In this article we will discuss about the four main phases of growth curve in bacteria. The phases are: 1. Lag Phase 2. Log Phase or Exponential Phase 3. Stationary Phase 4. Decline Phase.
1. Lag Phase:
After inoculation into the sterile nutrient medium, the bacterium first undergoes a period of acclimatisation. At that time, necessary enzymes and intermediate metabolites are synthesised, thereby bacterium reaches a critical stage before multiplication, multiplication takes place at this stage.
The duration of lag phase depends on the type of bacteria, quality of culture medium, size of inoculum and several environmental factors such as CO2, temperature, pH, etc. The average time of lag phase is 2 hours, although it varies from species to species (1-4 hours).
2. Log Phase or Exponential Phase:
In this phase, the bacteria undergo cell division and their population (number) increase exponentially at a logarithmic rate. The number of viable count, when plotted against time, gives a straight line of inclined fashion. The average time of log phase is 8 hours, though it varies in different species.
3. Stationary Phase:
In this phase, the growth i.e., cell division, almost ceases due to exhaustion of nutrients and also the accumulation of toxic products. At this stage the cell death starts at a slow rate and is compensated by the formation of new cell through cell division.
The total cell number increases at a slow rate, but the viable count remains almost constant. The duration of this phase is variable which ranges from few days to few hours. Secondary metabolites like antibiotics, toxins etc. are produced in this phase.
4. Decline Phase:
In the phase of decline, the total number of cells remains constant, but the number of viable cells gradually decreases due to exhaustion of nutrients and also the accumulation of toxic products. In some cases a few- cells remain viable for long time, even after death of most of the cells. These viable cells probably grow by utilising nutrients released from dead cells.
The cells attain maximum size at the end of lag phase and become smaller in log phase (exponential phase). In spore forming species, the sporulation occurs at the end of log phase (exponential phase) or in the early part of stationary phase.