Every September 17, the International Day of Microorganisms is celebrated. Such a celebration seems contradictory in the midst of a pandemic, as if something good could come out of these living beings. Germs cause us illness and even death, but let's not let the tree blind us to the forest: the vast majority of microorganisms are good people. What's more, they are necessary for our survival and that of all ecosystems on the planet, so they deserve to be celebrated.

But why September 17?

September 17, 1683 probably dawned cold and rainy in the small Dutch town of Delft, famous for its canals. Anton van Leeuwenhoek, first a cloth merchant and then a municipal employee , without any scientific training , decided that day to send a letter that would change the course of science. In that letter, addressed to the Royal Society of London, he described for the first time microorganisms, apparently simple microscopic life forms that he called "animalcules".

Leeuwenhoek was fond of constructing small magnifying glasses that merchants used to analyze the quality of fabrics. He polished his own biconvex lenses which he fixed between two sheets of brass and held very close to the eye. The samples were placed on a kind of pin, which could be moved closer or further away from the lens for focusing by means of screws. He had such a skill for polishing lenses that his magnifiers could reach more than 250 magnification and a resolving power (ability to differentiate between two closely spaced points) of 1.5 microns. This is almost the resolution of a modern optical microscope. He was, therefore, the first person to observe bacteria and other microorganisms.

Leeuwenhoek did not actually invent the microscope. It was probably another Dutchman, Zacharias Janssen (1588-1638), who built the first one, composed of two lenses. This consisted of a simple tube about 25 cm long and 9 cm wide with a convex lens at each end.

The Englishman Robert Hooke (1635-1703), a contemporary of Leeuwenhoek, published in 1665 the book Micrographia, in which he described the observations he had made with a microscope similar to Janssen's own design. This book contains for the first time the word "cell". Hooke discovered them by observing under his microscope a sheet of cork, realizing that it was formed by small polyhedral cavities that resembled the cells of a honeycomb.

However, those compound microscopes were only a magnifying glass capable of a few magnifications. Neither Janssen nor Hooke were able to observe what Leeuwenhoek would later describe using a single lens.

The world of microorganisms was hidden from science until Leeuwenhoek decided to focus his microscope beyond the tissues and fabrics of his trade.

Leeuwenhoek was the first to see red blood cells and spermatozoa. His drawings of bacteria published in 1684 are of excellent quality and allow us to recognize several common types of bacteria and their groupings: bacilli, cocci, etc.

He was very jealous of his microscopes. He did not share with anyone his way of polishing or carving the lenses and left no indication of his manufacturing methods. He destroyed most of his creations, of which only a dozen remain today. One of them is on display until December 8 at the National Museum of Natural Sciences in Madrid, on the occasion of the exhibition "Exploring beyond the visible" organized by the Spanish Society of Microbiology on the occasion of its 75th anniversary.

The first inhabitants of the planet

It took science almost two hundred years to redevelop a technique equivalent to Leeuwenhoek's. His observations demonstrated three characteristics of the microbial world: that it is composed of very small beings, that they are everywhere, and that they are very diverse. His observations demonstrated three characteristics of the microbial world: that it is composed of very small beings, that they are everywhere, and that they are very diverse.

Microorganisms have played and continue to play an essential role in our ecosystems. It is estimated that about 3.8 billion years ago life emerged on Earth. Since then, until the appearance of the first multicellular living beings some 900 million years ago, the planet has only been inhabited by microscopic beings. Bacteria, archaea, viruses and more complex but unicellular microorganisms.

This means that, for about 2.9 billion years, they have been the only inhabitants of the planet. They have preceded us and will most likely still be here when our species disappears.

They have been responsible for major changes at the planetary level: until the appearance of cyanobacteria (a subject of microorganisms that carry out oxygen-generating photosynthesis) some 2.8 billion years ago, the Earth was an anaerobic environment. Atmospheric oxygen is an invention of microorganisms. Therefore, not only has life on Earth been and will be fundamentally microbial, but the more complex beings, plants and animals, have evolved from microbial ancestors in a biosphere modified and conditioned by their activity.

When we talk about biodiversity conservation on the planet, we must not forget that the bulk of biodiversity on Earth is invisible. These tiny life forms have come to colonize virtually all terrestrial ecosystems and are able to survive the most extreme conditions. Even where at first glance life is impossible: Geogemma barossi is able to survive at 121 ⁰C in deep-sea hydrothermal vents. The bacterium Psychromonas ingrahamii is isolated from polar environments and grows at temperatures of -12 ⁰C. Picrophilus oshimae was isolated from volcanic vents at an acidic pH of 0.7. Halobacterium salinarum is isolated for example from the Dead Sea at saturating salt concentrations, incompatible with other life forms.

