- In higher organisms, growth means increase in size and volume of the organism whereas in case of bacteria, growth refers to the increase in their number.
- Most common mode of bacterial reproduction is transverse binary fission in which one cell divides to form two daughter cells.
- The time required by the cell to double its number is called generation time which is not same for all bacteria.
- The growth medium has a direct influence on the generation time of bacteria.
- When fresh liquid medium is inoculated with a given number of bacteria and incubated for sufficient time, it gives characteristic growth pattern of bacteria.
- If we measure the bacterial population intermittently at different intervals of time and ‘log’ of the number of viable population is plotted against time, we obtain a characteristic growth curve or growth cycle that has a hyperbolic shape.
- There are 4 phases in the growth curve of bacteria which are as follows:
- Lag phase
- Logarithmic (log) phase or exponential phase
- Stationary phase
- The phase of death or decline
- Lag phase:
- The bacterial population when inoculated into a new fresh medium doesn’t multiply immediately but takes some time to adjust to the new environment.
- This time period of adjustment during which bacteria don’t divide but synthesize the cellular components (are metabolically active) is called lag phase.
- The cell size increases and bacteria have the largest size at the end of this phase.
- Lag phase is induced by various reasons and its duration depends on various factors:
- If the bacterial cells are injured, bacteria take some time to repair the cells before division and lag phase becomes longer.
- If the bacterial inoculum is taken from old culture, the duration will be longer and if inoculated bacteria are fresh, young and healthy, lag phase becomes shorter.
- Similarly, if the new culture media is different from the one in which previous culture was done, the lag phase becomes longer because bacteria take some time to adjust to the new media.
- At the end of lag phase, bacteria become fully prepared for the cell division and the cell division starts.
- Log phase or exponential phase:
- In log phase, bacteria divide continuously at a constant rate and the number increases exponentially.
- In this period, all the bacteria are in their stage of division because of which they have the smallest cell size.
- Bacterial population is most nearly uniform in terms of their number, metabolic activity, chemical composition and other physiological properties of the cell.
- Generation time for bacteria in log phase is determined by the relation;
g=t/n
Where, ‘n’ is the number of generations,‘t’ is the total time of incubation, ‘g’ is the generation time, ‘N0’ is the initial number of bacteria and ‘N’ is the total number of bacteria after ‘n’ generations
- Generation time in log phase is not same for all the bacteria and is strongly dependent on the growth factors, concentration of nutrients and the density of bacteria present in the media.
- Stationary phase:
- When the bacteria grow continuously in log phase, their density increases, concentration of nutrients decreases and various secondary metabolic products (toxic products) begin to accumulate in the media.
- Production of antibiotics such as Penicillin, streptomycin etc. and enzymes by certain bacteria occur during this phase.
- These conditions bring the stationary phase of growth curve. In some cases, there is complete cessation of cell division and there is neither increase nor decrease in the number of bacteria. i. e no net increase in population.
- In other cases, some bacteria divide and some die and hence balance each other and the number of bacteria remains the same.
- In endospore forming bacteria, sporulation occurs as the bacteria enter stationary phase.
- Decline phase or death phase:
- In decline phase, the number of bacteria decreases continuously.
- The bacterial number decreases exponentially which is just the opposite of log phase.
- The death rate in decline phase is slower than the growth rate in the log phase.
- During this phase, total count of bacteria may remain constant but the viable count decreases.
- Various conditions like depletion of nutrients, accumulation of toxic products like acids bring about the decline phase.
- Not all the bacteria die at the same time in decline phase, some die faster and some which are more resistant (endospore forming bacteria) remain viable for a longer time.