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Bacterial Growth Curve

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Understanding the Bacterial Growth Curve - Foundation Figure 6.15
Bacteria reproduce by a process called binary fission. The length of time it takes for a cell to complete binary fission and form two cells is the generation time. Generation time can also be defined as the length of time required for a population to double.
When a liquid growth medium is inoculated with a few bacteria, the population can be counted at regular intervals, and the counts (logarithm of the number of bacteria) can be plotted over a length of time. In this way, we can develop a visual representation of growth, known as a bacterial growth curve. A typical bacterial growth curve shows four distinct phases: lag phase, log phase, stationary phase, and death phase. Bacterial growth cannot be sustained in this closed system because nutrients become depleted and waste products accumulate, inhibiting further growth. There are open systems, known as chemostats, that can provide a continuous supply of nutrients as well as remove older cells and waste products. Chemostats can sustain bacterial growth indefinitely as long as nutrients are provided and wastes and cells are removed.
Labeling the Phases of a Bacterial Growth Curve
Bacterial growth curves typically can be divided into four distinct phases: lag phase, log phase, stationary phase, and death phase.
Drag the following descriptions to the appropriate location on the bacterial growth curve.
Log of.. Bacteria are me.. period of most.. population, time rate of cell
Lag phase
Lag phase is a period of adjustment during which the number of cells is not increasing. Although the number of cells is not increasing, this is NOT a period of metabolic inactivity. During the lag phase, the cells adjust and produce the enzymes required for them to grow in the new environment.
Stationary phase
Stationary phase is another phase in which there is no increase in the number of cells. During this period, cells are dying at the same rate as news cells are being produced, so the population number stays the same.
Death phase
In death phase, more cells are dying than new cells are being produced, and the number of cells decreases rapidly.
Log phase
This is the phase of most rapid growth. The constant generation time produces a straight line when the log of the number of cells is plotted over time.
...
Lag Phase
When bacteria are inoculated into a new sterile nutrient broth, their numbers don't begin to increase immediately. Instead, there is a lag phase that may last for an hour or even several days. Why don't bacterial numbers increase immediately?
a. The medium contains inhibitors that prohibit rapid growth of the bacteria, and these must be inactivated before bacterial numbers will increase.
b. There are not enough nutrients for the bacteria to grow, and growth is delayed until there are some dead cells to cannibalize.
c.The bacteria must adjust to the nutrient content in the new medium, synthesizing necessary amino acids, growth factors, and enzymes.
d.The bacteria have to establish a bio film before their numbers can increase.
c. The bacteria must adjust to the nutrient content in the new medium, synthesizing necessary amino acids, growth factors, and enzymes.

A new medium may not have the same nutrients that were available in the medium from which an inoculum was taken. The bacteria may have to synthesize different amino acids, growth factors, or enzymes to enable them to grow in this new medium. Once those are synthesized, the growth rate is likely to increase, and the cells will move into log (exponential) growth phase.
What Is Happening during Stationary Phase?
After a period of rapid growth (log phase), bacterial growth rates will slow and enter the stationary phase. The number of viable cells no longer increases, but instead stays constant. In this activity you will indicate the statements that correctly describe what is happening during stationary phase.

a. The cells are likely running out of nutrients.
b. The number of cells that are dying is balanced by the number of new cells that are being formed.
c. Cells are not increasing in number because they have not yet adjusted to the nutrient availability in the new media.
d. The cells are dead; therefore, the number is staying constant.
e. Harmful waste products may be accumulating.
a. The cells are likely running out of nutrients.
b. The number of cells that are dying is balanced by the number of new cells that are being formed.
e. Harmful waste products may be accumulating.

