The Phases Of Growth Of Bacteria (Bacterial Growth Curve)

In microbiology, the bacterial growth curve describes the size or population of live cells or bacteria in a culture over time, which can be best described by the 4 phases: the lag phase, the log phase, the stationary phase, and the death phase.

Once the microorganisms have settled on the host or new environment, their population initially grows slowly. Commonly, bacteria reproduce by binary fission. This way, a single cell splits into two equal-sized cells. And then 4 cells, then 8 cells, then 16 cells, then 32 cells, so on and so forth (exponential growth). Fission, spore production, and budding are less common forms of asexual reproduction.

Once they have settled, Their rate of reproduction gradually enters an acceleration phase. At this point, rapid multiplication and then the equilibrium of population occur. This is especially true when food and other requirements that favor their growth are met satisfactorily. This behavior follows an S-shaped growth curve or sigmoid curve.

Let’s further discuss all of the 4 phases of growth of bacteria.

Bacteria growth curve
Bacterial growth curve

Lag phase

The lag phase is where it all starts. While in the lag phase, the cells adjust to their environment. Do the conditions in the environment favor them to grow? Is there sufficient food (medium)? The population size also remains constant, and the cells only grow in size.

Although the lag phase mainly involves adapting to the growth conditions, some processes are already occurring. These include the synthesizing of RNA to produce proteins necessary for exponential growth; and enzymes required for cell division. The lag phase can last from an hour up to a few days. Although the exact length varies depending on certain factors such as the growth medium, physiological history of the cells and the precise physiochemical environment of both the original and the new growth medium.

If the cells had been placed in a nutrient-rich medium, and then transferred to a poorer medium, the cell will need to produce enzymes first, hence the lag phase will be longer.

Injured cells will require more time to repair themselves before they proceed to reproduction.

If the new environment is kind of similar to the previous environment they were transferred from, the lag phase will take a shorter time. Otherwise, the adjustment time will take longer.

Log phase

Once the population enters the log phase, the growth continues, but at a faster rate. This eventually leads to an exponential or logarithmic growth phase wherein the cells reach the highest rate of reproduction. The population increases directly with time. If not limited, the growth of the bacterial population keeps on at a constant rate. The population increases directly with time. Plotting the movement will result in a straight line on a logarithmic graph.

The generation time will largely vary on the type of bacteria. However, when optimum conditions are met, the generation time for most common bacteria is around 20 to 60 minutes. Escherichia coli can double the number in 20 minutes in a nutrient-rich environment. However, some bacteria can generate much quicker. For example, the spore-forming Clostridium perfringens has a generation time of just 10 minutes.

The behavior of growth of bacteria is predictable in the log phase, especially in a controlled environment like a growth medium. The generation time can be calculated by the below formula.

Bacterie generation time formula

It is true that the log phase, among all the phases, is when the rate of the growth of bacteria is at the quickest. However, the rate of metabolic activity is also the reason why cells at this stage are highly susceptible to the action of antomicrobial agents such disinfectants.

Stationary phase

Once the environment is becoming worse, the growth transitions from log phase to the stationary phase. At this phase, the nutrients are starting to deplete rapidly, and the toxins or waste products are starting to accumulate. Physical and chemical stresses such as change in temperature and pH (acidity) also push the cells to enter this phase.

The cells still divide. However, other cells also start to die resulting in an equilibrium. The stationary phase results in a horizontal straight line when plotted on a logarithmic graph.

At this point, cells are no longer growing in number. However, they are still active metabolically as physiological, morphological, and gene changes take place to adapt to the worsening scenario. The cells change shape; they become spherical and shrink and size. They also activate a mechanism that reprograms the gene to adapt to the stress. In doing so, instead of consuming nutrients as they normally would in log phase, they synthesize amino acids to survive until the conditions of the environment improve.

During prolonged stationary phase, some bacteria enter Growth Advantage in Stationary Phase (GASP) phenotype. In this state, cells continue to grow even in a nutrient-deprived environment. This mutation allows living cells to scavenge nutrients from dead cells. Some bacteria including E. coli, Campylobacter, Pseudomonas are capable of entering this state.

Bacteria from the genera Bacillus and Clostridium, release spore especially during the stationary phase to help them combat the severing environment.

Death phase

The death phase is the last of all the phases of growth of bacteria. At this stage, the number of living cells decreases at an exponential rate.

This phase proceeds when the nutrients available have significantly depleted, the waste product accumulation has proceeded to a point of overcrowding, and the environment is no longer favorable to support the growth of cells. As a result, death rate is faster than cell regeneration. Plotting the movement of the death phase will result in a downward steep line on a logarithmic graph.

Although so-called death phase, it is worth noting that not all cells die at this stage. This is more likely because of their ability to mutate, which they usually start exhibiting in the stationary phase.

Another way they do to survive is by consuming nutrients from dead cells. Once the cells die, they are no longer able to reproduce. The cell membrane no longer works, and the lysate, the nutrients it holds becomes available to the surviving cells.

Website Link (Article by The Food Untold)

Published by RenSun Lee

Kia ora! Sustainability is at the core of my soul ever since I was a kid. I always strive to finish the food on my plate and live as a minimalist. I love to cut down on waste in order to live sustainably and harmoniously with our planet. This brings me to my passion as a Food Scientist to integrate new technologies into innovative and creative solutions to meet customer demands and market trends and to optimize products and processes for quality, savings and sustainability. To these goals, I have published a Journal on my work on sustainable packaging and patented a new Antimicrobial wash. Nothing is more satisfying than working hard and smart at the workplace and playing hard outside of working hours. I enjoy rejuvenating myself through spending quality time with my two adorable kids and my awesome soul mate and getting close to nature when possible, be it gardening, tramping or going to the beach. I also love to learn about our magnificent universe and how sustainability is working in the grand scheme of things. I strongly believe that Work, Life & Balance is the key to a healthy state of mind, both physically and mentally. I look forward to making a positive difference wherever and whenever I can. Through this Blog, I hope to catalog recent Food Trends and Food Technologies that I come across so that anyone who is interested can have access to it (articles and resources). Please use these resources at your discretion. On top of that, I would also like to share related news and technologies of the future that would help mankind advance towards a Type 1 Civilization. Please feel free to contact me if you would like to share and contribute to the “Resources“. I would like to thank you in advance for dropping by. I sincerely hope that you can benefit from the recent Food Trends and Food Technologies I catalogued. Kind regards | Ngā mihi RenSun Lee

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