Mycobacterium leprae
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 Microphotograph of Mycobacterium leprae taken from a skin lesion. Source: CDC
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| Mycobacterium leprae Hansen, 1874 |
Mycobacterium leprae, also known as Hansen’s bacillus, is the bacterium that causes leprosy (Hansen's disease). It is an intracellular, pleomorphic, acid fast bacterium. M. leprae is a gram-positive aerobic rod-shaped (bacillus) surrounded by the characteristic waxy coating unique to mycobacteria. In size and shape, it closely resembles Mycobacterium tuberculosis. Due to its thick waxy coating, M. leprae stains with a carbon finishing rather than with the traditional Gram stain. The culture takes several weeks to mature.
Optical microscopy shows M. leprae in clumps, rounded masses, or in groups of bacilli side by side.
It was discovered in 1873 by the Norwegian physician Gerhard Armauer Hansen, who was searching for the bacteria in the skin nodules of patients with leprosy.
The organism has never been successfully grown on an artificial cell culture media. Instead it has been grown in mouse foot pads and more recently in nine-banded armadillos. This can be used as a diagnostic test for the presence of bacillus in body lesions of suspected leprosy patients. The bacterium can infect armadillos, so it is studied in them (see Animal Rights). The difficulty in culturing the organism appears to be due to the fact that the organism is an obligate intra-cellular parasite that lacks many necessary genes for independent survival. The complex and unique cell wall that makes members of the Mycobacterium genus difficult to destroy is apparently also the reason for the extremely slow replication rate.
Virulence factors include a waxy exterior coating, formed by the production of mycolic acids unique to Mycobacterium.
M. leprae is sensitive to dapsone (diaminodiphenylsulfone, the first effective treatment which was discovered for leprosy in the 1940's), but resistance against this antibiotic has developed over time. Therapy with dapsone alone is now strongly contraindicated. Currently, a multidrug treatment (MDT) is recommended by the World Health Organization, including dapsone, rifampicin and clofazimine. In patients receiving the MDT, a high proportion of the bacilli die within a short amount of time without immediate relief of symptoms. This suggests that many symptoms of leprosy must be due in part, to the presence of dead cells.
