BACTERIAL GROWTH CURVE

BACTERIAL GROWTH

• In microbiology, growth refers the “Increase number of cells”.
• More than 2000 chemical processes were occurred during growth.
• Step wise reactions occur during the growth:

ü Synthesis of the macromolecules viz., DNA, RNA, protein, polysaccharides, lipids polymerization. 

ü  Assembly of macromolecules.

ü  Cell organelles formation

·     Growth rate: Increase of bacterial cell number per unit time is referred as growth rate.

·     Generation: The interval of formation of daughter cells from parental cell is called as generation.

·  Generation time: Time required to form two daughter cells from a single cell is called as Generation time.  This is also called as Doubling time.

GROWTH CURVE OF BACTERIA

·     Assuming a single bacterium has been inoculated to a liquid medium and the bacterium starts multiplying by Binary fission gives increase of population.

·       The growth curve of bacteria can be divided in to 4 phases.

a)     Lag phase

b)     Log phase or Exponential phase

c)     Stationary phase

d)     Death phase or Decline phase

·       In the beginning, the growth (increasing the population) will not occur leads to straight line (Lag phase), then the cells start multiply gives linear line (Log or Exponential phase) followed by straight line (Stationary phase) and decline linear line (Death phase/Decline phase).

Figure – 1: Growth curve of Bacteria (Source: microbenotes.com)

a) Lag phase

·       When a bacterium is introduced into a medium, it will not multiply immediately. 

·       After a period of time only the bacterium will start multiply.  This period is called as Lag phase. 

·       During this period, the cell increases its size.

·       It will synthesis new protoplasm and enzymes.

·       The bacterium will adjust the physiological environment.

·       At the end of Lag phase, cell will start reproduction.

b) Log phase or Exponential phase

·       After lag phase, the cell start multiplies steadily by Binary fission at constant rate. 

·       The portions of curve are linear and have constant Generation or Doubling time. 

·       All the cells are uniform chemical composition, metabolic activity and physiological characters.

·       At this time, the Generation time will be of constant.

·       This is the ideal stage to study the metabolic activity of the bacterium.

c) Stationary phase

·   By this time, the essential nutrients of the medium get depleted and some toxic substances produced by the organism leads to slow down the cell division.

·     At this time no net increase or decrease of cells in the medium occurs.  This stage is called as Stationary phase. 

·       During this stage, Cell division rate and Cell death rate is uniform

·    Cell death occurs due to low nutrients content, pH change, toxic wastes produced and oxygen content reduced.

d) Death phase or Decline phase

·       In this phase, few cells are alive and few died. 

·    The death rate is higher than the growth rate leads to decline the population. This phase is referred as Death phase or Decline phase.

·       The Cell lysis occurs during this stage.

·       Some species can die very rapidly and some can persists for months.