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Mycobacterium leprae, one of the causative agents of leprosy. As acid-fast bacteria, M. leprae appear red when a Ziehl-Neelsen stain is used.

Main article: Mycobacterium leprae

Biology

Mycobacterium leprae and Mycobacterium lepromatosis are the causative agents of leprosy. M. lepromatosis is a relatively newly identified mycobacterium isolated from a fatal case of diffuse lepromatous leprosy in 2008.

Mycobacterium leprae a gram-positive bacterium that causes leprosy (Hansen's disease). It is an intracellular, pleomorphic, acid-fast bacterium. M. leprae is a slow growing aerobic bacillus (rod-shaped), surrounded by the characteristic waxy coating unique to mycobacteria. Due to its thick waxy coating, M. leprae stains with a carbol fuchsin rather than with the traditional Gram stain. In size and shape, it closely resembles Mycobacterium tuberculosis.

Due to extensive loss of genes necessary for independent growth, M. leprae and M. lepromatosis are obligate intracellular parasites, and unculturable in the laboratory, a factor that leads to difficulty in definitively identifying the organism under a strict interpretation of Koch's postulates. The use of non-culture-based techniques such as molecular genetics has allowed for alternative establishment of causation.

Studies performed in animal models show that the ideal temperature for growth of the M. leprae is at 27 - 33ºC, which is compatible to the preferred location in the body by this pathogen, the skin, mucous membranes and nerves close to the skin. This also explains the growth of bacteria in armadillos, since those animals have a core temperature of 34ºC.

Tropism

Mycobacterium leprae, unlike other mycobacteria, has tropism for cells of the peripheral nervous system, such as schwann cells, as well as for cells of the reticuloendothelial system. This bacillus preferably infects macrophages, being collected in intracellular pockets called globi.

Natural reservoir

While the causative organisms have to date been impossible to culture in vitro, it has been possible to grow them in animals. Charles Shepard, chairman of the United States Leprosy Panel, successfully grew the organisms in the footpads of mice in 1960. This method was improved with the use of congenitally athymic mice (nude mice) in 1970 by Joseph Colson and Richard Hilson at St George's Hospital, London.

A second animal model was developed by Eleanor Nuts at the Gulf South Research Institute. Dr Storrs had worked on the nine-banded armadillo for her PhD, because this animal had a lower body temperature than humans and might therefore be a suitable animal model. The work started in 1968 with material provided by Waldemar Kirchheimer at the United States Public Health Leprosarium in Carville, Louisiana. These experiments proved unsuccessful, but additional work in 1970 with material provided by Chapman Binford, medical director of the Leonard's Wood Memorial, was successful. The papers describing this model led to a dispute of priority. Further controversy was generated when it was discovered that wild armadillos in Louisiana were naturally infected with leprosy.

Naturally occurring infection has also been reported in non-human primates including the African chimpanzee, sooty mangabey, and cynomolgus macaque.

Random notes

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