Bacterial Growth Curve: Definition, Phases and Significance

The growth of any organisms is defined by the increase in cell size and number. In case of bacteria, that are prokaryotic, single-celled microorganisms, and increase their populations by reproducing asexually by budding, multiple fission, binary fission or by spores formation.

As we know that instead of growing in size, the bacterial cell grow in numbers and this growth is in an exponential form, i.e. one cell divides in 2, then 4, then 8, 16, 32, 64 and so on.

The time taken for a bacterial cell to divide and double itself is known as generation time, that may vary from one species of microorganism to another along with the other environmental factors. For instance, Escherichia coli has a doubling time of 20 minutes, while Clostridium perfringers is seen as the fastest doubling bacteria, having generation time as 10 minutes. On the contrary, Mycobacterium tuberculosis is the slowest one, that has generation time of 12-16 hours.

Method of Obtaining Bacterial Growth Curve

To achieve the population of the any species of bacterium and study the growth curve, can be done by the growing pure culture of the that organism in a suitable environment, and temperature along with the nutrients.

After the inoculation in a sterilized environment (to avoid contamination), samples of the culture are collected at regular intervals and the the viable organisms from each sample can be checked.

The graph is plotted after the data collection, for this logarithmic graph paper is used. The aim of growing the bacterium is to study the molecular, biochemical and pathogenesis studies.

The logarithmic graph paper is plotted against the bacteria per milliliter of medium and time.

Phases of Bacterial Growth

Lag Phase

  • This is the kind of preparatory stage for the microorganisms, during this phase the microorganisms are inoculated or introduced to the fresh culture medium.
  • No sudden growth or increase in cell number was observed at this time and so this is known as lag phase.
  • As mentioned earlier, this phase is the only time where bacteria take time to prepare itself and get adjust to the new provided environment in terms of nutrients and other physical conditions such as temperature, pressure, etc. and therefore it can also be called as ‘adaptation period’.
  • Now this phase does not undergo any growth or multiplication is been observed, rather bacteria starts preparing for the cell division after adapting with the environment.
  • The bacteria start preparing for the division or multiplication by expanding the cellular factory and other essential factors such as preparation and expanding the enzymes, proteins, ribosomes, etc. that are required to conduct the molecular and physiological processes.
  • There is no specific time for the lag phase, it may depends on the conditions such a physical environment, and nutrients and also the type of bacteria.

Exponential/Log phase

  • This is the most productive phase, where the microorganism divide at their maximum rate by utilizing the most of the nutrients and other favorable conditions provided to them.
  • The bacterial division or multiplication takes place through the binary fission, where one bacterial cell produces two daughter cells. So, total number of population can be counted as 2n, where n is the number of generation or bacterial multiplication number.
  • The growth is at constant rate and consistent as the microorganisms keeps on dividing at the regular interval of time and resultant growth curve is smooth instead of the discrete form.
  • Th exponential phase reveals the balanced growth between the cellular constituents and production rates of bacteria.
  • The exponential phase is one the crucial phase for the researchers to study more about the bacteria.
  • It is also seen that if the concentration of nutrients and other conditions changes for any reason, it may drastically change the rate of growth of microorganisms and its number too.
  • This can be easily noticed, if one inoculates the microbe form the rich culture medium to the pure medium or vice versa, this may result in the shift up or down.
  • It can be said that, the increase or decrease in the number of cells is directly proportional to the concentration of the nutrient provided to them, and other physical factors.

Stationary Phase

  • After the lag and log phase, there comes the stationary phase.
  • This is kind of plateau in the growth curve graph.
  • It signifies the rate of the cell growth is equivalent to the rate of cell death.
  • The final population growth can be depicted by the nutrients that were provided and used by the microorganism, moreover on the type of microorganism as well.
  • This is the kind of saturation phase, and the cause can be numerous, but the most important is the nutrients that were provided, along with the other toxins that gathered or grow during the lag and log phase.
  • In case of aerobic microbes (that grow in presence of oxygen) lack of air can also be the major factor to enter into the stationary phase.
  • Formation of endospore can be easily observed if there is lack of nutrients or any unfavorable conditions.
  • Endospores formation produces the starvation proteins and begins their endospore formation pathway and also becomes resistant to the environmental or physical stress.
  • It is been observed that Salmonella thyphimurium and other pathogenic bacteria becomes active at the starved phase.

Death Phase

  • This is also known as the decline phase where the rate of death of bacterial populations is more than that of rate of division.
  • The ground reason for entering into the decline phase of the bacterial populations is the consumption of the nutrition provided to them for the growth and simultaneously accumulated toxins material.
  • With the passing time, the accumulation of toxin materials get increased and there is a depletion of nutrients.
  • This results in the increase in the death rate of the bacterial populations and therefore it shows exponential decrease in number of cells.
  • As the lag phase varies with the type of bacteria and nutrients provided, same is the case of the death phase, that the rate may vary as per the bacteria and medium.

Formula to calculate the Bacterial Growth

bacterial growth curve 1bacterial growth curve 2

Significance of the Bacterial Growth Curve

The main aim of growing the bacterial artificially or in laboratory is conduct the scientific research and studies and to attain the desired products. Following are the crucial points to understand the importance of the bacterial growth curve.

1. To know more about the specific bacteria and to understand their physical and nutritional requirements. This will be helpful in utilizing such bacteria after the isolation. These can be helpful in degradation of toxic materials, they can be plant growth enhancer, antibiotics and useful in some or the other ways.

2. To get more deep knowledge of the molecular and biochemical pathway of bacteria, growth kinetics.

3. To achieve the desired products, if the bacterial strain has industrial use and is needed to be grown at higher rate. This can be achieved by knowing the growth kinetics as we would be able to know that whether particular strain of bacteria are able to metabolize the certain substrates like oil pollution or industrial waste.

4. It is seen that antibiotics are produced mostly during the stationary phase, and to gain that it is important to study such phases. Moreover, the slope and shape of growth curve of particular strain of bacteria grown with industrial waste products can provide more opportunities to the researchers whether the particular substance, and its extended potential energy sources for the bacteria are present in the waste.

Sum up

All in all we can say that there is numerous purpose of growing the bacteria, by providing suitable environment to them. We can use these studies for industrial as well as educational purposes.

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