Bacteria Home > Mycobacterium Leprae

Mycobacterium leprae -- the bacteria responsible for leprosy -- grow slowly and mainly affect the skin, nerves, and mucous membranes. The bacteria are likely transmitted from person to person in respiratory droplets. Transmission from animals to humans is extremely rare. Today, the majority of cases can be treated without much difficulty. In 2002, 96 cases were reported in the United States.

What Is Mycobacterium Leprae?

Mycobacterium leprae are bacteria that cause leprosy, which is also known as Hansen's disease. Mycobacterium leprae are part of the family Mycobacteriaceae, which is the same family as the bacteria that cause tuberculosis (Mycobacterium tuberculosis). Mycobacterium leprae grow slowly and mainly affect the skin, nerves, and mucous membranes. The ability for Mycobacterium leprae to survive and cause damage within humans is poorly understood.


Dr. Gerhard Armauer Hansen, a Norwegian scientist, first discovered Mycobacterium leprae in 1873. Considerable progress has been made over the last 40 years so that today the majority of Mycobacterium leprae cases can be treated without too much difficulty and can counteract most of the fears generated by the folklore surrounding leprosy. Although, Mycobacterium leprae has never been grown in bacteriologic media or cell culture, it has been grown in mouse footpads.

Transmission of Mycobacterium Leprae

Despite first discovering Mycobacterium leprae in 1873, leprosy research scientists still do not completely understand the means by which Mycobacterium leprae is transmitted.
Most scientists believe that Mycobacterium leprae transmission occurs from person to person in respiratory droplets. While this may be one mode of transmission, over half of the people who develop leprosy have no confirmed contact with an infected person. The degree of susceptibility of the person, the extent of exposure, and environmental conditions are among the factors that could influence Mycobacterium leprae transmission.
Once Mycobacterium leprae transmission occurs, only about 10 to 29 percent of people will develop indeterminate leprosy, and only about 50 percent of those with indeterminate leprosy will develop full-blown leprosy.
Even in severe cases, Mycobacterium leprae destruction is limited to the:
  • Skin
  • Peripheral nerves
  • Front portion of the eyes
  • Upper respiratory passages
  • Testes
  • Hands
  • Feet.
Two nerves that Mycobacterium leprae seem to prefer are the ulnar nerve by the elbow and the peroneal nerve by the head of the fibula. In people with lepromatous leprosy, Mycobacterium leprae seem to prefer the liver, spleen, and bone marrow.
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Last reviewed by: Arthur Schoenstadt, MD
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