Anton van Leeuwenhoek (1632-1723) was a Dutch tradesman and scientist, considered the first microbiologist. He is best known for his work on the improvement of the microscope and for his contributions towards the establishment of microbiology. 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. Oil painting by Dutch painter Jan Verkolje.