Mycobacterium tuberculosis

Characteristics

          Mycobacterium tuberculosis is a rod-shaped (bacillus) bacterium that causes the disease tuberculosis in humans, as well as other primates, hamsters, dogs, and guinea pigs (Figure 1).  Since the organism is non-motile, it travels through the air on particles called droplet nuclei.  Droplet nuclei, which range in size from 1 to 5 μm, are introduced into the air when an infected person sneezes, coughs, etc.  Normal air currents keep the particles airborne so that they can spread throughout an area.  (See the “Pathogenesis” section for information on how an infection progresses.)  In addition to being non-motile, M. tuberculosis is an obligate aerobe, meaning that the bacterium can only survive in an environment that contains oxygen.  (See the “Pathogenesis” section to learn more about the environment in which M. tuberculosis lives.)
 

Although Mycobacterium tuberculosis possesses a Gram-positive type cell wall, a cell wall with extensive peptidoglycan and no outer membrane, the bacterium does not stain with Gram stain reagents. Gram stain reagents are unable to penetrate the cell wall of the bacillus because layers of lipids surround the peptidoglycan in mycobacteria.  Unlike most Gram-negative bacteria, which have a 5-20% lipid content by weight, M. tuberculosis and other mycobacteria are composed of up to 60% lipids.  Many of these lipids are in the form of mycolic acids.  (See the “Adaptations” section for more on the importance of a high lipid content.)

            Since the Gram stain method proves ineffective on Mycobacterium tuberculosis, acid-fast staining must be used to make the bacilli visible under a microscope.  In the Ziehl-Neelsen procedure, bacteria from a sputum (mucus coughed up from the lungs) sample are flooded with a basic solution of carbolfuchsin, a magenta dye.  After heating the slide in a flame, the sample is washed with water and treated with acid-alcohol to decolorize the bacteria.  Then, a counterstain of methylene blue is applied to the sample.  When the process is complete, M. tuberculosis bacilli appear pink because they retain the carbolfuchsin during the acid-alcohol decolorization step (Figure 2).  Thus, mycobacteria are classified as acid-fast bacilli.  Bacteria that are not acid-fast appear blue after the procedure.

Figure 2.  Mycobacterium tuberculosis bacilli stained using the Ziehl-Neelsen method.