4 Phases of Bacterial Growth Curve

 When bacteria are inoculated in to a fresh medium and are allowed to grow, they show a characteristic pattern of growth. This is known as a normal growth curve. This characteristic pattern is observed in batch or closed culture system.

  Batch culture is the one, where bacteria once inoculated in the medium, are allowed to complete their growth without further adding any nutrients in the medium. This culture is also known as closed culture because once organisms are inoculated in the culture medium they are allowed to complete their growth in the same medium. The normal growth curve of bacteria appears sigmoidal in nature, when growth is plotted graphically.

Typical bacterial growth curve. 
A lag phase; B log(logarithmic) or exponential phase, C stationary phase; D death or decline phase

 The curve can be divided in to four phases as under :

  1. Lag phase or initial phase
  2. Log phase or logarithmic phase or exponential phase.
  3. Maximum stationery phase.
  4. Death phase. 

The occurrence of different growth phase is due to constantly changing environment in the medium due to bacterial activity.

Lag phase

 When bacteria are inoculated into a fresh medium, initially for some time, there is no visible increase in cell number and cell mass. This is referred to as lag phase of growth This is attributed to various reasons.

  1. If the inoculum is obtained from old culture or culture from death phase , it may carry with it toxic metabolites which may continue to Inhibit the growth.
  2. Such cells may also be metabolically weak , having very few numbers of ribosome, RNA and proteins.
  3. Much often, the nutrient environment in fresh medium may be quite different than the one that might be in the old / previous culture. Hence organisms may require sometime to adapt to the new environment. 
  4. If the inoculum is obtained from spore forming bacteria and consists mainly of spores, they must germinate and get converted to vegetative cells before starting the growth. This time for spore germination may also account for the lag phase of the growth.

Changes in cells during lag phase  

 Though there is no visible increase in cell number during lag phase. metabolic and physiological changes do occur in the bacteria, to prepare them to undergo rapid growth Hence, we cannot consider lag phase as idle phase. During this phase,
  1. The number of ribosome, amount of RNA,DNA and proteins increase in cell. This shows physiological activation of the weak cells. 
  2. The overall rate of metabolic activity and respiration Increases. 
  3. The cell size also increases.

Factors affecting the length of lag phase

The length of lag phase is influenced by a number of factors They may include :

1). The type of Inoculum
  • If the inoculum is obtained from previous culture occurring in log phase. It will show a shorter lag phase. 
  • This is because cells from log phase are metabolically more active than those occurring in death phase. 
  • Similarly, Inoculum containing large number of spores will show longer lag phase. 

2). Medium composition
  • If the composition of fresh medium is similar to that of previous one or if it is rich, a smaller lag phase in obtained.

3). Size of Inoculum
  • If the inoculum is large the culture shows shorter lag phase and vice versa. 

Exponential or logarithmic phase of growth (log phase)

 By the end of lag phase, most of organisms in the culture are active and large enough to divide. Hence they start dividing causing logarithmic increase in cell number.

 Since the environmental and nutrient conditions are most favorable, these bacteria will show maximum rate of growth and minimum generation time.  

  Usually, the medium contains nutrients in excess. Therefore even though nutrients are being continuously depleted from the medium due to their utilisation during the growth Initially, the change in their concentration has very little effect on growth. Hence the bacteria continue to grow logarithmically for many generations.

  However, during the growth the chemical environment of the medium undergoes a constant change and gradually becomes unfavorable for growth. 

 This results in to gradual decline in growth rate. Hence, the increase in growth slows down, allowing it to other stationery phase.

The development of unfavorable chemical and physical environment of medium occurs due to following reasons:

  1. During growth, bacteria consume nutrients as well as dissolved oxygen. Therefore, gradually, their supply becomes limiting and unfavorable for growth
  2. Further, as growth increases, the need for nutrients Increases more and more. Hence this depletion becomes more significant during later part of log phase.
  3. Increase in cell mass due to growth causes increase in Viscosity of medium significantly. This also provides unfavorable condition to the organisms for uptake of nutrients as well as O2 by the organisms.
  4. Even enough space is also not available for the organisms to have free movement in the medium dut a to increased cell density.
  5. Due to respiratory and metabolic activities of the organisms during growth, a drop in pH of medium is normally observed. This also contributes to the development of unfavorable condition for the organisms. 
  6. The metabolic activities of organisms results in to excretion of various waste metabolites by the organisms. These metabolites may have toxic effect on the microorganisms and they have unfavorable effect on growth and tend to retard growth.

Stationary phase

 When the bacteria are allowed to continue their growth in the same medium for prolonged time , condition in medium becomes more and more unfavorable for the growth. This is due to 

  • Depletion of nutrients from the medium, due to their utilisation in log phase.
  • More and more accumulation of toxic waste metabolites. 
  • Increased cell density can cause limiting space available for the organisms and increase in viscosity. 
  • Greater changes in pH to unfavorable value etc.

As a result, growth of the bacteria becomes Inhibited and their growth rate decreases and finally organisms stop dividing. This results in to development of stationary phase. 

  Perhaps, the rate of new cell being formed may also become equal to the death of cell. Thus the equivalent growth rate and death rate helps in establishing stationery growth phase.

Death or Decline phase of growth

 Following stationary phase, the bacterial growth curve enters death phase, if they are still continued to remain in the same medium. Due to the development of still more vigorous unfavorable conditions in the medium, bacteria stop growing. Their death rate exceeds growth rate and hence number of living cells from the culture decrease.

  The decrease in number of bacteria may even become logarithmic. Hence, this phase of growth may also be called as negative logarithmic or negative exponential growth phase.

The factors that contribute death of bacteria include :

  1. Sharp decrease in essential nutrients.
  2. Excessive accumulation of toxic waste metabolites. 

  However, death rate will not be same for organisms of all species. It may differ with the type of organisms. Usually gram-negative bacteria decline sharply and hence very few bacteria may survive at the end. While gram - positive bacteria die slowly and viable cells may persist for prolonged periods in the culture.

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