In 1674, a cloth merchant in the Dutch city of Delft did something no human being had ever done before. He pressed a tiny glass bead — a lens he had ground himself, by hand, in his back room — up to a drop of lake water and looked through it.
What he saw would change the course of human history.
Antonie van Leeuwenhoek was not a scientist. He had no university degree, no formal training in natural philosophy, no institutional backing. He was a draper — a seller of fabrics who had learned to grind lenses to inspect the thread count of his cloth. But Leeuwenhoek had an obsessive curiosity and an almost supernatural patience for detail, and when he turned that lens toward the natural world, he became the first human being to see life that had been invisible to every person who had ever lived before him.
He called them diertjes — "little animals." We call them microorganisms. And the world he cracked open in that back room in Delft is the same world Flore is exploring, 350 years later, one personalized microbiome at a time.
The Draper Who Saw the Invisible
Leeuwenhoek was born in Delft, Netherlands on October 24, 1632 — the same year Galileo was forced to recant his model of the solar system before the Inquisition. He never left the Netherlands. He worked his whole life as a fabric merchant, a city official, and an obsessive, tireless tinkerer with glass.
The microscopes of his era were crude things — novelties, really, capable of magnifying objects perhaps 20 or 30 times. Leeuwenhoek ground his lenses differently. He developed techniques he kept largely secret, polishing glass beads to a perfection that allowed magnifications of up to 270 times. No one would match his lenses for decades after his death.
He built over 500 microscopes during his lifetime. He died with 26 in his possession, which he bequeathed to the Royal Society of London — the scientific body he had corresponded with for over four decades, entirely by letter, in Dutch, because he never learned another language.
The First Microbiome: Scraped From His Own Teeth
In 1683, Leeuwenhoek did something remarkable. He scraped material from between his own teeth, mixed it with rainwater, and put it under his lens.
He found, to his astonishment, a bustling city of invisible life.
In a letter to the Royal Society dated September 17, 1683, he described what he saw with the precision of a man who had been looking at the world this closely for a decade:
"I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving. The biggest sort had a very strong and swift motion, and shot through the water (or spittle) like a pike does through the water. The second sort oft-times spun round like a top."
This was the first recorded observation of bacteria. The "animalcules" he described — by shape, by movement, by behavior — were different species of oral bacteria, living on and between human teeth. It was, without Leeuwenhoek knowing it, the first microbiome study.
He compared samples from different people — himself, an old man who had never cleaned his teeth ("a very little white matter, which is as thick as if 'twere batter"), a woman with clean teeth, a child. He found more animalcules in some mouths than others. He was, in effect, running the world's first microbiome diversity comparison.
The Gut: Humanity's First Glimpse Inside
Leeuwenhoek did not stop at the mouth. He examined samples from the intestines of frogs, horses, and sheep. He looked at his own stool. He observed that during a bout of diarrhea, the composition of what he found in his gut changed — fewer of some types of animalcules, more of others.
He did not have the language for what he was seeing. He did not know about the immune system, about inflammation, about the gut-brain axis. He did not know that the "animalcules" in his gut would outnumber his own human cells, that they would produce neurotransmitters, metabolize his food, train his defenses against disease. He knew only that they were there, that they were alive, that they were different from person to person and from moment to moment — and that this difference seemed to matter.
He was right.
The 350-Year Gap — and What Filled It
After Leeuwenhoek's death in 1723, microbiology advanced slowly. Louis Pasteur proved, two centuries later, that microorganisms caused fermentation and disease. Robert Koch isolated individual pathogens. For most of the 19th and 20th centuries, the dominant paradigm was germ theory: microorganisms were enemies to be killed.
The revolution came quietly. In the 1990s and 2000s, new sequencing technologies made it possible to identify microorganisms not by growing them in a culture — which most gut bacteria refuse to do — but by reading their DNA directly from a sample. Suddenly, the full scope of the gut microbiome became visible for the first time.
What the Human Microbiome Project (2007–2016) found was staggering:
- The average human gut harbors 100 trillion microorganisms from over 1,000 species
- The microbial genes in a human body outnumber human genes roughly 150 to 1
- No two people share the same microbiome — it is as individual as a fingerprint
- The gut microbiome influences immunity, metabolism, mood, cognition, skin, hormones, and cardiovascular health
- ~70% of the immune system is gut-associated; ~90% of the body's serotonin is produced in the gut
Leeuwenhoek's little animals, it turned out, were running much of the show.
From Diertjes to Precision Probiotics
The distance from Leeuwenhoek's lens to Flore's sequencing lab is not just technological — it is philosophical. Leeuwenhoek was the first person to take the invisible world seriously on its own terms: not as contamination, not as disease, but as life, with structure and variety and behavior worth documenting carefully.
That is exactly what Flore does. We sequence the microbial community living in your gut — every species, every relative abundance — and we use that data to build a probiotic formulation that is specific to you. Not a shelf product. Not a quiz-matched recommendation. A one-of-one capsule blend, compounded from your actual microbiome data.
In Leeuwenhoek's world, knowing the animalcules were there was the discovery. In ours, we can read their genomes, understand their functional roles, identify which species are missing or depleted, and supply targeted reinforcements — down to the strain level, because strain specificity is what separates a meaningful intervention from a generic supplement.
Over 40,000 unique formulations. 94.2% reported success rate. All of it built on a curiosity that a Dutch cloth merchant expressed through a glass bead in 1674: What is really living in here?
What Leeuwenhoek Would Think of Your Microbiome Report
Leeuwenhoek spent the last 50 years of his life writing letters to the Royal Society describing what he saw — 190 letters in total, painstakingly detailed, because he understood that accurate observation was everything. He described the shapes of his animalcules. He sketched them. He noted which ones moved in spirals, which ones darted, which ones tumbled end over end.
He never knew their names. He never knew their functions. He never knew that the "animalcules" he found in abundance in a healthy mouth were different species from the ones he found in a diseased one, and that this difference was the difference between protection and harm.
We know now. We know the names — Lactobacillus, Bifidobacterium, Akkermansia, Faecalibacterium prausnitzii. We know that F. prausnitzii is a primary producer of butyrate, the short-chain fatty acid that feeds the cells lining your colon and moderates gut inflammation. We know that low Akkermansia muciniphila is associated with metabolic dysfunction. We know that the ratio of Firmicutes to Bacteroidetes shifts with diet, stress, antibiotic use, and age.
Leeuwenhoek saw the world he opened for the first time and was, by his own account, struck with wonder. We hope, when you see your own microbiome data for the first time — a map of the invisible life that has been living in and with you your entire life — you feel something of the same thing.
Wonder is where science starts.
Ready to See Your Own Microbiome?
Antonie van Leeuwenhoek spent 50 years mapping invisible life with tools he built himself, driven by pure curiosity. You can see yours in three weeks with a home test kit — and get a personalized probiotic formulation built from what's actually living in your gut.
Leeuwenhoek would have found that remarkable. So do we.