Leeuwenhoek Discovers Microorganisms (1676): The Animalcules

In October 1676 Antonie van Leeuwenhoek, a cloth merchant in Delft in the Dutch Republic, sent the Royal Society in London a long letter describing “little animals” swimming in drops of water. Seen through his home-made single-lens microscopes, they were thousands of times smaller than anything visible to the naked eye. The letter is the first detailed record of microorganisms, including protists and bacteria, and it marks the beginning of microbiology.

Background: a draper with a lens

Antonie van Leeuwenhoek was born in Delft on 24 October 1632. He trained in the cloth trade in Amsterdam and returned to Delft around 1654 to run his own drapery business. Cloth merchants used magnifying glasses to count threads, and this may have sparked his interest in lenses. He also held municipal posts, including chamberlain of the sheriffs’ assembly chamber, which gave him a steady income.

He visited England in 1668, and it is often suggested that he saw Robert Hooke’s Micrographia, the famous illustrated book of microscopic observations. From the early 1670s he began making his own microscopes and examining everything he could find.

His microscopes

Leeuwenhoek’s instruments were very different from the large compound microscopes of Hooke. Each consisted of a single tiny lens, often less than a few millimetres across, mounted between two small metal plates of brass or silver. The specimen was placed on a pin in front of the lens and moved with screws; the user held the device close to the eye and looked towards the light.

Grinding and polishing such small lenses with high precision was extremely difficult. He made hundreds of lenses over his lifetime, and about a dozen of his microscopes are known to survive. The best of them magnify about 270 times. He kept his techniques secret, especially his methods of lighting specimens, and historians still debate how he achieved some observations.

First contact with the Royal Society

In 1673 the Delft physician Regnier de Graaf wrote to Henry Oldenburg, secretary of the Royal Society, praising Leeuwenhoek’s microscopes. Oldenburg invited him to send his observations. Leeuwenhoek knew no Latin or English and wrote in Dutch, so his letters were translated, often abridged, and published in the society’s Philosophical Transactions.

His first letters described mould, the sting of a bee and the mouthparts of a louse. They were careful and lively, but not yet revolutionary.

The letter of 9 October 1676

In 1674 Leeuwenhoek examined water from a lake near Delft and noticed tiny moving creatures among green streaks, probably the alga Spirogyra and protists. Over the following two years he studied rainwater collected in a tub, well water, sea water, and infusions of pepper in water, which he made while trying to find out why pepper tastes hot.

His letter of 9 October 1676, sometimes called Letter 18, described these observations at length. He found creatures of many shapes and sizes moving in the water, some with what looked like little feet or hairs, others smaller still. He estimated their size by comparing them with a grain of sand or the eye of a louse, and suggested that some were more than a thousand times smaller than a sand grain. Many of the larger forms were protozoa; the smallest, in the pepper water, were very likely bacteria.

Oldenburg published an English translation of much of the letter in the Philosophical Transactions in 1677. It was the first time anyone had described the vast world of microscopic life.

Doubt and confirmation

Fellows of the Royal Society were impressed but uncertain. The creatures were so small, and Leeuwenhoek’s claim that a single drop could hold many thousands of them so extraordinary, that some suspected an error. Leeuwenhoek sent statements from Delft notaries and clergymen who had looked through his lenses and confirmed what he saw.

Robert Hooke, the society’s Curator of Experiments, tried to reproduce the observations. After early failures he succeeded, and in November 1677 he showed the animalcules in pepper water to Fellows of the society. Leeuwenhoek was elected a Fellow of the Royal Society in 1680, an unusual honour for a tradesman who had never attended university.

Later discoveries

Leeuwenhoek continued working for nearly fifty more years. In 1677 he and the medical student Johan Ham described spermatozoa. In a letter of 17 September 1683 he described and drew several kinds of bacteria scraped from the plaque on his own teeth and from the mouths of others, including an old man who had never cleaned his teeth.

He also described red blood cells, blood flowing through capillaries in a tadpole’s tail, the life cycle of fleas and ants, and the structure of muscle fibres. His observations helped undermine the old belief that small creatures such as fleas arose by spontaneous generation from dirt. In total he wrote around 190 letters to the Royal Society between 1673 and 1723, now preserved in its library in London.

Fame and legacy

By the 1680s Leeuwenhoek had become one of the sights of Delft. Scholars, diplomats and royalty came to look through his lenses; visitors included Queen Mary II of England and, in 1698, Tsar Peter the Great of Russia during his journey through Western Europe. Leeuwenhoek was wary of visitors and rarely let anyone use his best instruments, but he showed them selected specimens.

He left no students and no school. On his death he bequeathed a cabinet of 26 silver microscopes to the Royal Society, but these were later lost. Because he guarded his methods, his observations were difficult to repeat, and for over a century few scientists matched them. Only after improvements in lens design in the 19th century did microbiology become an organised science.

Modern researchers who have tested surviving Leeuwenhoek microscopes have confirmed that they can show bacteria and other structures he described. His letters, with their careful measurements and detailed drawings made by hired illustrators, are now regarded as a model of patient observation.

Key people

• Antonie van Leeuwenhoek (1632–1723): Delft draper, civic official and self-taught microscopist.
• Regnier de Graaf (1641–1673): Delft physician who introduced him to the Royal Society.
• Henry Oldenburg (c. 1619–1677): Royal Society secretary who translated and published his letters.
• Robert Hooke (1635–1703): confirmed the observations before the society in 1677.

Myths and facts

• Myth: Leeuwenhoek invented the microscope. Microscopes had existed for decades; his innovation was a superior single-lens design and the skill to use it.
• Myth: Leeuwenhoek understood that microbes cause disease. He did not link his animalcules to illness. The germ theory of disease was established only in the 19th century by Louis Pasteur, Robert Koch and others.
• Fact: Leeuwenhoek and the painter Johannes Vermeer were both born in Delft in October 1632, and Leeuwenhoek acted as trustee of Vermeer’s estate after the painter’s death in 1675. Claims that he was Vermeer’s model are speculation.

Timeline

• 1632, 24 October: Antonie van Leeuwenhoek born in Delft.
• 1673: Regnier de Graaf introduces him to the Royal Society.
• 1674: He first sees tiny creatures in lake water near Delft.
• 1676, 9 October: Letter describing animalcules in rainwater and pepper water.
• 1677: Letter published in the Philosophical Transactions; Hooke confirms the observations in November.
• 1680: Leeuwenhoek elected a Fellow of the Royal Society.
• 1683, 17 September: Letter describing bacteria from dental plaque.
• 1723, 26 August: Leeuwenhoek dies in Delft, aged 90.

Why it matters

Leeuwenhoek revealed that the world is full of living things too small to see, a discovery that underpins microbiology, medicine and much of modern biology. His patient, careful observations, much like Galileo’s work with the telescope, showed how a new instrument could transform what people knew about nature.