17 September 2026 marks 343 years since a letter from Antonie van Leeuwenhoek to Francis Aston, secretary of the Royal Society in London. In the document dated 17 September 1683, the Delft tradesman and municipal official described microscopic organisms seen in the whitish matter on teeth, examined with simple microscopes he built himself.

The Royal Society archives hold the signed autograph (EL/L1/69). An English extract appeared in Philosophical Transactions (vol. 14, no. 159, 1684). Leeuwenhoek was not a physician and did not use modern bacterial vocabulary; he wrote of “animalcules” and the “scurf of the teeth”. The letter is often cited as one of the first unambiguous descriptions of microorganisms in dental material, not as the invention of microbiology on a single day.

He had already studied microorganisms in water and other samples. The 1683 letter belongs to a series of communications to the Royal Society, not to a nineteenth-century laboratory treatise. The original drawings attached to the letter have not survived.

At a Society meeting the letter was read as an account of several kinds of organisms found in material from the teeth, mixed with saliva or rainwater. Leeuwenhoek noted that some died when vinegar was added. Scientific classification of bacteria developed much later.

His instruments were simple lenses with high magnification for the period, but far from modern microscopes. The document’s importance lies in the detail of the observation and in its transmission to a scientific institution, not in a sudden discovery of the entire microbial world.

Three hundred and forty-three years later, the letter of 17 September 1683 remains a landmark in the history of microscopy and microbiology, provided it is described accurately: a documented observation of tiny organisms in dental material, not the instantaneous invention of the discipline.

Image: Jan Verkolje, portrait of Anthonie van Leeuwenhoek, c. 1680–1686, Rijksmuseum. Public domain. Cropped to 16:9.

Source consulted: Royal Society — Letter from Antoni van Leeuwenhoek to Francis Aston, 17 September 1683 (EL/L1/69); Philosophical Transactions — abstract of the 17 September 1683 letter (DOI 10.1098/rstl.1684.0030).