Which organism did Koch identify as the cause of tuberculosis?

• A. Mycobacterium leprae
• B. Mycobacterium tuberculosis
• C. Bacillus anthracis
• D. Streptococcus pneumoniae

Answer: (B)

The question tests the identification of the organism that Koch tied to a specific disease, demonstrating how a single microbe can cause a particular illness. Koch showed that tuberculosis is caused by a distinct bacterium, Mycobacterium tuberculosis. He visualized acid-fast bacilli in tuberculous tissue using staining, then demonstrated that the same organism could be isolated and, when introduced into a susceptible host, produce TB. This body of work, aligning with his criteria for establishing causation, established Mycobacterium tuberculosis as the cause of tuberculosis. The organism is an acid-fast, slow-growing rod with a waxy cell wall rich in mycolic acids, characteristics that help explain both its pathology—forming granulomas in the lungs—and its challenging, slow culture. The other listed organisms cause different diseases—leprosy, anthrax, and pneumococcal disease—so they are not the etiologic agent of TB.