GROWTH CURVES -

S-shaped or sigmoid -

Described by Verhuhst in 1839.

It shows 5 phases -

(i) Lag phase

(ii) Positive acceleration phase

(iii) Exponential phase

(iv) Negetive accesleration phase

(v) Stationary

1.       Lag phase -

• Early stage of curve.
• When new individuals come in new environment so minimum growth occur in these phase.
• It is due to absence of new resources.
• It is due to lack of adaptations.

2.       Positive acceleration phase -

• Growth rate is increased.
• Birth rate increases.
• Death rate is minimum.

3.       Exponential phase -

• Individuals become adaptated.
• Rapid growth occur.
• Birth rate very high.
• Death rate very low.

4.       Negetive accesleration phase -

• Growth rate decreases.
• Available sources become limited.

5.       Stationary -

• Resources are limited.
• Birth rate is equal to death rate.
• So growth rate is zero.
• In the growth curve of bacteria after stationary phase growth decreases due to unavailability of resources.

If semi log curve is drawn for bacteria curve is in the form of straight line.