Every year on September 17, International Microorganism Day draws attention to microorganisms, whose biomass outweighs that of all the elephants on Earth. The day challenges the idea that microbes are merely threats to human health and highlights how they sustain life. They also advance medicine and support ecosystems.
The event began in Portugal and has been coordinated by the Federation of European Microbiological Societies (FEMS) since 2020. By 2026, it had grown, with new public events and announcements in several countries, reflecting its wider reach and recognition among scientists.
Microbes are everywhere, and their numbers are hard to grasp. In the oceans, they make up between half and nearly all of the total biomass. The mass of microbial cells in the world’s waters is about the same as that of 240 billion African elephants. A single gram of surface soil can hold more than a billion bacterial and archaeal cells, along with trillions of viruses. It can also contain tens of thousands of protozoa and microalgae, plus 200 meters of fungal hyphae. This hidden world is far larger than all visible life combined.
Still, most people think of microorganisms mainly as sources of disease. The picture is much broader. Of the estimated one trillion microbial species, only about 1,400, including bacteria, viruses, fungi, protozoa, and helminths, are known to cause disease in humans. That’s less than 1% of all known species. The rest are either harmless or helpful.
The human body is home to about 40 trillion bacterial cells, which outnumber human cells. This relationship is impressive. Microbes help digest fiber and make vitamins, while also training the immune system, among other jobs. Without them, basic processes that keep us alive would break down.
Medicine and pharmacology rely heavily on these tiny organisms. Microbes are a basic source of therapeutic molecules. Studies of the gut microbiome are opening new ways to treat chronic inflammatory diseases and metabolic disorders, as well as some cancers. Viruses that target bacteria are also being reconsidered as a tool against antibiotic-resistant infections.
Microorganisms are also basic to global food systems. Soil microbes are critical for agriculture and food production. Beyond the fields, they are used to make bread, cheese, fermented drinks, yogurt, vitamins, enzymes, bioplastics, and biofuels. Their influence reaches into almost every part of daily life and industry.
On a global scale, microbes help fight climate change and support the circular economy. Their ability to recycle nutrients and break down waste is basic to reducing environmental impact and using resources more efficiently. Research published in Nature in 2026 describes how marine bacteria work together to break down complex carbohydrates from brown algae, showing their practical role in the carbon cycle and ocean ecosystems. The research highlights the importance of marine microbes in global nutrient recycling and climate regulation.
The story of microorganisms in science began with a letter. On September 17, 1683, Antonie van Leeuwenhoek, a Dutch merchant, reported his observations of microorganisms in dental plaque to the Royal Society of London. In the letter, he described the first observed unicellular organism, what he called an “animálculo,” using microscopes he had built himself. His work laid the foundation for modern microbiology and changed how people understood the living world.
Recognizing the constant work and value of microorganisms means shifting from fear to respect for the invisible engines that keep life running on Earth. As research uncovers more about their influence, it’s clear that the future of medicine and agriculture, along with environmental care, will depend on how well we understand and use the power of these microscopic giants.
