Mycobacterium tuberculosis is the bacterium responsible for tuberculosis, a disease that affects millions globally. Understanding its physical attributes provides insight into how this organism functions and persists within its environment and a host.
Its Rod-Like Form
Mycobacterium tuberculosis is characterized by its rod-like or bacillary shape. This means individual cells are elongated and cylindrical. These bacilli are typically slender and can be slightly curved. They are relatively small, generally measuring about 0.2 to 0.5 micrometers in width and approximately 2 to 4 micrometers in length. The bacterium is also non-motile, meaning it lacks structures like flagella that would allow it to move independently.
How Cells Group
The arrangement of Mycobacterium tuberculosis cells is generally less organized than some other bacteria. These cells are commonly observed as single units or in small, irregular clumps. A notable form of aggregation, however, is “cording,” where cells appear wrapped or tangled together. This unique arrangement is attributed to the presence of fatty acids within their cell walls, which cause the individual bacilli to stick to each other.
Unique Cell Wall and Visualization
The distinctive shape and stability of Mycobacterium tuberculosis are largely due to its unique and complex cell wall. This cell wall is rich in lipids, making it waxy and hydrophobic. Mycolic acid is a major component contributing to this waxy nature. This specific composition gives the bacterium a property known as “acid-fastness.”
Acid-fastness means that once the bacterial cells are stained with certain dyes, they resist decolorization by acid-alcohol solutions. This property is fundamental for its identification in laboratory settings, primarily through the Ziehl-Neelsen staining method. In this technique, a primary stain, carbol fuchsin, is applied and penetrates the waxy cell wall, often with the aid of heat. Subsequently, when acid-alcohol is used, the mycolic acids in the cell wall prevent the stain from being washed out, causing the M. tuberculosis cells to retain the red color. Non-acid-fast bacteria, lacking this waxy protection, would be decolorized and then take up a blue counterstain, allowing clear differentiation under a microscope.
How Morphology Aids Survival
The rod shape and unique cell wall of Mycobacterium tuberculosis are crucial for its survival and ability to cause disease. The robust, waxy cell wall provides a protective barrier, allowing the bacterium to endure harsh environmental conditions, including desiccation. This lipid-rich composition also contributes to its inherent resistance against many disinfectants and a wide range of common antibiotics, making treatment challenging.
The cell wall also helps M. tuberculosis evade the host’s immune system. It can resist being effectively destroyed by immune cells, such as macrophages, after being engulfed. The components of this cell wall can even modulate the host’s immune responses, contributing to the bacterium’s persistence within the body.
Furthermore, the small, rod-like shape of M. tuberculosis facilitates its transmission. When an infected person coughs, speaks, or sneezes, the bacteria are expelled in tiny airborne droplets. These small particles can remain suspended in the air for extended periods and be easily inhaled deep into the lungs of another individual, initiating a new infection.