The vinegar killed the bacteria it could reach

On September 17, 1683, Antoni van Leeuwenhoek wrote to the Royal Society about the white matter lodged between his teeth. The letter is often cited as the first unambiguous description of bacteria.

Leeuwenhoek introduced the specimen with a small boast. Every morning he rubbed his teeth with salt and rinsed with water. After meals he used a toothpick and vigorously rubbed his molars with a cloth. Few people his age, he thought, had teeth so clean and white.

Still, under magnification, he found matter “about as thick as batter.” When he scraped it out and mixed it with clean rainwater or saliva, tiny creatures shot about like pike, spun like tops or swarmed like gnats.

The scraping mattered. Leeuwenhoek had repeatedly examined saliva alone without finding animalcules. He got the same result from two women who cleaned their mouths daily and from an eight-year-old child. Their saliva appeared empty; material picked from between their teeth contained multitudes. The organisms were not conveniently distributed through the mouth. They were concentrated in something attached to its surfaces.

He then performed a neat vinegar experiment. Leeuwenhoek took strong wine vinegar into his mouth, clenched his teeth and forced the liquid between them several times. After rinsing three times with water, he scraped his teeth again. He still found an “incredible number” of living animalcules, especially in matter from between his molars.

But when he added a little vinegar directly to the diluted samples, the animalcules died at once.

Leeuwenhoek did not conclude that vinegar was ineffective. He concluded that it had failed to penetrate the matter firmly lodged around his teeth. It killed the creatures in the outermost parts and left those farther in alive.

The surviving manuscript records an early sighting of bacteria, but also a distinction that the sighting made possible. A substance can kill an organism perfectly when mixed with it and still fail as a rinse. On the glass, every animalcule met the vinegar. In the mouth, the plaque had depth.