The person credited with first discovering bacteria is Dutch tradesman and scientist Antony van Leeuwenhoek. He greatly improved the microscope, discovered protozoa (which he called animalcules) and set the very foundations of microbiology.
Antony van Leeuwenhoek, born in 1632, defied the odds with his discoveries, as he was relatively untrained in the sciences. Despite his lack of a formal education, he was the first person to observe and record single-cell organisms, bacteria, muscle fibers and spermatozoa. Because of his work, other scientists have built upon his foundation, contributing greatly to the fields of modern medicine and biology.