Bacterial growth curve

• Nutritional environment regulates the bacterial growth.
• When a bacterium inoculated in a suitable liquid medium is incubated, its growth leads to an increase in the number of bacterial cells which follows a definite course.
• When bacterial count of such culture is determined periodically and plotted, a curve is obtained i.e. called growth curve.
• The growth curve is hyperbolic due to exponential bacterial growth pattern.
• Four main phases of growth are generally recognized. They are:

A) Lag phase

• There is no appreciable multiplication of cells in this period.
• But, they may increase considerably in size and show marked metabolic activity.
• The condition and the number of cells in the inoculum, type of bacterial species, quality of culture medium, phase culture determine the duration of this phase.
• This might be due to the time taken for the organism to adapt itself to growth in the fresh medium.
• The cells in an inoculum may lack enzymes, metabolic intermediates and other factors for which time is required to build up those materials to their optimal levels.
• New enzymes may require to be synthesized by the process of induction or by the selection of mutants if the composition of new medium differs significantly.
• Towards the end of lag phase, bacteria attain maximum size.
• Environmental factors such as temperature and carbon dioxide also play vital role.
• The time taken by lag phase is 1-4 hours.

B) Log (logarithmic) or exponential phase

• The cells divide at a constant rate at this phase.
• There is a linear relationship between time and the logarithm of the number of cells as a result of growth by binary fission.
• It is very important to realize the potential of this enormous or explosive growth.
• If the number of bacterial cells in a culture increases ten-fold in a given period of time, they will increase 100-fold in twice that time.
• Similarly, the increase in number will reach 1000-fold in three times the given period and so on although the rate of division is much slower in vivo.
• As the time goes on, it will be very difficult to eradicate the bacteria if an infection overwhelms the body defenses and gets out of control.
• Bacteria must have high rate of metabolism to maintain such high rates of growth.
• The particular environmental conditions that prevail determine the direct relationship between the actual rate of growth and the generation time of bacterium.
• Some of the environmental factors that affect the rate of growth are O2, CO2, temperature, moisture and dessication, osmotic pressure, etc.
• The average duration of this phase is 8 hours.
• The bacteria become smaller in size and typical organisms are formed in log phase.
• Sporulation of spore forming species of bacteria occurs either at the end of log phase or early in stationary phase.

Image source: thebiologynotes

C) Stationary phase

• The rate of multiplication gets decreased and ceases altogether in due course as the exponential phase is no longer possible.
• Now the cells pass into the stationary phase.
• A stable number of cells can be reached by the establishment of an equilibrium between the rates of growth and death which is possible theoretically.
• But the common situation is one in which neither growth nor death occurs but the cells remain in a state of suspended animation.
• Some metabolic activity will be going on though the normal metabolism and growth no longer exists.
• This metabolism is called endogenous metabolism which provides the cell with energy and intermediates required to maintain life.
• Cessation of growth is due to the exhaustion of essential nutrients and accumulation of toxic waste products.
• Various factors may induce the beginning of stationary phase, due to which cells can exhibit a corresponding variation in morphology and physiology between themselves and in comparison with exponential phase cells.
• Stationary phase cells may have high levels of intracellular storage polymers such as polysaccharides (glycogen) and lipids (poly-beta-hydroxybutyrate) which are often virtually absent in exponential phase cells.
• Gram positive organism may become gram negative in this phase.
• Many secondary metabolites are produced by bacterial species at the onset of this phase that have stopped dividing.
• Secondary metabolites are diverse in nature and each kind has a restricted taxonomic distribution.
• Antibiotics and endotoxins are produced as secondary metabolites.
• The initiation of sporulation generally occurs at the end of exponential phase or early in the stationary phase in case of spore forming species.
• Since, many of the antibiotics and exotoxins producing bacteria are spore forming organisms, there may be the relation between sporulation and production of these metabolites.

Image source: clinicalgate

D) Death or decline phase

• The cells in a culture begin to die after a variable period of time in stationary phase.
• They become incapable of growth when taken to a fresh medium.
• There will be an increasing divergence between the total count (living and dead cells) and viable counts (living cells only).
• There are various reasons for this death or loss of viability which might also have cause the cessation of growth at the onset of stationary phase.
• There may be a brief stationary phase followed by a rapid death rate if the reason of cessation of growth is due to accumulation of toxic wastes.
• There will be rapid fall in the total and viable count in some cases which is due to the lysis and liberation of cytoplasmic contents into the environment as microorganism are very prone to digest themselves.
• This process is called autolysis which is the reason for the occurrence of bizzare cell shapes quite different from the normal range seen in exponential growth.
• Some microorganisms may stay in the stationary phase for some days despite of the rapid degeneration and death.
• The rapidity of the onset of death phase is an important factor that may influence the spread of infection.
• Duration of this phase varies from a few hours to a few days.

References: 

i) https://clinicalgate.com/bacterial-growth-genetics-and-virulence/

ii) https://theory.labster.com/bacterial-growth-curve/

Bacterial growth curve