Bacterial growth curve, phases, factors

The Bacterial growth it is a complex process that involves many biochemical reactions and that results in bacterial cell division. If we had to define it more precisely, we would say that it is an increase in the number of bacteria in a population, not the size of each individual bacterium..

Bacteria are prokaryotic organisms, lacking a nucleus or any other intracellular membranous compartment. They are unicellular, microscopic organisms, naturally distributed in all ecosystems on the biosphere: in soils, bodies of water, animals, plants, fungi, etc..

Compared to many eukaryotes, bacteria generally spread much more rapidly, which can occur both in the natural context of each species and in controlled experimental environments (in vitro).

Article index

• 1 How does cell division occur in bacteria?
• 2 Bacterial growth curve
  • 2.1 What is the bacterial growth curve?
• 3 Phases of bacterial growth
  • 3.1 Latency phase or lag phase
  • 3.2 Exponential or logarithmic phase (log)
  • 3.3 Stationary phase
  • 3.4 Decline or death phase
• 4 Factors that influence bacterial growth
• 5 References

How does cell division occur in bacteria?

As in the rest of cellular organisms, cell division in bacteria is a process that occurs under strict control, both spatially and temporally, which includes:

- the replication or duplication of DNA (the genetic material)

- its distribution between the two future daughter cells (opposite poles of the dividing cell)

- the separation of the two resulting cells thanks to the formation of a "septum" or middle wall in the dividing cell

In these organisms, such cell division is known as binary fission and is the process that leads to an increase in the number of bacterial individuals in a population, that is, bacterial growth..

As each cell during division has to duplicate its genetic material and, consequently, increase its size, this implies that binary fission is a biochemically active event, which requires energy investment, that is, synthesis reactions and degradation reactions..

The growth of a bacterial population can be plotted as the increase in the number of cells as a function of time and this graph draws a curve called the "bacterial growth curve", in which several phases are distinguished where different characteristic processes occur..

Bacterial growth curve

Many authors have described the growth of a bacterial population as an exponential or geometric process, since each division cycle (also known as generation) causes 2 to emerge from 1 initial cell, then 4 emerge from these two, then 8, then 16 and so on.

The time necessary for each of these generations to form is known, therefore, as the generation time or doubling time, which can be easily calculated, which is generally constant and almost always species-specific..

In order to E. coli, For example, one of the model organisms among prokaryotes, the doubling time is plus or minus 20 minutes, while other species such as Clostridium perfringens or Mycobacterium tuberculosis have doubling times of 10 minutes and more than 12 hours, respectively.

It is important to mention that the generation time and, therefore, bacterial growth, can be altered depending on various factors, which we will talk about later..

What is the bacterial growth curve?

Through the years, scientists have managed to describe the phenomenon of bacterial growth using graphic methods, and this is how what we know today as the bacterial growth curve came to light..

This curve is nothing more than a graph showing the increase in the number of cells in a culture (plotted on a logarithmic scale) per unit of time, from which valuable information is obtained..

Generally, all experimentally grown bacteria in vitro which are provided with all the necessary nutrients to grow exhibit a similar growth pattern, which can be easily observed when the growth curve is plotted.

In this growth curve, several stages or phases are distinguished, which are very characteristic and for which microbiologists have achieved plausible biological explanations..

Phases of bacterial growth

As we have already mentioned, a population of bacteria grows exponentially, so the growth curves are plotted on a logarithmic scale..

Since the behavior during bacterial growth is not uniform, that is, it does not describe a straight line always rising, in a typical growth curve four phases are observed, which are known as:

- lag phase (lag)

- exponential or logarithmic phase (log)

- Stationary phase

- decline or death phase

Latency phase or phase lag

To start a bacterial culture, a small cell inoculum is started. When this inoculum is introduced into a complete fresh culture medium, that is, with all the necessary nutrients to grow the given bacterial species, initially no changes are observed in the number of individuals..

It has been shown that during this "latency" phase, in which there appears to be no cell growth, bacteria increase their size and are metabolically very active, since they are synthesizing nucleic acids, proteins and enzymes, etc..

The duration of this phase in time depends on some intrinsic factors of the population and some environmental factors. For example:

- of the initial inoculum size

- of the previous environmental conditions of the inoculum

- of time to synthesize the elements necessary for the division

Exponential or logarithmic phase (log)

When bacteria are ready to begin dividing, an exponential increase in the number of cells per unit volume per unit time is observed. They are, then, in the exponential or logarithmic phase of the curve.

During this phase, most bacteria are considered to be undergoing binary fission events at a constant rate and it is in this phase that scientists calculate the doubling time..

Like all phases of bacterial growth, the exponential or logarithmic phase and the doubling time of a population depends not only on the species, but on whether the bacteria in the culture medium find all the necessary nutrients and the right conditions to its growth.

Stationary phase

The exponential growth of bacteria is not infinite and this is because the culture medium, which is a closed growth system, sooner or later runs out of nutrients (bacteria consume everything).

In addition to nutrients, an increase in the number of cells in a constant volume (increase in cell concentration) is also synonymous with an increase in the concentration of metabolites or waste products that can have inhibitory effects on growth..

A greater number of cells in a finite space also implies that eventually there will not be enough space for more cells, which results in an inhibition of growth..

In this phase, called the stationary phase, some cells continue to divide, but others begin to die at a similar rate, so the curve flattens out..

Decline or death phase

After the stationary phase, it is observed as a dish On the growth curve, the phase of death or decline continues, where the bacteria begin to die and the curve declines.

During the death phase the bacteria die exponentially, so it is considered a "reverse" stage that the exponential phase.

Factors influencing bacterial growth

There are numerous factors that influence bacterial growth, many of them related to the environment where they grow.

Like all living organisms, bacteria need certain "basic" conditions to survive, which go beyond food. Thus, we can list some of the main factors that can alter or affect the appearance of a bacterial growth curve:

- the composition of the growing medium: in terms of carbon sources and in terms of essential elements

- the pH

- the temperature of the medium

- the concentration of ions and minerals

- gas concentration

- the availability of water

- the number of cells

- the presence of metabolites

- the presence of antibiotics and other potentially bactericidal substances

References

1. Bramhill, D. (1997). Bacterial cell division. Annual review of cell and developmental biology, 13 (1), 395-424.
2. Monod, J. (1949). The growth of bacterial cultures. Annual review of microbiology, 3 (1), 371-394.
3. Pepper, I. L., Gerba, C. P., Gentry, T. J., & Maier, R. M. (Eds.). (2011). Environmental microbiology. Academic press.
4. Vedyaykin, A. D., Ponomareva, E. V., Khodorkovskii, M. A., Borchsenius, S. N., & Vishnyakov, I. E. (2019). Mechanisms of bacterial cell division. Microbiology, 88 (3), 245-260.
5. Widdel, F. (2007). Theory and measurement of bacterial growth. Di dalam Grundpraktikum Mikrobiologie, 4 (11), 1-11.
6. Willey, J. M., Sherwood, L., & Woolverton, C. J. (2011). Prescott's microbiology (Vol. 7). New York: McGraw-Hill.