If a suitable and a fixed volume of liquid medium is inoculated with a small bacterial inoculum or bacteria cells taken from a culture that has previously been grown to saturation and the number of viable cells per milli liter is determined periodically and plotted, a curve of growth is obtained.
The phase of the bacterial growth curve is reflection of the events in a population of cells, not in *individual* cells.
The changes of slope in the graph indicate the transition from one phase of development to another.
Phases of bacterial growth curve:
1. Lag phase:
After a liquid culture broth is inoculated, the multiplication of bacteria does not start immediately. It takes some time to multiply. The time between inoculation and beginning of multiplication is known as lag phase.
In this phase, the inoculated bacteria become acclimatized to the environment, switch on various enzymes, and adjust to the environmental temperature and atmospheric conditions.
During this phase, there is an increase in size of bacteria but no appreciable increase in number of bacterial cells.
The cells are active metabolically. The duration of the lag phase varies with the bacterial species, nature of culture medium, incubation temperature, etc.
It may vary from 1 hour to several days.
2. Log phase:
Regular growth of bacteria occurs in this phase which is also of short duration since the nutrients present the medium are utilized by the bacteria and daughter cells.
Once exhaustion of nutrients occurs, slowing down of growth also takes place and bacterium passes onto stationary phase. The morphology of bacteria is best developed in this phase and organisms manifest typical biochemical characters.
This phase will continue as long as cells have adequate nutrients and the environment is favorable. Hence, this phase can be prolonged by continuous supply of fresh nutrients at a constant and predefined rate. Chemostat and turbidostat are examples of techniques by which this phase can be prolonged, since in both continuous availability of the nutrients is assured.
3. Stationary phase:
After log phase, the bacterial growth almost stops completely due to lack of essential nutrients, lack of water-oxygen, change in pH of the medium, etc. and accumulation of their own toxic metabolic wastes.
 |
| Bacterial Growth Curve |
Many bacteria start producing exotoxins in this phase.
Figure: Bacterial growth curve. The viable count shows lag, log, stationary and decline phases. In the total count, the phase of decline is not evident.
4. Decline phase:
During this phase, the bacterial population
declines due to death of cells.
declines due to death of cells.
The decline phase starts due to
(a) accumulation of toxic products and autolytic enzymes and
(b) exhaustion of nutrients.
(a) accumulation of toxic products and autolytic enzymes and
(b) exhaustion of nutrients.
Involution forms are common in this stage.
Growth rate during different phases of bacterial growth curve is summarized in the following Table.
Growth rate during different phases of bacterial growth curve.
Phase
|
Growth
|
Lag phase
|
Zero
|
Log or exponential phase
|
Constant
|
Stationary phase
|
Zero
|
Decline phase
|
Negative
|
*The continuous culture is a method of culture useful for industrial and research purpose.*
[This is achieved by using a special device for replenishing nutrients and removing bacterial population continuously so that bacteria growth is not inhibited due to lack of nutrients or due to accumulation of toxic bacterial metabolites.]
[Short answers for examination]
Bacterial growth curve
If a suitable and a fixed volume of liquid medium is inoculated with a small bacterial inoculum or bacteria cells taken from a culture that has previously been grown to saturation and the number of viable cells per milli liter is determined periodically and plotted, a curve of growth is obtained. The phase of the bacterial growth curve is reflection of the events in a population of cells, not in individual cells.
The changes of slope in the graph indicate the transition from one phase of development to another.
Phases of bacterial growth curve:
The bacterial growth curve can be divided into four major phases (i) lag phase (ii) log phase (iii) stationary phase and (iv) decline or death phase.
These phases reflect the physiologic state of the organisms in the culture at that particular time.
i) The lag phase: The lag phase represents a period during which cells, depleted of metabolites and enzymes as the result of unfavorable conditions that exist at the end of their previous culture history, adapt to their new environment. Enzymes and intermediates are formed and accumulate until they are present in concentrations that permit growth to resume. The duration of lag phase varies with the bacterial species, nature of the culture medium, incubation temperature etc. It may vary from 1hour to several days.
ii) The log phase: The phase is characterized by rapid exponential cell growth (i.e. 1to2, 2 to 4, 4 to8 and so on. Following the lag phase, the cells start dividing and their numbers increase exponentially with time. If the logarithm of the viable count is plotted against time, a straight line will be obtained. The lag phase is the time when cells are the most active metabolically. During their log phase of growth, microorganisms are particularly sensitive to adverse conditions such as antibiotics and other microbial agent.
 |
| Figure: Bacterial growth curve |
Growth rate during different phases of bacterial growth curve.
Phase
|
Growth
|
Lag phase
|
Zero
|
Log or exponential phase
|
Constant
|
Stationary phase
|
Zero
|
Decline phase
|
Negative
|
iii) Stationary phase: After log phase, the bacterial growth almost stops due to essential nutrients, such as lack of water, oxygen, change in pH of the medium etc and accumulation of their own toxic metabolic wastes. Death rate of bacteria is equal to the formation rate of bacteria cells. The number of progeny cells formed is just enough to replace the number of cells die. The growth curve becomes horizontal. The viable remains stationary as an equilibrium exists between the dieing cells newly formed cells.
iv) Decline phase: During this phase, the bacterial population declines due to death of cells. The decline phase starts due to an accumulation of toxic products and antolytic enzymes and exhaustion of nutrients. Finally after a variable period, all cells die and culture becomes sterile. When the total count is plotted, it parallels the viable count up to the stationary phase, but it continuous steadily without any phase of decline. Even the total count shows a phase of decline with antolytic enzymes
The end]
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