Final answer:
Antonie van Leeuwenhoek was the first to observe bacteria, which he called "animalcules," in pond water using his homemade microscopes.
Step-by-step explanation:
Antonie van Leeuwenhoek is the scientist who was the first to observe "animalcules" under the microscope. These "animalcules" were what we now refer to as bacteria. Leeuwenhoek discovered bacteria in several places including pond water, which is noted for the presence of many microscopic organisms. This finding was pivotal in the development of microbiology. Not only did he find bacteria in pond water, but he also scraped plaque from his own teeth and observed it under the microscope, identifying bacteria in dental plaque as well.