September: The discovery of 'animalcules'

On 17th September 1683, Dutch scientist Antoni van Leeuwenhoek wrote to Francis Aston of the Royal Society relating his discovery of microscopic living 'animalcules' (live bacteria). His 6 volumes of collected letters form part of the RSC historical collection.

The observations he made were on the plaque between his own teeth which he described as:

"a little white matter about as thick as batter."

Using his microscope to view the bacteria, Leeuwenhoek reported on their activity:

"I then again and again saw to my great astonishment, that there were many very small animalcules in the said matter, which moved very prettily.  The big 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... often spun round like a top. these were far more in number."

He explains that greater numbers of bacteria appear to exist in the mouths of people with poor oral hygiene:

".especially in those who never clean their mouths, owing to which such a stench comes from the mouth of many that one can hardly bear talking to them."

In his letter, he makes further observations on the types and movements of these bacteria when he has mixed his saliva and other scrapings from his mouth and body with various different liquids such as pure water and vinegar.