Mycobacterium smegmatis is a species of bacteria from the Mycobacterium genus. First identified in 1884, this non-pathogenic microbe is common in the environment and does not cause disease in healthy humans. It has become a subject of intense scientific interest primarily due to its relationship with more dangerous members of the same genus, such as the bacterium that causes tuberculosis.
Biological and Structural Features
Mycobacterium smegmatis is a rod-shaped bacterium measuring between 3.0 and 5.0 micrometers in length. Its defining feature is a complex cell wall. This multi-layered envelope is rich in long-chain fatty acids called mycolic acids, which create a waxy, hydrophobic coating that makes the cell surface impermeable to many substances.
This waxy coat is responsible for the bacterium’s ability to be “acid-fast.” During a procedure called the Ziehl-Neelsen stain, dye penetrates the mycolic acid layer with heat. The cell wall then holds the dye so strongly that it resists being decolorized by an acid wash. This property is a hallmark of the Mycobacterium genus, allowing for specific identification under a microscope.
The cell wall’s composition provides protection for the bacterium, acting as a physical barrier against certain antibiotics and disinfectants. This resilience is a survival mechanism. The cell envelope also contains glycopeptidolipids (GPLs), which are associated with sliding motility. This form of movement allows colonies to spread across surfaces without structures like flagella.
Growth and Habitat
M. smegmatis is notable for its rapid growth rate. It can double its population in just a few hours, forming visible colonies on a laboratory plate within 3 to 5 days. This speed is a significant distinction from many of its pathogenic relatives.
In nature, M. smegmatis is a saprophyte, feeding on decaying organic matter. It is found in various environmental niches, including soil, dust, and water sources. Its ability to thrive in diverse settings is partly due to its capacity to form biofilms, which are structured communities of bacterial cells that adhere to a surface.
Biofilm formation offers a collective defense mechanism for the bacterial population. By encasing themselves in a self-produced matrix, the bacteria gain increased resistance to environmental stressors. This communal living arrangement allows them to persist in otherwise hostile environments.
Interaction with the Human Body
For healthy individuals, M. smegmatis is considered non-pathogenic. It can exist as a commensal organism, meaning it lives on the human body without causing negative effects. It is sometimes found as part of the skin’s microbiota, particularly in moist areas, where its presence does not lead to illness.
In rare instances, M. smegmatis can act as an opportunistic pathogen. This occurs almost exclusively in individuals with severely compromised immune systems, such as those with advanced HIV/AIDS or patients undergoing chemotherapy. Infections can also follow traumatic injuries where bacteria from the environment are introduced deep into tissues.
Significance in Scientific Research
M. smegmatis serves as a model organism for studying the pathogenic M. tuberculosis. Because it is non-pathogenic, researchers can use it as a safe substitute to investigate the biology of tuberculosis without requiring high-level biosafety precautions. Its rapid growth rate is another advantage; it forms colonies in days, a stark contrast to M. tuberculosis, which can take weeks to cultivate.
This bacterium shares a large portion of its genetic material with M. tuberculosis, with some estimates suggesting a 75% similarity. Many genes required for the survival of M. tuberculosis have functional equivalents in M. smegmatis. This genetic overlap allows scientists to study the functions of these genes in a safer context to understand how the pathogen operates.
M. smegmatis is frequently used in the early stages of drug development for tuberculosis. New compounds can be tested on it to determine if they can penetrate the mycobacterial cell wall and inhibit growth. The development of a specific strain, mc²155, which is easy to work with genetically, has advanced mycobacterial research on topics ranging from antibiotic resistance to fundamental life processes.