Bacterial colony shows a definite pattern in its growth if it is grown in optimal environmental conditions. To get a bacterial colony growth curve, we need to inoculate a bacteria to enriched media and incubate it. Then bacterial count is taken after at intervals from inoculation and plotted in relation to time, a growth curve is obtained. Typically bacterial colony growth curve shows following phases:
This is the first phase of bacterial colony growth curve. It is obtained just after the inoculation of bacteria. It represent that there is no appreciable increase in numbers of bacterial cell. This occur because at initial time bacterial cell intake nutrient and increase in size. When they reach to the critical size they tend to divide. During lag phase there may be increase in size but there is no considerable increase in number of bacterial cells. The duration of lag phase depend on the bacterial species, size of inoculum, nature of culture medium and environmental factors like temperature, light (if required) etc.
Log (logarithmic) or exponential phase:
This phase appear after lag phase. Here bacterial cells start dividing and their number increases exponentially or by geometric progression with time. If this is plotted against time on a graph paper, a straight line is obtained.
As we are considering bacterial growth in vitro. There is limitation of nutrients. This is the cause of a stationary phase of bacterial colony growth curve. Here cell division stops due to depletion of nutrients and accumulation of toxic substances, produce by bacterial cells during their metabolic activities. In this phase bacteria maintain its population cells divide but just to replace the dead cells of bacteria. The viable count remains stationary as an equilibrium exists between the dying cells and the newly formed cells.
Phase of decline
When bacterial population start decreasing due to cell death, forms a phase of decline. Beside nutritional exhaustion and toxic accumulation, cell death may also occur due to autolytic enzymes.
The various stages of the growth curve are associated with morphological and physiological alterations o the cells. Bacterial cells in the lag phase have the maximum cell size. In the log phase cells are smaller and stain uniformly. in the stationary phase, cells frequently are Gram variable and show irregular staining due to the presence of intracellular storage granules. Sporulation may occur in this phase. Phase of decline occur because many bacteria develop exotoxins and antibiotics which lead to mass cell death.