Bacterial Growth Curve And Different Phases

Bacterial growth curve

The term growth is more commonly used to refer to growth in the size of a population/Bacterial Cultures.

Population growth is often studied by analyzing the growth of microbes in liquid (broth) culture. When microorganisms are cultivated in broth, they usually are grown in a batch culture; that is, they are incubated in a closed culture vessel like a test tube or a flask with a single batch of medium.
When we inoculate a fresh medium with a given number of cells, determine the bacterial population intermittently during an incubation period of 24 h (more or less), and plot the logarithms of the number of cells versus time, we obtain a curve, which is referred to as the bacterial growth curve.

From this curve, it can be seen that there is an initial period of what appears to be no growth (the lag phase), followed by rapid growth (the exponential or logarithmic phase), then a leveling off ( stationary phase), and finally a decline in the viable population (death or decline phase).
Therefore, the Bacterial growth curve consists of 4 different phases such as the lag phase, the exponential or logarithmic phase, stationary phase, and death or decline phase.
Between each of these phases, there is a transitional period (curved portion). This represents the time required before all cells enter the new phase.

Bacterial Growth Curve and Different Phases

Obtaining Bacterial Growth Curve

The bacterial growth curve or microbial growth curves of a particular species of bacteria can be obtained by the following steps;

The growth curve of a population of a particular species can be obtained by growing a pure culture of the organism in a liquid medium at a constant temperature.
The Samples (bacterial cultures) are collected from the culture at fixed intervals (e.g., every 30 minutes), after that the number of viable organisms is determined.

The collected data are plotted on logarithmic graph paper, where the logarithm of the number of bacteria per milliliter of medium is plotted against time.

Different Growth Phases of Bacterial Growth Curve

The study of bacterial growth is done since they are started to grow in the lab in a optimal growth condition. In the laboratory, they are grown in a closed system or batch culture system in a predictable pattern where no foods are added, and no wastes are removed, resulting in a growth curve consist of four distinct growth phases: the lag phase, the exponential or log phase, the stationary phase, and the death or decline phase. Additionally, this growth curve can generate the generation time for a distinct organism – the number of times it takes for the population to double.

The features connected with each growth curve such as the number of cells, length of each phase, rapidness of growth or death, the overall number of times will fluctuate from organism to organism or even with different circumstances for the same organism. But the pattern of four different phases of growth will typically continue.

The Lag phase

The lag phase is also known as the adaptation period, during this phase the bacterial cells start to adjust to their new conditions.
In lag phase, bacterial cell take some time to get adapt themselves to the new growth conditions.
The time period of the lag phase may vary based on how complex the environments are from the environments that the bacteria came from, as well as the condition of the bacterial cells themselves. If the growing cells are shifted from one type of media into the same type of media, which has the same environmental conditions, will have the shortest lag period.

Those cells are damaged and will have a long lag period because they must repair themselves before they start reproduction.
The addition of inoculum to a new medium is not followed immediately by a doubling of the population. Instead, the population remains temporarily unchanged.
During this phase, the bacterial cells start to increase in size and physiologically they are becoming very active and are synthesizing new protoplasm.

The bacterial cells also synthesize enzymes and coenzymes which are lacking in the new environment to operate the chemical machinery within the cells.
During this step the cells start their metabolic activity but don’t divide.
In this phase the bacterial cells also synthesis RNA, enzymes, and essential metabolites, as well as adjusting to environmental changes such as changes in temperature, pH, or oxygen availability.

At the end of this phase, the cells start to divide, as a result, the number of cells in the population begins to increase.

Exponential Phase or Log phase

After the accumulation of essential components which are required for the growth of the cells, they proceed into cell division, where 1 cell becomes 2 cells, becomes 4, becomes 8, etc.
The maximum growth rate is obtained during the exponential growth phase.

During this phase, the cells are divided steadily at a constant rate, that’s why the log of the number of cells plotted against time results in a straight line.
Besides, the population is most nearly uniform in terms of the chemical composition of cells, metabolic activity, and other physiological characteristics.
In presence of optimal conditions, the bacterial cells will show a very rapid growth (and a steeper slope on the growth curve) if the condition does not meet the optimal requirements for cell growth, the cells will show a slower growth rate.

Due to the predictability of growth in this phase, this phase can be used to mathematically calculate the time it takes for the bacterial population to double in number, known as the generation time (g). Microbiologists use this information in basic research, as well as in industry.
The generation time g (the time required for the population to double) can be determined from the number of generations n that occur in a particular time interval t. Using the following equation [equation A]. (for n, the generation time can be calculated by the following formula [equation B])

The generation time may vary from species to species; for example, E. coli, it may be 15 to 20 minutes; for others, it may be many hours. Similarly, the generation time is not the same for a particular species under all conditions. It is strongly dependent upon the nutrients in the medium and on prevailing physical conditions.

