With his superior light adjusting techniques, Leeuwenhoek was able to construct microscopes which could magnify over 200 times, enabling him to perceive small "animalcules" or bacteria in water.
Furthermore, in a letter, he reports how he had discovered in the plaque of two old men who had never brushed their teeth in their lives “an unbelievably great company of living animalcules, a-swimming more nimbly than any I had ever seen up to this time. The biggest sort. . . bent their body into curves in going forwards. . . Moreover, the other animalcules were in such enormous numbers, that all the water. . . seemed to be alive.”