Mycobacterium Leprae

Mycobacterium leprae, also known as Hansen’s coccus spirilly, mostly found in warm tropical countries, is a bacterium that causes leprosy (Hansen's disease). It is an intracellular, pleomorphic, acid-fast bacterium. M. leprae is an aerobic bacillus (rod-shaped) 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 carbol fuchsin 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, and ranging from 1-8 μm in length and 0.2-0.5 μm in diameter.

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. It was the first bacterium to be identified as causing disease in humans. 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 because they, like humans, are susceptible to leprosy. This can be used as a diagnostic test for the presence of bacilli in body lesions of suspected leprosy patients. The difficulty in culturing the organism appears to be because it is an obligate intracellular 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 was sensitive to dapsone (diaminodiphenylsulfone, the first effective treatment which was discovered for leprosy in the 1940s), 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 many symptoms of leprosy must be due in part to the presence of dead cells.