigan et al., 2003)
3. Stationary phase. No net growth occurs because cell growth balanced by cell death. This phase may occur if the environment is running out of essential resources and/or accumulating too much of the waste products of the population.
4. Death phase. During the death phase, the rate of cell death is greater than the rate of growth so population size decreases. This phase may occur if resources are exhausted and/or waste levels are too high or if there are significant changes to environmental conditions that limit growth or stress cells such as changes in temperature or pH.

These phases are not always observed (Monod, 1949). For example, if system were open to inflow of critical resources and outflow of waste, population turnover may persist, preventing the occurrence of a death phase. The illustration of growth phases shown in Figure \(3.7\), therefore, is an idealized model.

Lastly, we should consider the question: are these phases observed in cultures relevant to microbial populations in natural environments? Natural environments are commonly not steady-state systems (Schlesinger and Bernhardt, 2013), meaning that controls on population growth are not stable over time. They change with the seasons, the daily cycle of the sun, hydrological variability, and more. When favorable conditions emerge, a population is not necessarily going to launch immediately into exponential growth. There may be a lag phase when they heal and gather resources. And their growth may not extend without limit. Eventually the population may exhaust its resources or the environment may accumulate too much waste. Thus, the answer to our question is yes, the pattern observed in cultures does have relevance to microbial populations in nature.

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