Bacterial Population Growth

Most bacteria reproduce by a process called binary fission, during which the cell duplicates its
components and divides into two cells. Under favorable conditions, individual bacteria rapidly
proliferate, forming macroscopically visible colonies (or clones) consisting of millions of genetically
identical cells on the surface of an agar plate or turbidity representing millions to billions of cells per
mL in a broth solution.
The growth of bacterial populations progresses through four phases: lag phase, exponential (log)
phase, stationary phase, and death phase (Figure 1). The student must remember that these growth
phases refer to a bacterial population, not to individual cells. Individual cells may or may not be
following the overall growth pattern of the population at any one time. For example, in the
stationary phase, the population is in a “leveling off” phase; however, many cells are still actively
growing and metabolizing.
Lag phase
The lag phase is the period where a population is adapting to its environment. It is sensing what
nutrients are available and producing the enzymes necessary to extract energy and divide during the
next phase. There is no growth in number of cells during this phase.
Log or exponential phase
The log or exponential phase is the term used to describe the pattern of population growth in which
the number of cells doubles during each unit time. One of the characteristics of exponential growth
is that the rate of increase in cell numbers is slow during the initial stages but increases at an everfaster rate. During the later stages of growth, this results in an explosive increase in cell population
numbers. Mathematically, this is known as exponential growth. The log growth phase is used to
calculate generation time and it’s the time it takes for a population to double. When the log of the
number of cells is plotted over time, there will be a linear increase in this phase.
Stationary phase
The stationary phase is the period immediately after the log phase. The cells are beginning to
exhaust the available nutrients, and waste products are beginning to accumulate. There is no net
growth; the number of dying cells is equal to the number of reproducing cells.
Biology 15 8.2
Death phase
The death phase is the period after the stationary phase. Nutrients are exhausted in this stage, and
waste products have been building up to toxic levels. There is a net decrease in the number of cells;
the number of dying cells exceeds the number of reproducing cells. This will appear as a linear
decrease in the number of cells over time on a semi-log plot.
Figure 1 The 4 phases of bacterial growth, Lag phase, exponential (log) phase, stationary phase, and
death phase. These phases are only revealed once cell numbers have been plotted on a log scale. In
the graphic determination of generation time, a doubling is measured using values from the y and xaxis. In this case, the generation time is determined to be: 133 – 117 = 16 minutes.
Generation time
The generation time (g) is the time required for a complete growth cycle or, in other words, the generation of
two cells from one cell. Thus, the generation time or doubling time for bacteria is the time required to double
the number of cells in a population. This time is highly variable in the microbial world and depends on
nutritional, genetic, and physical factors. Under the best nutritional and physical conditions, the generation
time of E. coli is about 20 minutes. Some microbes can grow faster than this, but many grow much more
slowly.
Biology 15 8.3
Determination of generation time graphically

1. To determine the generation time of an organism, you first need to graph the number of cells over time on
   a semi-log plot to define the log phase of growth clearly. The log phase will be the graph area where there
   is a linear increase in the number of cells.
2. Draw a “best-fit” line that best represents the log phase data only. You will connect the dots as best you
   can with an equal number of points above and below the line.
3. Pick a number from the y-axis within the log phase and multiply it by 2. In Figure 1, see how 1x108 and
   2x108 are used. You do not need to pick actual data points here.
4. Find where these two values intersect the best fit line in the log phase and then determine the amount of
   time (on the x-axis) that has elapsed between these two points by subtracting the smaller time from the
   larger one, as shown in Figure 1 with the red dashed lines.
   Calculation of generation time by the equation:
   Generation time can be calculated using the following equation shown below.
   ( )( )
   ( ) ( )
   − = −
   0.301
   g log log
   f i
   f i
   t t
   N N
   The variables in the above equation are defined as:
   • g = generation time
   • tf = final time point
   • ti = initial time point
   • Nf = final population size
   • Ni = initial population size
5. To use this equation properly, you first need to graph the number of cells over time on a semi-log plot to
   define the log phase of growth clearly