Throughout Earth's history, microorganisms have existed long before humans appeared.
Perhaps that is why plants, animals, and humans could not survive without microorganisms.
Microbes influence not only plant growth but even human mood.
Recently, it has been gradually revealed that gut microbes influence the production of hormones that affect human emotions.
It is said that when microorganisms do not function properly, humans become emotionally sensitive and have difficulty controlling their emotions.
"Bacteria That Save, Bacteria That Kill, Bacteria That Help Each Other" is the second story about the interaction between microorganisms and plants and animals, told by Dr. Ryu Chung-min, who garnered the attention of many readers with his previous work, "Good Bacteria, Bad Bacteria, Strange Bacteria."

The main function of microorganisms on Earth is to decompose organic matter.
Since there are no microorganisms on Mars or the Moon, food thrown in the trash can stay there forever.
Perhaps it is a blessing that organic matter on Earth rots and creates a smell.
In the Earth's ecosystem, when organic matter decomposes and carbon dioxide is created, plants photosynthesize to create sugars.
When an animal eats this, microorganisms break it down in the animal's intestines.
When animals poop, microorganisms break it down again and create carbon dioxide again.
What would happen if we completely removed all the microbes from this process? We must not forget that microbes sustain much of life on Earth.

--- p.17~18

One of the experiments I focused on after returning to Korea after completing my PhD was that plants secrete a special substance from their roots to combat insect attacks, attracting beneficial microorganisms that can ward off insects.
These beneficial microorganisms attracted by plants either directly kill insects or boost the plant's immunity to ward off insect attacks.

--- p.75

Research has also reported that, rather than being caused by disease, environmental stress can cause microorganisms to exist in the soil that increase the resistance to environmental stress in the next generation.
The soil remembers exactly what stress the plants were under last year.
--- p.78

Plants release 30 percent of the glucose they produce through photosynthesis into the soil through their roots.
I still don't quite understand why I'm doing such a useless thing.
Plants are self-proclaimed philanthropists who simply sprinkle the food they have worked so hard to produce onto the soil.
Why has it been this way for so long? Most of the nutrient users are microorganisms within a millimeter of the roots.
So, within this 1 millimeter, microorganisms that are well adapted to plants have come to live together.

--- p.80

When plants are stressed and phenolic substances are released into the soil, only microorganisms that can overcome them will survive and continue to interact with the plants.
The reason plants choose these to grow near them is probably because it is advantageous for the plants.
Interactions can sometimes benefit only one party, but in many cases they are mutually beneficial.
So, symbiosis can be a great advantage for both organisms to survive.

--- p.83

It is still amazing to see how plants retain fragments of their memories through microorganisms in the soil.
It's as if I'm looking at an obelisk, carved into the soil by plants that have no written language to record history for their descendants.
If we think about the tens of thousands of years of memories stored beneath our feet as we walk on the ground, wouldn't the feeling of touching the ground be new?
--- p.84

Nature doesn't like one microbe to dominate.
If a phenomenon occurs in which one microorganism becomes dominant in our body, we will no longer be able to survive because of that microorganism.
Most pathogens follow this path.
Human immunity has a strong tendency to prevent a single microorganism from multiplying rapidly.
So, living things live in diversity, at an appropriate density, and in cooperation with each other.
The only exception on Earth is humans.

--- p.92

Plants always give off a scent.
If we approach a plant with a little sensitivity and bring our nose close, we can smell the plant's unique scent.
Tomatoes, which emit a variety of scents, are a representative example.

--- p.98

It is a well-known fact that when plants are attacked by insects, they emit a smell to warn neighboring plants and prepare them in advance.
The odor created at this time is called 'herbivore-induced plant volatile, HIPV'.
The significance of our study is that it shows that something similar can happen with microorganisms.
So we coined the term 'Microbe-induced plant volatile (MIPV)' and used it for the first time in our laboratory.
Now scientists doing similar research will use this word.

