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Mycobacterium smegmatis is a common microorganism which, for a number of reasons, has become one of the most important bacteria for biological study. It is easy to culture and reproduces rapidly. It is non-pathogenic to humans and other animals. Its basic structure and metabolism are prototypical of other species in the genus mycobacterium, some which are the agents of intractable and devastating diseases. If the secrets of M. smegmatis can be uncovered, some of the oldest of maladies to bedevil humanity might one day become treatable.
The taxonomic phylum actinobacteria are among the most common life forms on Earth. Most species are aerobic, mixing oxygen with organic nutrients, and therefore, despite their single-celled size, actinobacteria are a vital link in the planet’s carbon cycle. In the respiratory process, they produce secondary metabolites peculiar to each species. These natural compounds, including Nobel Prize-winning antibiotics, are of medical and commercial significance.
The genus category mycobacterium is characterized by a “waxy” shell. Instead of an outer cell membrane, the organisms of this group have a thick cell wall composed of lipids, or fatty acids. This protective layer makes them hydrophobic, nearly impervious to water, and contributes to their hardiness. They can survive exposure to acids, alkalis, detergents and even antibiotic assaults. Among the pathogenic species, the two most notorious mycobacteria are parasitic and responsible for the diseases tuberculosis and leprosy.
Mycobacterium smegmatis is a non-motile bacillus, a rod-shaped bacteria 3-5 micrometers long with no means of locomotion. They grow rapidly into a colony called a biofilm, doubling in population through cell division every three to five days. They are classified as Gram-positive bacteria, with cell walls that absorb standard violet stains used in lab work. They tolerate a large range of temperatures and thrive on the simplest of substrates that at least contain nitrogen, carbon and phosphorus nutrients. Mycobacterium smegmatis is a model organism, easily managed in a laboratory setting.
Except in the most unusually compromised cases, Mycobacterium smegmatis is harmless. If ingested or otherwise introduced into a human body, it is normally eventually expelled. By contrast, Mycobacterium tuberculosis can be both deadly and contagious to humans. Insofar as M. smegmatis has cellular structures, particularly the cell wall, that are similar to those of M. tuberculosis and other species, it has become the stand-in surrogate of mycobacterial research.
When the technology to sequence a complete genome had been realized, among the organisms at the top of the wish list were Homo sapiens and Mycobacterium smegmatis. Completed in 2006, two-thirds of its DNA, almost 7 million nucleotides long, was discovered to consist of the base pairs guanine and cytosine. About 90 percent of the DNA has been encoded for more than 6,700 proteins whose sum make the whole of the organism.
Since 2006, microarrays have been produced capable of quickly reading the deviations from norm of the nucleotides M. smegmatis. In addition, DNA manipulation techniques such as gene insertion, inactivation and expression reporting have advanced the laboratory study of the bacteria’s metabolic imperatives. As suspected, it shares more than 2,000 homologs, nearly identical nucleotide sequences, with the tuberculosis bacteria’s genome.
One of the focal, most promising directions of laboratory research is to disrupt the biosynthesis of Mycobacterium smegmatis’ protective cell wall, an operative tactic of many antibiotics, including penicillin. Another area of study is to determine how M. smegmatis captures and transports free phosphate molecules from its environment through its thick shell and into its cell interior, where it is required for reproduction. Scientists have begun to apply these studies toward a potential treatment for tuberculosis, leprosy and other diseases caused by one of the most common types of microbes.