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Importance and phases of growth of bacteria
Biochemistry (Rajiv Gandhi University of Health Sciences)
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Bacterial growth is regulated by nutritional environment. When suitable environment is
there that time bacterium is incubated, its growth leads to increase in number of
cells which allow definite course.
The growth curve has got four phases:
■ Lag phase
■ Log phase(logarithmic) or exponential phase
■ Stationary phase
■ Decline phase
Phases of Growth:
Lag phase:
bacteria adapt themselves to growth conditions.
■ It is the period where the individual bacteria are maturing and not yet able to divide.
■ During the lag phase of the bacterial growth cycle, synthesis of RNA, enzymes and
other molecules occurs.
■ Length of this phase depend on type of bacterial spesis, culture medium, and
environmental factors.
Phases of Growth:
Log Phase: it is a period characterized by cell doubling.
■ The number of new bacteria appearing per unit time is proportional to the present population.
■ If growth is not limited, doubling will continue at a constant rate so both the number of cells
and the rate of population increase doubles with each consecutive time period.
■ For this type of exponential growth, plotting the natural logarithm of cell number against time
.
produces a straight line
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Plotting growth on graphs:
Phases of Growth:
■ The slope of this line is the specific growth rate of the organism, which is a measure of the
number of divisions per cell per unit time.
■ The actual rate of this depends upon the growth conditions, which affect the frequency of cell
division events and the probability of both daughter cells surviving.
■ Under controlled conditions, cyanobacteria can double their population four times a day.
■ Exponential growth cannot continue indefinitely, however, because the medium is soon depleted
of nutrients and enriched with wastes.
Generation time
■ The time required to for a population to double (doubling time) in number.
■ Ex. Escherichia coli (E. coli) double every 20 minutes
■ Ex. Mycobacterium tuberculosis double every 12 to 24 hours.
Mean Generation Time and
Growth Rate:
◻ The mean generation time (doubling time) is the amount of time required for the concentration
of cells to double during the log stage. It is expressed in units of minutes.
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◻ Growth rate (min-1) =
◻ Mean generation time can be determined directly from a semilog plot of bacterial
concentration vs time after inoculation
Phases of Growth:
Stationary phase :
The "stationary phase" is due to a growth-limiting factor; this is mostly depletion of a nutrient,
and/or the formation of inhibitory products such as organic acids.
■ newly formed cells per time = dying cells per time Secondary metabolites are synthesised during
the stationary phase of growth
Phases of Growth
Death phase– death exceeds division. bacteria run out of nutrients and die although number
of cells remain constant.
■ The decline phase is brought by exhaustion of nutrients, accumulation of toxic products and
autolytic enzymes
■ Sometimes a small numbers of survivors may persist for month even after death of majority of
cells these few surviving cells probably grow at expense of nutrients released
Growth in Batch Culture
1.
Bacteria growing in batch culture produce a growth curve with up to four
distinct phases.
2.
Batch cultures are grown in tubes or flasks and are closed systems where no fresh
nutrients are added or waste products removed.
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3.
Lag phase occurs when bacteria are adjusting to them medium. For example,
with a nutritionally poor medium, several anabolic pathways need to be turned on,
resulting in a lag before active growth begins.
4.
In log or exponential phase, the cells are growing as fast as they can, limited only
by growth conditions and genetic potential. During this phase, almost all cells are
alive, they are most nearly identical, and they are most affected by outside
influences like disinfectants.
5.
Due to nutrient depletion and/or accumulation of toxic end products, replication
stops and cells enter a stationary phase where there is no net change in cell
number.
6.
Death phase occurs when cells can no longer maintain viability and numbers
decrease as a proportion.
Basic Chemostat System:
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