The following points highlight the four main phases of growth in bacteria. The phases are: 1. Lag Phase 2. Log or Exponential Growth Phase 3. Stationary Phase 4. Death or Decline Phase.
1. Lag Phase:
Lag phase represents a period of active growth during which bacteria prepare for reproduction, synthesizing DNA, various inducible enzymes, and other macromolecules needed for cell division. Therefore, during this phase, there may be increase in size (volume) but no increase in cell number. The lag phase may last for an hour or more, and near the end of this phase some cells may double or triple in size.
The lag phase is necessary before the initiation of cell division due to variety of reasons. If the cells are taken from an old culture or from a refrigerated culture, it might be possible that the cells may be old and depleted of ATP, essential cofactors and ribosomes.
If the medium is different from the one in which the microbial population was growing previously, new enzymes would be needed by the cells to use new nutrients in the medium.
However, these deficiencies are fulfilled by the cells during lag phase. It is, therefore, the lag phase is generally longer if the cells are taken from an old or refrigerated culture. In contrast, if the cells are taken from young, vigorously growing culture (microbial population) and inoculated to a fresh medium of the identical composition, the lag phase may be short or even absent.
2. Log or Exponential Growth Phase:
Bacterial cells prepared for cell division during lag phase now enter into the log phase or exponential growth phase during which the cells divide at a maximal rate and their generation time reaches a minimum and remains constant.
The growth in this phase is quite balanced (i.e. all cellular constituents are synthesized at constant rates relative to each other) hence, the most uniform in terms of chemical and physiological properties, the log phase cultures are usually used in biochemical and physiological studies.
Since the generation time is constant, a logarithmic plot of growth during log phase produces an almost a straight line. This phase is called log phase because the logarithm of the bacterial mass increases linearly with time, and exponential growth phase because the number of cells increases as an exponential function of 2n (i.e. 21 , 22 , 23, 24 ,25 and so on).
The log phase also represents the time when bacterial cells are most active metabolically, and in industrial production, this is the period of peak activity and efficiency.
3. Stationary Phase:
Since the bacteria are growing in a constant volume of medium of batch culture, and no fresh nutrients are added, the growth of bacterial population eventually ceases and the growth curve becomes horizontal. Such a phase of growth in bacteria is attained at a population level of around 109 cells per ml.
The ceasation of growth may be because of the exhaustion of available nutrients or by the accumulation of inhibitory end products of metabolism. The ceasation of growth may also be due to O2 availability particularly in case of aerobes. Oxygen is not very soluble and may be depleted so quickly that only cells on the surface of the culture may find necessary oxygen concentration for adequate growth.
Sooner or later, the bacterial cells start dying and the number of such cells balances the number of new born cells, and the bacterial population stabilizes. This state of growth, during which the total number of viable cells remains constant because of no further net-increase in cell number and the growth rate is exactly equal to the death rate, is called stationary phase.
The transition between the log and exponential and stationary phases involves a period of unbalanced growth during which the various cellular components are synthesized at unequal rates. Consequently, cells in the stationary phase have a different chemical composition from those in the exponential phase.
4. Death or Decline Phase:
After a while, the number of dying cells begins to exceed the number of new-born cells and thus the number of viable bacterial cells present in a batch culture starts declining.
This condition represents the death of decline phase which continues until the population is diminished to a tiny fraction of more resistant cells, or it may die out entirely. Like exponential growth, death is also exponential, but inverse, as the number of viable bacterial cells decreases exponentially.