Mycobacterium leprae

Mycobacterium leprae is a specialized bacterium that causes Hansen's disease, commonly known as leprosy. This chronic infectious disease targets the skin, eyes, nose, and muscles, and it can damage peripheral nerves.

The bacterium is a Gram-positive, rod-shaped organism. It is also described as an acid-fast bacterium. This means it possesses a waxy coating of mycolic acid. This unique cell wall makes the organism difficult to destroy. It also contributes to an extremely slow replication rate. Scientists classify it as an obligate intracellular parasite. This means it must live inside the cells of a host to survive.

Because it is an obligate intracellular parasite, Mycobacterium leprae cannot be grown in standard laboratory media. It lacks the ability to survive in a cell-free environment. Instead, it relies heavily on its host for nutrients and metabolic intermediates. The bacterium primarily infects macrophages and Schwann cells. Within these host cells, the bacteria are often found in clumps called "globi." They may also appear organized in structures known as a palisade. The bacteria prefer cool temperatures between 27 and 30 degrees Celsius. This temperature preference explains why they target the skin, nasal mucosa, and peripheral nerves.

This reliance on a host is the result of a process called reductive evolution. Over time, the bacterium's genome has experienced significant gene deletion and decay. This evolution has impaired many of its metabolic abilities. For example, it has lost the ability to use common carbon sources like acetate and galactose. Its ability to degrade lipids is also impaired due to missing enzymes. However, its anabolic pathways remain largely unaffected. It can still synthesize most amino acids, though it cannot make methionine or lysine. It also retains the ability to produce essential genetic material like nucleotides. This reduction in genome size is an extreme case of evolutionary downsizing.

The genome of Mycobacterium leprae is notably small. The first genome sequence was completed in 2001. A second sequence from a strain in Tamil Nadu, India, was finished in 2013. This Indian strain contains 3,263,203 base pairs. It has an average G+C content of 57.8 percent. This is much lower than its relative, Mycobacterium tuberculosis. In Mycobacterium leprae, less than half of the genome contains functional genes. More than 1,000 genes are actually pseudogenes. These are non-functional segments of DNA that resulted from mutations. This high number of pseudogenes is the highest among published bacterial genomes.

History shows that Mycobacterium leprae was a landmark discovery in medicine. In 1873, the Norwegian physician Gerhard Armauer Hansen identified the bacterium.

This was the first time a bacterium was identified as the cause of a human disease. For many years, the antibiotic dapsone was used as a treatment. However, the bacterium developed resistance to dapsone alone during the 1960s. To combat this, the World Health Organization now recommends multidrug therapy (MDT). This treatment combines dapsone with rifampicin and clofazimine.

Using multiple drugs prevents the bacteria from developing resistance.

While humans are the primary hosts, the bacterium has a narrow host range. The only other natural hosts are nine-banded armadillos and red squirrels.

Contact with armadillos can lead to zoonotic transmission in humans. In the United States, studies have linked leprosy cases to armadillo contact. Scientists have also detected the bacterium's DNA in environmental sources. This includes soil in Bangladesh and armadillo holes in Suriname. The disease has a global distribution, but it is most prevalent in sub-Saharan Africa, Asia, and South America. High infection rates have been noted in armadillos in the Brazilian state of Pará.

Since the introduction of multidrug therapy in the 1980s, the impact has been massive. The prevalence of leprosy cases has declined by 95 percent globally. Because of this success, the World Health Organization declared leprosy eliminated as a public health problem. This status is defined as having fewer than one case per 10,000 people. Research into the bacterium's evolution continues to provide new insights. Studies of medieval skeletons have even allowed scientists to sequence ancient DNA. These studies help us understand how the disease spread from East Africa to Europe and the Americas.