Mycobacterium leprae

Mycobacterium leprae, 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 rod-shaped surrounded by the characteristic waxy coating unique to mycobacteria tuberculosis. Due to its thick waxy coating, M. leprae stains with a carbol fuscin rather than with the traditional Gram stain.

Gerhard Armauer Hansen first discovered it in 1873. It was the first bacterium to be identified as causing disease in humans. It has never been successfully grown on an artificial cell culture media. It has been grown in mouse foot pads and more recently in nine-banded armadillos since they are susceptible to leprosy.

Dapsone is often used to treat M. leprae; however, over time a resistance has developed. Generally a multi-drug treatment is recommended. In treatment a high proportion of the bacilli die in a short amount of time without immediate relief of symptoms.

It has the longest doubling time of all known bacteria and has so far been impossible at culture in the laboratory. Less than half the genome contains functional genes. The genome sequence has been completed. Between M. leprae and M. tuberculosis share 1500 common genes. Comparative analysis suggests that they developed from a common ancestor.

Diagnostic skin tests can be developed from information from the completed genome. Diagnosis is primarily a clinical one. It is generally based on 1 of 3 signs: hypopigmented or reddish patches with definite loss of sensation, thickened peripheral nerves, Acid-fast bacilli on skin smears or biopsy material.