Bacterial Growth curve

Bacterial Growth curve

Bacterial growth can be observed by inoculating bacteria into a liquid culture media and measuring population size at regular time intervals.If an old culture of bacteria is inoculated into a fresh medium and the cell concentration is periodically measured.A curve describing the change in cell number against time can be drawn.This curve is called bacterial growth curve.This growth curve can be divided into several distinct phases i.e log phase(exponential growth phase) lag phase stationary phase and death phase.When microorganisms are cultivated in a liquid medium they usually are grown in a batch culture or closed system ie they are inoculated in a closed culture vessel with a single batch of medium.Because of no fresh medium is provided during incubation nutrient concentration will be declined and the concentration will be increased.The growth of microorganism produced by binary fission can be plotted as the logarithm of no. of the viable cell is incubation time.

Lag phase

When a bacterial population is inoculated into the new fresh medium,it does not divide immediately.Bacteria take some time to adjust the new environment and it synthesizes necessary cellular component required for cell division in this period.This time period of adjustment during which bacteria remain metabolically active but do not divide is called lag phase.In lag phase, the various cellular material is synthesized and cell size increase so bacteria have large size at the end of the lag phase.For e.g if bacteria are injured in this case bacteria take some time to repair before division.In contrast to inoculated bacteria is healthy and young.lag phase is shorter.Similarly, of new media is different from the one in which bacteria were cultivated previously,it takes some time to adjust to new media.in this case log phase is longer.In contrast, if new cultivation media is some as previous one lag phase may be absent or of shorter duration.

• This phase is necessary for a variety of reason
• The cell may be old and devoid of various essential constituents which must be synthesized before growth can begin.
• The cells that have been damaged by treatment with heat radiation or toxic chemicals require time to recover.
• The medium may be different from the one microorganisms which were growing previously. On transfer to the new medium, time is required for synthesis of new enzymes.
• At the end of the lag phase, each microorganism divides and all cells are capable of dividing at regular intervals.

Log phase

In log phase, bacteria divide continuously at a constant rate and no of bacteria increase exponentially.In this period all bacteria population is most nearly uniform in term of their metabolic activity, the chemical composition of the cell and other physiological characteristics.The lag phase ends when cells have adapted to their new environment and begin to divide.When all the cells in culture are in the exponential phase or log phase of growth.The growth rate is strongly influenced by the physical environment and composition of culture media.It is also influenced by genetic factor and determine the speed at which material from the culture media can be absorbed and assimilated by each type of organism.

G=t/n=t/3.3 (log N-log N_o)

Where n is no of generation and t is total time of incubation.No is initial no of bacteria and N is no of bacteria after time t.The generation time of bacteria in log phase is not same for all bacteria.It is strongly dependent on growth factor present in media.In log phase growth rate of bacteria remain same.It is calculated by relation

Growth rate(R)=l/n= 3.3 (log N-log N_o) /t

Time and duration of log phase depend on the concentration of nutrients and density of microorganisms in the nutrients.

Stationary phase

• Microorganisms continued to multiply at a logarithmic rate until either of the two factors terminate the log phase.
• The supply of essential nutrients are depleted.
• Toxic products from the microbial metabolism accumulated to the inhibitory level.

When microorganisms grow continuously in log phase, the density of microorganisms increases, the concentration of nutrients and various metabolic toxic products being to accumulate.These condition i.e exhaustion of nutrients,accumulation of toxic product and increase in cell density bring stationary phase of growth in which there is no net increase or decrease in the number of bacterial population with time.In some case,complete cessation of cell division occurs and hence there is no net divide and some die and hence balancing each other,so no of bacteria remain same.

Death phase

In decline phase,no. of bacteria continuously decreases.Bacterial number decrease exponent i.e just reverse of log phase.If incubation continues after a population reaches a stationary phase,the cell may die and the population is said to be in the death phase.A variety of condition contributes to the bacterial death that is depletion of nutrients and accumulation of waste products.In some cases, death is accompanied by actual cell lysis which is mediated by autolysis.This mechanism is seen in streptococcus pneumonia which produces autolysis.The death phase is also called decline phase and no of viable cells decrease exponentially i.e the inverse of growth during log phase.

Survival phase

Some cells may survive and may again start the natural growth cycle if they obtain essential nutrients for their physiological process.This phase is seen among spore forming bacteria.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.