Pasteur thought bacteria caused disease but was never able to isolate the bacteria. In 1882, Robert Koch announced his discovery of the tubercle bacilli, which caused an incredible stir. Koch also discovered the anthrax bacilli. The tubercle bacilli didn't stain as easily as other bacteria and Koch had to come up with a staining method that penetrated the cell walls of the bacteria. Koch invented usage of solid medium and petri dishes.
Pure cultures are now possible because of solid medium usage.
A petri dish could be filled with solid matter, and injected with different bacteria to grow distinct colonies of bacteria. Mixed samples would grow isolated colonies. This was key to isolating tubercle bacilli. Koch's methodology in identifying the bacteria is now known as Koch's postulates. Koch worked on a cure for TB in a lab the German government created for him.