Growth curve

Bacterial reproduction takes place by binary fission in which two daughter cells are produced from a bacterium. When bacteria are grown in a single medium and in a close culture vessel, it is known as batch culture. The growth of bacteria in a batch culture can be plotted as logarithm of number of viable cells in Y-axis and incubation time in X-axis.

This curve has four distinct phases and they are described in detail below:

Lag Phase: When microorganisms are added to the fresh medium, they do not divide immediately. They require some time to adapt themselves to the new environment and to prepare themselves for division. Hence, it is called lag phase. Although cell division does not take place, they are synthesizing RNA, enzymes and other molecules.

Lag phase is important as it allows microorganisms to prepare themselves for cell division. They replicate their DNA, increases their mass and synthesizes required cofactors.  When organisms are transferred from one medium to another, there may be need of some other enzymes to use that nutrient. They can synthesize that during lag phase. Also, if microorganisms are injured they can recover during this time period.

The duration of lag phase varies depending on the nature of medium and condition of microorganism. The lag phase is of short duration when young, exponential phase culture is transferred from one medium to another of same composition. On the contrary, lag phase will be of longer duration if bacteria are transferred from old culture or one which is refrigerated or transferred into the medium of chemically different composition.

Log Phase: This is also known as exponential phase. During this period, cell divides and doubles in number. Doubling occurs at a constant rate i.e. microorganisms are dividing and doubling in number at regular time interval. And because the rate of growth is constant, it results in a straight line. The slope of the line is the growth rate of organism i.e. the measurement of number of divisions per cell per unit time. However, exponential growth cannot continue for long because over the time period nutrient will deplete and waste will accumulate in the medium and eventually population growth will cease.

Stationary Phase: After exponential phase, stationary phase begins during which the growth curves become horizontal.  This phase is marked by the accumulation of waste products and depletion of nutrients in medium. During this phase, growth rate and death rate are balanced. Some organisms are dividing and some are dying keeping the total number of viable cell a constant.

Death Phase: Bacteria begins to die during this phase. Detrimental environmental condition like toxic waste builds up in the medium and nutrients depletions leads to the decline in the number of viable cells. Like the growth of bacteria during log phase, the death of bacteria is also logarithmic and results in a straight line. Most of the microbial population dies but some resistant cells can survive.