During exponential growth, the growth rate (i.e., the number of generations per hour), termed R, is the reciprocal of the generation time g. It is also the slope of the straight line obtained when the log number of cells is plotted against time. The growth rate can be determined by this formula;

Generation time (g) can be represented by t/n, with t being the specified period of time in minutes, hours, days, or months. Thus, if one knows the cell concentration at the start of the exponential phase of growth and the cell concentration after some period of time of exponential growth, the number of generations can be calculated.
The relationship between the initial number of cells at the start of the exponential phase and the number of cells after some period of time is expressed by:N = N02^n.Where N is the final cell concentration, N0 is the initial cell concentration, and n is the number of generations that occurred between the specified period of time.

When the bacterial cells within a closed system such as a batch culture, run out of all the required nutrients/chemicals or microbial growth are inhibited by the waste products which is produced by them or lack of physical space, leading the cells to enter into the stationary phase.
During this phase the total number of viable microorganisms remains constant. This may result from a balance between cell division and cell death, or the population may simply cease to divide but remain metabolically active. In this phase the growth curve becomes horizontal.
There are different reasons that microbes enter into the stationary phase. One of the most important reasons is nutrient limitation; if an essential nutrient is severely depleted, population growth will slow and eventually stop.

Aerobic organisms often are limited by O2 availability. Oxygen is not very soluble and may be depleted so quickly that only the surface of culture will have an O2 concentration adequate for growth.
There are other reasons is the accumulation of toxic waste products, which may cease Population growth. For example, streptococci can produce so much acid from sugar fermentation that growth is inhibited
During this stage, the cell Physiologically becomes quite different to adapt to their new starvation conditions. Some of the cells are produced smaller in size, with bacilli becoming almost spherical in shape. The plasma membrane grows less fluid and permeable, with more hydrophobic molecules on the surface that promote cell adhesion and aggregation.

In this adverse condition, Some cells protect their genetic element in a different way such as; nucleoid becomes condensed and the DNA becomes bound with DNA-binding proteins from starved cells (DPS). All of these changes help them to survive for a longer period of time in adverse conditions.
During the stationary phase, the bacterial cells also produce secondary metabolites or metabolites produced after active growth, such as antibiotics.

Death Phases or Decline phase

After the stationary phase, the bacterial species enters into the death or decline phase where the number of viable cells or cell density decreases in a predictable (or exponential) fashion.

In batch culture, the cells cannot remain in the stationary phase indefinitely, that’s why they enter into the death phase. During this phase, the number of viable cells declines exponentially, with cells dying at a constant rate.
The depletion of essential nutrients and the accumulation of toxic wastes; such as acids are the main reasons for the Death or Decline phase of growing bacterial cells.
It is thought to be under specific conditions the dead cells might be revived, this condition is known as viable but nonculturable (VBNC).

Long-Term Stationary Phase

Long-term growth experiments reveal that after a period of exponential death some microbes have a long period where the population size remains more or less constant, this phase is known as the Long-Term Stationary Phase.
This long-term stationary phase is also known as the extended stationary phase can last months to years.
During this time, the bacterial population continually evolves so that actively reproducing cells are those best able to use the nutrients released by their dying brethren and best able to tolerate the accumulated toxins.

This dynamic process is marked by successive waves of genetically distinct variants. Thus natural selection can be witnessed within a single culture vessel.

Application of Bacterial Growth Curve

The bacterial growth curve is used when trying to utilize or inoculate known amounts of the bacterial species, such as to improve plant growth, enhance the biodegradation of toxic organics, or produce antibiotics or other natural products at an industrial scale.
Growth kinetics used for evaluating whether distinct strains of bacteria are accommodated to metabolize particular substrates, such as industrial garbage or oil pollution.

The bacterial growth kinetics and bacterial numbers in a culture medium are essential information for the researchers and commercial point of view.
Bacteria that are genetically engineered to clean up oil spills, for example, can be grown in the presence of complex hydrocarbons to ensure that their growth would not be repressed by the toxic effects of oil.

Generation time

The Generation time is the amount of time needed for bacteria to develop as well as divide i.e. the complete division of a cell. Some microbes can divide at a rate of once after 12 or 15 minutes. Other microbes require many hours. A some bacteria that grow very slowly could require longer than 24 hours for each cell division.

Since there is no fresh medium available during the incubation process as a result, the levels of nutrients drop and the concentration of wastes rise. The development of microorganisms reproduced through binary fission is represented as the logarithms of the number of viable cells per the time of incubation. The resultant curve is composed of four distinct phases.

The growth of microorganisms can be measured using:

an increase of size, but this is not a good indicator of growth.

increase the number of bacteria by count of the number of living cells (viable count) or by counting all cell count (total count).
testing of a part of the cell structure like DNA or protein as a sign of an increase in microbial activity (growth) or decline (death).

FAQ

What is lag time?

Lag time is defined as the initial period in the life of a bacterial population when cells are adjusting to a new environment before starting exponential growth.

References

Microbiology by Pelczar.
Prescott’s Microbiology by Joanne Willey, Linda Sherwood Adjunt Professor Lecturer, Christopher J. Woolverton Professor.

https://open.oregonstate.education/generalmicrobiology/chapter/microbial-growth/
https://microbenotes.com/bacterial-growth-curve-and-its-significance/
https://en.wikipedia.org/wiki/Bacterial_growth