Anton Von Leeuwenhoek, in the early 1600s, saw these tiny microbes and called them “animalcules” and “wee beasties”.
I think it was either Robert Hook or Anton Van Leevwen Hoek
Antonie van Leeuwenhoek viewed moving objects from pond water under his microscope and named them “animalcules. Antonie van Leeuwenhoek was a scientist and the first microscopist to observe protozoa and bacteria. He created microscopes that possess a single high-quality lens of small focal length and isolated tiny animalcules (microbes) from various sources such as pond water, well water and rain water. Through his experiments, he also calculated the sizes of the animalcules.