Final answer:
Antonie van Leeuwenhoek first observed microorganisms, including bacteria and protists, in 1675 using a microscope he had crafted, a breakthrough in the budding field of microbiology.
Step-by-step explanation:
The first microorganisms to be observed under a microscope were discovered by Antonie van Leeuwenhoek in 1675. Utilizing his own handcrafted simple microscope with exceptional magnifying capabilities, Leeuwenhoek observed single-celled organisms in a drop of rain water, which he affectionately termed "animalcules" or "wee little beasties."
These microorganisms, documented in a series of letters to the Royal Society of London, were later identified as bacteria and protists, marking a significant milestone in the field of microbiology and the study of life at a microscopic level.