--- p.101

How do plants, unlike animals, perceive sound? They don't have eardrums or antennae.
And how can humans know if plants can hear? While we can tell if moving animals respond to sound by their movements, how can we know if immobile plants perceive sound?
--- p.106~107

If we could slow down the ripening of tomatoes with sound, consumers might perceive it as an environmentally friendly method.
Another advantage is that it doesn't cost any extra.
This is because Hertz only needs to produce a constant 'zing' mechanical sound.
All you need is a speaker.

--- p.110~111

We exposed Arabidopsis plants in a quiet box to continuous sound at 10 kilohertz and 90 decibels for three hours a day for ten days, and then watered the roots with pathogenic bacteria.
Usually, after 10 days, the plants begin to show symptoms of wilting, and after 20 days, they all dry up and die.
However, in the case of Arabidopsis plants that were given sound, even if they were sick, the symptoms were definitely less, and there were also healthy Arabidopsis plants with almost no disease symptoms.

--- p.115~116

So why do plants hear sound? In nature, pea roots can be observed growing in the direction of the sound of flowing groundwater.
Corn roots were also observed to respond to 100 to 300 Hz and move and grow in that direction.

--- p.116

Experiments have shown that plants produce sounds ranging from 20 to 2,000 kilohertz.
Water is important for plants to grow, but they make a special sound when there is no water.
The longer the days went by without watering, the louder and stronger the sound became.
Besides drought, it also makes a special sound when its leaves are cut with scissors or when it is infected with a virus.

--- p.118

Plants infected with phytoplasma exhibit characteristic symptoms.
Common symptoms include death, wilting, or yellowing of the plant, but the consequences of phytoplasma infection are quite different.
Rather, the leaves grow more lushly.
It also produces an excessive number of leaves that are pointed in shape (like pine needles) rather than the typical leaf shape.

--- p.135~136

Could it be that many aspects of our thoughts (free will) are being controlled by unknown entities? Shouldn't we also consider microbes, not just humans, as part of our species, and consider their role to be a part of our own actions and thoughts? This is because microbes likely existed long before humans and likely attempted to communicate their experiences to multicellular organisms in various ways.

--- p.137

Some causes of depression include toxins produced by the gut bacteria Clostridium.
So, if these bacteria are eliminated with antibiotics, depression improves.
Ultimately, the story is that one of the causes of mental illness is an imbalance in gut microbes.
--- p.139

The discovery of antibiotics by humans and the emergence of superbugs are only natural outcomes.
Since bacteria are also living things, they try to survive by changing their DNA through mutations (even though they have no bones) and making efforts to cut through bones.
So for now it looks like they're winning the fight against humans.
If bacteria win completely, humans will have no choice but to return to the pre-antibiotic era.

--- p.189

It was truly a stroke of luck that, thanks to my close observation of the unlikely event of the escape of the larvae of the wax moth, I discovered an enzyme that could decompose plastic and even developed the technology to apply it.
Above all, overcoming the bluebird syndrome, the belief that what happens in the lab is not separate from me but could be exactly what I'm looking for, played a big role in solving the problem.
In science, things that were thought to be mere coincidences are inevitably intertwined.

--- p.209

To confirm our findings, we performed additional experiments using vancomycin-killed bacteria isolated from the wax moth.
If metamorphosis accelerates when Gram-positive bacteria are absent, could forcibly introducing more Gram-positive enterococci slow the rate of metamorphosis? So, when Gram-positive bacteria isolated from the intestines were added, metamorphosis was delayed by 2-4 days.
Interestingly, introducing a bacillus (known as natto bacteria) into the gut of the wax moth, which had not been found in the gut, was able to delay metamorphosis similarly.

--- p.234

To find acne-causing bacteria inside masks, we decided to check the very basics one by one.
We looked into where bacteria like Staphylococcus aureus, which cause skin inflammation, live.
They reside deep in the skin's hair follicles, where they multiply and produce toxins that attack skin cells.
Skin cells then try to kill the bacteria by causing inflammation to solve the problem.
During this process, an inflammatory reaction occurs in the skin, which is called acne.
As a microbiologist, my first question was, 'Why are these bacteria hiding so deep under the skin?'

--- p.245~246