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Write down the different stages of bacterial growth curve with their importance.

Bacterial growth curve : 

If a small number of bacteria is inoculated into a liquid nutrient medium and bacteria is counted of  frequent intervals and a graphical demonstration is produced by putting log number of cells along the Y-axis and time along the X-axis, then it is known as bacterial growth curve.

Phases of bacterial growth curve:

1. Lag phase or preparatory phase 

2. Log phase or exponential phase

3. Stationary phase 

4. Decline phase/Death phase

Description of each phase:

1.Lag Phase:

This is the time or period required for bacteria for adjustment in human body.


  •  No appreciable multiplication of bacteria occurs in this phase.
  • Metabolic activity increases. 
  •  Cell size may increase
  • Time requires 1-4 hours


Membrane acting antibiotics such as polymyxin, amphotericin-B can be used in this phase. Detergents, soaps and other surface acting agents act better in the lag phase.

2. Log Phase: 


  • Rapid cell multiplication & cell number increases in geometric process.
  •  Active synthesis of the cell wall occurs.
  •  Metabolic activity increase at a very high rate 
  • Time requires 1-4 hours.


  • Antibiotics act better at this phase as cell wall growth is very active during this phase.
  •  At this phase disease-producing capability of bacteria is highest, and if not treated the disease properly at this stage, dreadful conditions like septicaemia can result.

3. Stationary phase:


  • At this stage, multiplication rate & death rate of bacteria are equal, so net growth rate is zero.
  • Cell dies because of exhaustion of nutrients in the bacteria & accumulation of toxic products in themedium.
  •  Exotoxin production starts at this stage.
  •  Time requires – few hours to days.


  • Release of exotoxin starts. 
  • Spore forming bacteria starts formation of spores.
  • Cell wall acting antibiotics may be used. 
  • Gram positive bacteria may be transformed into Gram negative bacteria by erosion of

peptidoglycan layer.

4. Decline phase/Death phase:


  • The death rate is greater than the multiplication rate. 
  • Accumulation of toxic products occurs.
  • Time requires a few hours to days.


  • Exotoxin of C. diphtheria is produced in this phase.
  • Bacteria may develop L-forms, which are resistant to antibiotics. 
  • Sporulation starts in some bacteria.
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