What was the source of the microscopic "animalcules" described by leeuwenhoek?

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.”