Dutch-born self-taught scientist Antonie van Leeuwenhoek and pioneer of microscopy was the first to observe (and experiment with!) microbes. He originally referred to them as dierkens, diertgens or diertjes, dutch words for "small animals". They were translated to Animalcules in english. We also accepted "small animal(s)" as an answer!
So, Animalcule (from Latin animal + the diminutive suffix -culum) is an old term for microscopic organisms that included bacteria, protozoans, and very small animals. Today, the word is not used anymore, but it is a beautiful way to think about microorganisms!
A 1795 illustration of van Leeuwenhoek's animalcules by an unknown artist.