Growth Curve of Bacterial Population: 8 Phases

This article throws light upon the eight main phases in which the growth curve of bacteria is divided. The phases are: 1. 1. Initial Stationary 2. Accelerated Growth 3. Logarithmic Growth 4. Decreasing Growth Rate 5. Maximum Stationary 6. Increasing Death Rate 7. Logarithmic Death 8. Survival or Decreasing Growth Rate.

Phase # 1. Initial Stationary:

This phase is characterized by a period during which there is no increase in the number of cells. It is the phase of cell enlargement sometimes known as the lag phase. The latter name is somewhat misleading, because it implies inactivity or dormancy, which is contrary to the actual situation.

Though there is no increase in the number of cells, the organisms are very active physiologically, and are synthesizing new protoplasm.

There is an increase in total protein, ribonucleic acid, and cell phosphorus. It is a phase of adjustment necessary for the synthesis of the internal supply of intermediate metabolites, enzymes and coenzymes. Time is also required for adjustments in the physical environment around each cell. The duration of this phase varies with conditions and species.

When a medium is inoculated with cells obtained from a culture growing in its logarithmic phase, the culture displays little lag in population increase. This phase is prolonged, however, in a medium inoculated with dormant cells from an older culture. In short, the organisms are metabolizing and growing, but there is a lag in cell division.

Phase # 2. Accelerated Growth:

After the end of first phase, each organism starts dividing. However, since not all organisms have completed the first phase simultaneously, there is a gradual increase in the population until the end of this period. On plotting, the rate of multiplication increases with time i.e. the time required for each cell to divide gradually decreases.

Finally, the rate of multiplication reaches a maximum at the end of this phase. This is the transition period between the initial stationary phase and logarithmic growth phase. During this period the cells are unusually sensitive to unfavourable environmental conditions such as extremes of temperature, high osmotic pressure, or disinfectant chemicals.

Phase # 3. Logarithmic Growth:

During this phase the cells divide steadily at a constant rate, and the log of the number of cells plotted against time results in a straight line. In this phase the cells are in a state of balanced growth.

During this state the mass and volume of the cells increase by the same factor, in such a manner that the average composition of cells and the relative concentration of metabolites remain constant for a certain period of time. Cells are smaller in this phase because they are constantly dividing.

It is a phase of physiological youth where cells are actively growing and multiplying, and the entire population is uniform with respect to cellular activity. Biochemical and physiological properties that are commonly used for identification of organisms are usually most manifest during this phase. Also, organisms are highly sensitive to various physical and chemical agents.

In short this phase is characterized by the following:

(i) Growth rate is maximum and constant.

(ii) Generation time is shortest and constant.

This phase lasts for several hours, depending upon the type of species and conditions of growth.

Phase # 4. Decreasing Growth Rate:

The organisms continue to multiply, but at a slower rate than during the logarithmic growth phase. The decline in rate of multiplication is due to a number of factors. The most important factors are the depletion of nutrients and the accumulation of toxic waste products. This is a transitionary phase between the logarithmic growth phase and the maximum stationary phase.

Phase # 5. Maximum Stationary:

In this phase a constant high population of cells is maintained by a balance between cell division and cell death. The net viable population remains unchanged for some time. The total count (living plus dead) continues to increase slowly, and can be used to calculate the rate of death.

Phase # 6. Increasing Death Rate:

In this phase there is a decrease in number of viable organisms with increase in time. The rate of death gradually increases, and reaches a maximum at the end of the phase. This is a transitionary phase between the maximum stationary phase and the logarithmic death phase.

Phase # 7. Logarithmic Death:

The number of organisms decreases exponentially during this phase, i.e. half the surviving cells die in each successive equal time interval. For example a population decreases from 1 million to 1/2 million in the first hour, to 1/4 million in the next hour to 1/8 million in the third hour, and so forth.

Thus the rate of death is constant, and the rate can be calculated by the same formula used to calculate the rate of exponential growth. The result in this case is a minus quantity, and indicates the time required to diminish the population by 50 per cent. The same method can be used to compare the effectiveness of germicidal agents.

A variety of conditions contribute to bacterial death, but the most important are depletion of nutrients and accumulation of toxic waste products. Bacteria die at different rates, just as they grow at different rates. Some species, e.g. Neisseria gonorrhoea, die very rapidly.

This organism is particularly susceptible to autolysis, presumably the result of digestion by enzymes present in the bacterial cells. The steepness and duration of the death phase depends in part upon the nature and concentration of the toxic waste products.

Phase # 8. Survival or Decreasing Growth Rate:

In this phase the rate of death decreases, and finally equilibrium is reached, such that both the rate of death and the rate of growth tend to balance each other at a very low level of population. Survival of the cell depends upon the types of organisms. Some die off with 3 to 4 days, while other types may remain viable for months and even years.

The formation of the spore provides a resistant form of the cell that may survive long after all the vegetative cells are dead. It often happens that mutant cells, which are present in small numbers, find environmental conditions more suitable for rapid growth than the parent types. Altered environmental conditions create selective conditions favouring the more rapid growth of certain mutant cells.