They are everywhere! Microscopic fungi and bacteria have been isolated in high layers of the atmosphere, more than 15 km high. They are found in the deep sea at depths of more than 10 000 meters and even several hundred meters below the earth's surface.

Ninety percent of marine biomass is microbial and they are responsible for half of the CO₂ fixed and half of the O₂ produced. Therefore, microorganisms can also influence climate change and vice versa: changes in temperature and humidity can alter the biology of these living beings and, in turn, that can change habitat conditions.

The soil we walk on, without going any further, is one of the most complex ecosystems. It is estimated that one gram of soil can contain more than 10 billion microorganisms, more than there are human beings on the planet. They are responsible for fill in all the biogeochemical cycles of the subject. For example, they fix atmospheric nitrogen (in symbiosis with leguminous or free-living plants in the soil) and transform it into ammonium, nitrite and nitrate. Without microorganisms, the nitrogen cycle, essential for life as we know it, would not exist.

Although it sounds drastic, it is very likely that the extinction of the emperor penguin (although an incalculable loss) would not mean the collapse of the planet, but the extinction of bacteria such as Nitrosomonas or Nitrobacter, which are involved in the nitrogen cycle, would mean the immediate collapse of life. In essence, without microorganisms the macroscopic life that we see with the naked eye, our own life, would not be possible.

Half human, half microbe

Today, new metagenomics (massive sequencing) techniques, which surpass traditional culture methods, allow us to verify the enormous microbial biodiversity hidden in nature.

Scientists know in some detail the biology of far less than 1% of the microorganisms that actually exist. The habitat of many of them is the surface or the interior of other living beings. This is what we know as the microbiota of plants, animals or humans. We ourselves are half human, half microorganism: for each of our human cells, we have at least one microbial cell. They are on our skin and in all our mucous membranes: in the mouth, in the intestines, in the vagina, in the respiratory tract, etc.

We are a walking collection of microbial ecosystems in which a multitude of interactions occur between our cells and microorganisms. The balance of these ecosystems is essential for our health. These tiny beings prevent colonization of our skin and mucous membranes by other pathogenic microorganisms. These outsiders must resort to complex virulence mechanisms to impose themselves in an environment that is well defended by the colonists.

The microorganisms that make up our microbiota help maintain the intestinal barrier and contribute to digestion by degrading bile salts, proteins and polysaccharides. They also modulate and train our immune system, regulate inflammatory processes, synthesize vitamins and other compounds necessary for our health, degrade drugs and toxins or produce neurotransmitters and hormones.

When the balance between our microbes and our organism is disturbed (dysbiosis), pathologies can occur.

Dental caries and periodontitis are direct examples of "diplomatic problems" with our microbiota, but recently it has been described the relationship of multiple pathologies with an alteration of our microbiota: from obesity, diabetes, allergies, asthma, inflammatory diseases, to depression, Alzheimer's and even autism.

Thus, 21st century medicine has a new system in the human body that is essential for health: the microbiota. Similarly, when we study the function of the human genome, we should not overlook the fact that the system is completed by the microbiome, the set of genes encoded in the genomes of the hundreds of microbial species that are part of us. Microbial ecology enters into the equation of our well-being and biomedicine faces new challenges.

Yeast, yogurt, cheese and PCRs

If you still think these reasons are not enough, let us add that thanks to microorganisms our life is easier and even more pleasant. Saccharomyces, the yeast used ancestrally in food fermentation, is a unicellular fungus thanks to which we have bread, beer and wine on the table.

Dairy products such as yogurt and cheese are the result of bacterial fermentation. Fermented foods and beverages, antibiotics, enzymes, vitamins, hormones, amino acids (additives, sweeteners, antioxidants...) are products of the metabolism of microorganisms. Biotechnology also relies on them for green energy production, pest control, animal welfare and decontamination.

The famous PCR technique that has been an essential element during the pandemic is possible thanks to a thermostable enzyme obtained from Thermus aquaticus, one of those bacteria capable of surviving at very high temperatures.

subject Today we are able to modify microorganisms at laboratory to manufacture all kinds of drugs or essential products in biomedicine and biotechnology. We can use them to develop transgenic plants capable of resisting drought, to produce biofuels, to degrade polluting compounds, and even to use them as vaccines to control a pandemic.

In that famous letter of September 17, 1683, there was an exquisite description of the first observation of live bacteria present in dental plaque, accompanied by drawings of the microorganisms observed and their movements. That day began a new era for science that would take two centuries, already in the time of Louis Pasteur and Robert Koch, to develop as a scientific discipline , and allows us to know and study the world of microorganisms, which has so much influence on our planet. We live in a microbial world.

Celebrate with us the International Day of Microorganisms.

This article was originally published in The Conversation. Read the original.