Antonie van Leeuwenhoek

Leeuwenhoek observes and reports bacteria-like microorganisms in water infusions

To make an almost unbelievable claim credible, Leeuwenhoek recruited eight ministers, lawyers and other reputable Delft men to sign testimonials that they had seen it too.

Six months of pepper water

From April through September 1676, in Delft, Antonie van Leeuwenhoek repeatedly examined rain, well, sea and river water and infusions of pepper and other spices. He observed several kinds of moving microorganisms, including forms now identifiable as bacteria.

There was no single discovery day. The observations accumulated across those months and were then reported in one letter.

Letter L-040

On 9 October 1676 he sent Henry Oldenburg of the Royal Society an eighteen-folio Dutch letter containing 145 observations from 19 series of liquids, describing the organisms' abundance, dimensions, shapes and motion.

It describes bacteria-sized organisms, some around one micrometre long, though neither the biological category nor the word bacteria yet existed. For one drop of pepper-water he offered a conservative estimate of more than a million living creatures, and later said the true figure might be eight times greater.

What 1676 is and is not the year of

The year is defensible for his first documented observations of bacteria, but not for microorganisms in general: he had already reported protozoa-like animalcules in water in 1674, and Hooke had published microscopic observations of mould in Micrographia in 1665.

The Royal Society sent no delegation to Delft. Leeuwenhoek gathered the eight local witnesses himself, and Hooke later reproduced the pepper-water observations in London. Translation and scrutiny took months: an excerpt appeared in Philosophical Transactions on 25 March 1677.

Sources

Researched 23 Aug 2026 5 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree about when this happened.

What the sources leave uncertain

The year 1676 is defensible for Leeuwenhoek's first documented observations of bacteria, but not for “microorganisms” generally: he had already reported protozoa-like animalcules in water in 1674.
There was no single discovery day. The relevant observations accumulated between April and September, then were reported in the letter dated 9 October 1676.
Priority language requires care: Robert Hooke had published microscopic observations of mould in Micrographia in 1665; Leeuwenhoek supplied the first detailed descriptions of living protists and bacteria.