The Classification and Identification
of Bacteria of Medical Importance

The mycobacteria are a group of bacteria that are classified with other Gram-positive bacteria on the basis of their cellular architecture, but they possess a very waxy cell wall, and they rarely stain using conventional protocols such as the Gram stain. They require special staining techniques in order to be observed easily under the microscope. In the Ziehl Neelsen technique, a strong solution of carbol fuchsin is applied to the microscope slide which is then heated and held near boiling point for at least five minutes. This is to allow the stain to penetrate the mycobacterial cells. Following this treatment the stain will remain in the cell, even when the slide is treated with a mixture of inorganic acid and alcohol. To visualise any other material on the slide, methylene blue is applied as a counterstain. Because mycobacteria resist decolourisation with acids and alcohol they are sometimes called acid alcohol-fast bacilli. Important examples include Mycobacterium tuberculosis and Mycobacterium leprae . The former causes tuberculosis, and the latter is the cause of leprosy. It takes 6-8 weeks to grow Mycobacterium tuberculosis in artificial culture, and as yet Mycobacterium leprae cannot be grown in artificial culture.