Who first observed bacteria under a microscope?

The first person credited with observing bacteria under a microscope was Antonie van Leeuwenhoek. In the 1670s, the Dutch microscopist used powerful single-lens microscopes of his own design to view microorganisms that had previously been invisible to science.

Van Leeuwenhoek reported bacteria in 1676, describing the tiny moving forms as “animalcules.” He did not understand them as a distinct biological group in the modern sense; that classification came much later. Christian Gottfried Ehrenberg introduced the word bacterium in 1828.

His achievement depended on unusually effective handmade lenses. Surviving instruments can magnify roughly 275 times, and his observations included bacteria, protozoa, sperm cells, red blood cells, and muscle fibers. Robert Hooke was an important earlier microscopist who documented mold structures in 1665, but he is not generally credited with the first observation of bacteria. Van Leeuwenhoek’s work helped launch microbiology.