Valid application of Koch’s postulates involves several critical details Each culture must be pure, observed microscopically, and identified by means of characteristic tests; the first and second isolate must be identical; and the pathologic effects, signs,and symptoms of the disease in the first and second subject must be the same. Once established, these postulates were rapidly put to the test. Within a short time, they had helped determine the causative agents of tuberculosis, diphtheria,and plague. Today,most infectious diseases have been directly linked to a known agent.
Many infectious agents such as viruses and obligately parasitic bacteria can be difficult to culture or cannot be cultured. For these pathogens, the postulates have been modified to take this into account. It is now considered significant to repeatedly observe the infectious agent in tissue samples taken from people with the same disease. Likewise, the presence of specific genetic profiles or genes for virulence are viewed as highly valid evidence.