Slow growth makes culturing a lengthy process and colony formation can take several weeks. TB bacilli form clumps, making them difficult to work with and to count. Because this is a dangerous airborne pathogen, their study requires special safety equipment. The bacteria's primary host is the human. When an infectious person talks, coughs, sings, or spits, tiny aerosolized droplets containing a few bacteria are spewed into the air and inhaled into the lungs of an uninfected person.
Viable bacteria can remain in the air for a long time.
The bacteria replicate within the macrophages for 2 to 3 weeks before spreading throughout the body. However, the bacteria are not completely killed and can remain dormant for years. Granulomas prevent spread of infection by confining bacteria within a compact collection of several types of immune cells and activated macrophages, some of which fuse together. These cells work in various but specific ways to isolate, inhibit the replication of, and destroy the bacteria. At the center of this aggregate, toxic substances released by some of the immune cells create an unfavorable environment for the bacteria and most of them die.