9.4.1 Cell Growth

Stages of cell growth in a batch reactor are shown schematically in Figures 9-21 and 9-22. Initially, a small number of cells is inoculated into (i.e., added to) the batch reactor containing the nutrients and the growth process begins, as shown in Figure 9-21. In Figure 9-22, the number of living cells is shown as a function of time.

Figure 9-21. Increase in cell concentration.

Figure 9-22. Phases of bacteria cell growth.

Phase I, shown in Figure 9-22, is called the lag phase. There is little increase in cell concentration in this phase. In the lag phase the cells are adjusting to their new environment, carrying out such functions as synthesizing transport proteins for moving the substrate into the cell, synthesizing enzymes for utilizing the new substrate, and beginning the work for replicating the cells’ genetic material. The duration of the lag phase depends upon many things, one of which is the growth medium from which the inoculum was taken relative to the reaction medium in which it is placed. If the inoculum is similar to the medium of the batch reactor, the lag phase can be almost nonexistent. If, however, the inoculum were placed in a medium with a different nutrient or other contents, or if the inoculum culture were in the stationary or death phase, the cells would have to readjust their metabolic path to allow them to consume the nutrients in their new environment.²⁰

Phase II is called the exponential growth phase, owing to the fact that the cell’s growth rate is proportional to the cell concentration. In this phase the cells are dividing at the maximum rate because all of the enzyme’s pathways for metabolizing the substrate are now in place (as a result of the lag phase) and the cells are able to use the nutrients most efficiently.

Phase III is the stationary phase, during which the cells reach a minimum biological space where the lack of one or more nutrients limits cell growth. During the stationary phase, the net cell growth rate is zero as a result of the depletion of nutrients and essential metabolites. Many important fermentation products, including many antibiotics, are produced in the stationary phase. For example, penicillin produced commercially using the fungus Penicillium chrysogenum is formed only after cell growth has ceased. Cell growth is also slowed by the buildup of organic acids and toxic materials generated during the growth phase.

The final phase, Phase IV, is the death phase, where a decrease in live cell concentration occurs. This decline is a result of the toxic by-products, harsh environments, and/or depletion of nutrient supply.