During log phase, bacteria are actively growing and metabolizing. If the bacteria are being cultured for industrial purposes, such as producing amino acids or some other desirable product, they are generally most productive during log phase. For this reason, it is desirable to extend or prolong log phase.
Stationary phase
The cells are likely running out of nutrients.
The number of cells that are dying is balanced by the number of new cells that are being formed.
Harmful waste products may be accumulating.
Log Phase
During log phase, bacteria are actively growing and metabolizing. If the bacteria are being cultured for industrial purposes, such as producing amino acids or some other desirable product, they are generally most productive during log phase. For this reason, it is desirable to extend or prolong log phase.
Prolonging Exponential Growth
A chemostat is continuous culture system that is designed to promote and prolong exponential growth and prevent bacteria from entering stationary phase. How might this work?

a. Chemostats promote the formation of endospores, which enable the long-term survival of the bacteria.
b. Chemostats provide a continued source of fresh nutrients and remove wastes and dead bacterial cells.
c. Chemostats include detoxifying agents that inactivate growth-inhibiting toxins that develop during continued microbial growth.
d. Chemostats include growth-promoting agents that prolong microbial growth.
b. Chemostats provide a continued source of fresh nutrients and remove wastes and dead bacterial cells.
Calculating Population Increases during Exponential Growth
This activity asks you to calculate the size of a bacterial population during exponential growth.
A broth medium has been inoculated, and microbial numbers will be counted periodically to generate a bacterial growth curve. At 2 hours after inoculation, the culture has progressed through lag phase and is now in log phase. At this point, the population size is 1 million cells. The generation time is 30 minutes. Assuming the continuation of log growth, how many cells would there be at 2 hours of growth in log phase?

a. 8 million
b. 2 million
c. 4 million
d. 16 million
e. 32 million
d. 16 million

There would be approximately 16 million cells. In the 2-hour interval of log growth, there are four 30-minute generations. This is four doublings of the 1 million organisms present at hour 2.

1 million cells x 24 = 16 million cells

A given bacterial species will not always exhibit the same generation time when growing exponentially under different growth conditions. Generally the species will exhibit the shortest generation time and grow the fastest under its ideal growing conditions. However, if the growth medium provides a different complement of nutrients or if the temperature or pH is altered, the generation time of the bacteria may change. For example, under optimal growth conditions (nutrients, pH, temperature) a species may have a 20-minute generation time, but if the temperature is lowered by 5 degrees C, it may exhibit a generation time of 3 hours.
Growth Rate and Generation Time
This question asks you to make comparisons regarding the slope of an exponential growth curve, based on generation time.
Each of the three graphs shown below includes data collected during exponential growth of a species of bacteria grown in three different growth conditions. Which growth condition resulted in the longest generation time?

Hint 1. Relationship between generation time and growth rate
The shorter the generation time of a bacterial species growing under a certain set of culture conditions, the faster it is growing. The faster a bacterial culture is growing, the steeper the slope during log phase.
A bacterial culture with a longer generation time will double more slowly. The slower growth rate, the shallower the slope during log phase.
c. is Correct
The graph of exponential growth under growth condition C illustrates the slowest rate of growth and, thus, the longest generation time.

There are a number of ways to count or estimate numbers of bacteria; some are relatively convenient, and others are quite labor intensive. Some counting methods count only those cells that are living (viable count, such as a plate count), whereas other methods count the number of all cells without discriminating whether they are living or dead (such as a direct microscopic count). Microbial numbers can also be estimated indirectly by turbidity measurements, using a spectrophotometer.
Counting Bacterial Populations to Prepare a Growth Curve
This activity asks you to consider various methods of counting or estimating the size of bacterial populations and to select the method that will provide the most accurate data for plotting a bacterial growth curve.
Which of the following methods would be most appropriate for gathering data to plot a bacterial growth curve throughout the four phases?

a. direct microscopic count
b. Any of these methods will provide reliable data for plotting a growth curve.
c. indirect estimate based on turbidity
d. plate count
e. electronic cell counter
d. plate count

A plate count will provide the number of living cells. These data are most appropriate for generating a plot of a bacterial growth curve.
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