There isn't one single person who "discovered" the bacteria kingdom. It was a gradual process of understanding that involved many scientists over a long period of time.

Here's a breakdown of key contributions:

* Anton van Leeuwenhoek (1670s): He was the first to observe bacteria using a simple microscope, calling them "animalcules." He didn't fully understand their nature but his observations were crucial.

* Louis Pasteur (mid-1800s): His experiments disproved the theory of spontaneous generation and showed that bacteria were responsible for fermentation and disease.

* Robert Koch (late 1800s): He established the "Koch's Postulates," a set of criteria to prove that a specific microbe causes a specific disease. He also identified the bacteria responsible for anthrax and tuberculosis.

* Ferdinand Cohn (mid-1800s): He made significant contributions to bacteriology, including describing endospores, classifying bacteria based on their shape, and establishing the foundation for bacterial taxonomy.

Therefore, it's more accurate to say that the understanding of the bacteria kingdom evolved over time through the work of many scientists.