The term 'animalcules' to oral microorganisms is given by:
Animalcule ('little animal', from Latin animal + the diminutive suffix -culum) is an old term for microscopic organisms that included bacteria, protozoans, and very small animals. 
The word was invented by Antonie van Leeuwenhoek to refer to the microorganisms he observed in rainwater.
W D Miller
Leeuwenhoek
Robertson
Socransky