Bacteria are so "fertile", will the earth be occupied by them one day?

Produced by: Science Popularization China

Author: Shi Zhihui

Producer: China Science Expo

Bacteria have lived on Earth for billions of years and have attracted the attention of scientists since Leeuwenhoek first observed them in 1683.

Bacteria reproduce very quickly. Take Escherichia coli as an example. They reproduce a generation every 20 minutes on average. In just one day, E. coli can grow from one cell to 4722×1021 cells. If these bacteria were spread out on the surface of the earth, they would completely cover the earth.

So, if this rate of division and reproduction continues, will the Earth be taken over by bacteria?

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There is a strong enemy outside: the threat of "eating bacteria"

Bacteria divide so quickly, but have not yet become a disaster. This is because they still have natural enemies on Earth. Just like the natural enemy of mice is weasels, the natural enemy of bacteria is bacteriophages, which are viruses that can "kill" bacteria.

Bacteriophages are microorganisms that are smaller than bacteria. If bacteria are compared to footballs, they are about the size of ping-pong balls. For example, E. coli is about 2 μm long, while T4 bacteriophage is about 0.2 μm.

The appearance of a bacteriophage is like a tadpole, with a round or polygonal head and a tubular tail with six tail filaments at the end.

Phages can inject their own nucleic acids into bacteria like a syringe, and then use the bacteria's nucleotides, enzymes and other raw materials to synthesize progeny phages. Taking T2 phage as an example, it only takes about 40 minutes at 37°C to produce 100 to 300 progeny phages.

After using bacteria to synthesize offspring, the bacteriophage will lyse the bacteria, so that the bacteriophage not only kills the bacteria, but also synthesizes a large number of offspring. The offspring phage will continue to kill more bacteria. Therefore, the existence of bacteriophages effectively curbs the growth of the number of bacteria.

Bacteriophage infection process

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In addition to bacteriophages, macrophages are also natural enemies of bacteria, and they can "eat" bacteria. Macrophages are a type of phagocytic cell that is located in tissues and is derived from monocytes.

Macrophages hunt down harmful bacteria that infect animals, but sometimes macrophages do not fight alone, they also have comrades-in-arms who work with them - neutrophils.

Macrophages engulf bacteria

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Neutrophils sacrifice themselves to produce neutrophil extracellular traps (NETs), which are like a spider web that fixes bacteria, thus enhancing the bactericidal ability of the spider macrophage. Therefore, the existence of "eating bacteria" can also curb the growth of the number of bacteria.

Internal strife: limitations of space and energy

Bacteria reproduce very quickly, but it also depends on whether the environment allows them to continue to divide and multiply. On the one hand, it is the carrying capacity of the space. Although bacteria are so small that we cannot see them, if they reproduce continuously for 24 hours, the bacteria can cover the entire earth. However, if they are in a specific space, due to space limitations, bacteria cannot multiply indefinitely.

On the other hand, bacteria need to take in nutrients from the outside world to multiply and divide, such as carbon sources, nitrogen sources, inorganic salts, etc.

As bacteria continue to multiply, the nutrients in the environment gradually decrease, making it increasingly unfavorable for growth. Intraspecific competition among bacteria becomes increasingly severe, and harmful substances in the living environment also increase. Finally, if environmental conditions do not improve, the shape of individual bacteria will change and begin to become deformed or die, and the number of bacteria will decrease significantly and finally approach zero.

For example, actinomycetes release some chemicals (such as antibiotics) during their growth process to inhibit the survival of other bacteria. They may also die due to lack of oxygen and nutrients in the environment due to their own massive proliferation. Therefore, a variety of environmental factors will affect the number of bacteria.

Image source: biorender

Own defects - short life cycle

Although bacteria reproduce quickly, their life cycle (individual) is relatively short. The growth cycle of bacteria is mainly divided into four periods: lag phase, logarithmic phase, stable phase, and decay phase.

Taking E. coli as an example, under normal circumstances, E. coli enters the decay phase after being cultured at 37°C for 17-20 hours after inoculation. When bacteria enter the decay phase, the number of dead bacteria increases significantly, and physiological metabolic activities tend to stagnate.

There are two main aspects of bacteria entering the decay stage: on the one hand, there are genes that control aging inside biological cells, and the same is true for bacteria. When these genes reach a certain level, they are activated, and eventually the bacteria age and die.

On the other hand, some bacteria, like eukaryotes, also have programmed cell death (PCD). Under the induction of environmental pressure, bacteria undergo stress adaptation, leading to functional differentiation, death or dormancy of cells. For example, adverse conditions such as starvation can cause some cells to die.

Bacterial growth curve

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From this we can see that the number of organisms is not simply increasing, but is presenting an approximate state of equilibrium under the constraints of external environmental factors and internal conditions. So there is no need to worry. Although bacteria reproduce very quickly, they will not occupy the earth due to various factors.

Editor: Wang Tingting

References:

Zhou Deqing. Microbiology Textbook. 3rd Edition[M]. Higher Education Press, 2011.

Madigan M. Brock Biology of Microorganisms, 13th edn[M]. Upper Saddle River, Nj. 2012.

Image: Self-made image elements from https://biorender.com/