Which of the following is associated with Antonie van Leeuwenhoek which became a cornerstone in microbiology?

Antonie van Leeuwenhoek first observed and repoed bacteria. Using his handcrafted microscopes, he was the first to observe and describe single-celled organisms, which he originally referred to as animalcules, and which we now refer to as microorganisms. Ref: Textbook of Microbiology by Ananthanarayan, 6th edition, Page 1.