6.3: Bacterial Growth Dynamics

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Learning Outcomes

Discuss the growth dynamics of a bacterial culture
Identify phases of bacterial growth curve

Bacterial Growth Dynamics

Bacterial growth refers to an increase in bacterial numbers, not an increase in the size of the individual cells. In most bacteria, growth first involves increase in cell mass and number of ribosomes, then duplication of the bacterial chromosome, synthesis of new cell wall and plasma membrane, partitioning of the two chromosomes, septum formation, and cell division. This asexual process of reproduction is called binary fission and results in two daughter cells that are genetically identical. This is accomplished by the process of binary fission, where a single bacterial cell divides into two.

Watch Video 1: Bacterial Growth

Watch Video 1: Bacterial cell division (00:15). URL: https://youtu.be/gEwzDydciWc

The dynamics of bacterial growth follow a predictable pattern visualized as a bacterial growth curve. This growth curve is generated by plotting the increase of cell number versus time. The curve can then be used to determine the generation time, the time required for a microbial population to double in cell number. There are typically four growth phases in a closed bacterial culture vessel, like a flask or tube. The phases are lag, log/exponential, stationary, and death phases.

Lag Phase: immediately after inoculation of the cells into fresh medium, the population remains temporarily unchanged (notice the line here is flat, no change in cell number). Although there is no apparent cell division occurring, the cells may be growing in volume or mass, synthesizing enzymes, proteins, RNA, etc., and increasing in metabolic activity. The cells are also adapting and adjusting to this media and growth condition, different genes may be turned on to start metabolizing different substrates. There is some repair processes going in, cell is re-synthesizing damaged cell constituents in preparation for binary fission. The length of the lag phase is dependent on a wide variety of factors, including the size of the inoculum; time necessary to recover from physical damage or shock in the transfer; time required for synthesis of essential coenzymes or division factors; and time required for synthesis of new (inducible) enzymes that are necessary to metabolize the substrates present in the medium, the age of the inoculum (microbe introduced into a culture medium to initiate growth), an “old” culture will probably a lot of dead/aged cells and may take longer to adjust to this medium.

The Log (Exponential) Phase. The exponential phase of growth is a pattern of balanced growth wherein all the cells are dividing regularly by binary fission, and are growing by geometric progression. The cells divide at a constant rate depending upon the composition of the growth medium and the conditions of incubation. The rate of exponential growth of a bacterial culture is expressed as generation time, also the doubling time of the bacterial population. This phase is usually relatively short in the scheme of the entire growth curve. Cells in this phase are most active metabolically, and is preferred for industrial purposes where, for example a product needs to be produced efficiently. Exponentially growing cells are typically at their healthiest and are thus most desirable for studies of their enzymes or other cell components. Because the generation time is constant, a logarithmic plot of growth during the log phase is a straight line.

Exponential growth cannot be continued forever in a batch culture. Population growth is limited by one of three factors: 1. exhaustion of available nutrients; 2. accumulation of inhibitory metabolites or end products; 3. exhaustion of space, in this case called a lack of "biological space". Eventually the growth rate slows, the number of microbial deaths balances the number of new cells, and the population stabilizes. This period of equilibrium is called the Stationary phase. Bacteria that produce secondary metabolites, such as antibiotics, often do so during the stationary phase of the growth cycle (Secondary metabolites are defined as metabolites produced after the active stage of growth).

It is during the stationary phase that spore-forming bacteria have to induce or unmask the activity of dozens of genes that may be involved in sporulation process to prepare for a dormant period. If incubation continues after the population reaches stationary phase, a death phase follows, in which the viable cell population declines. During the death phase, the number of viable cells decreases geometrically (exponentially), essentially the reverse of growth during the log phase. This phase continues until the population is diminished to a tiny fraction of the number of cells in the previous phase or until the population dies out entirely.

growth curve.JPG

Image 1: Bacterial growth curve showing 4 phases. Image by Michał Komorniczak (Poland). https://upload.wikimedia.org/wikiped..._growth_en.svg