Bacteria were the first microorganisms observed with a microscope, in observations reported by Antonie van Leeuwenhoek in 1676.
Using hand-built microscopes with carefully made lenses, Leeuwenhoek examined water, dental plaque, and other samples. He described tiny moving organisms as animalcules. His reports provided some of the earliest direct evidence that an invisible microscopic world existed.
The modern word bacterium came later, and early microscopists did not understand the cellular distinctions now used to separate bacteria, archaea, and eukaryotic microbes. Leeuwenhoek’s observations therefore belong to the history of microscopy rather than to modern microbial classification.
A common mix-up is identifying Leeuwenhoek as the discoverer of bacteria in the sense of understanding their role in disease. That came much later, especially through nineteenth-century work by Louis Pasteur and Robert Koch. Leeuwenhoek primarily observed and described microorganisms.