Mycobacterium leprae is the bacterium responsible for leprosy, also called Hansen's disease. It is classified among the mycobacteria and is best recognized by its waxy, lipid-rich cell wall that confers acid-fast staining properties rather than reliable Gram staining. For taxonomic summaries and basic references see taxonomy resources.

Microbiology and distinguishing features

M. leprae is a slender, rod-shaped (bacillary) organism with abundant mycolic acids in its outer envelope. This composition makes the organism acid-fast: it retains carbol-fuchsin dye after an acid wash and is visualized with Ziehl–Neelsen or Fite–Faraco stains. Although sometimes described as gram-positive on the basis of cell wall structure, it does not stain well with conventional Gram methods. Laboratory and diagnostic notes are summarized at diagnostic guidance.

Clinical disease and transmission

The bacterium causes a chronic infection that primarily affects the skin and peripheral nerves, leading to hypopigmented or reddish skin lesions, sensory loss, and, when advanced and untreated, nerve damage and disability. Transmission is not fully understood but is thought to occur mainly by prolonged close contact and likely by respiratory droplets; infectivity is relatively low compared with many other pathogens. Clinical management and public-health considerations are discussed in guideline summaries such as clinical guidelines.

History and discovery

M. leprae was the first bacterium conclusively linked to a human chronic disease when Norwegian physician Gerhard Armauer Hansen identified it in 1873, an important milestone in medical bacteriology. Historical overviews and biographical context can be found at historical sources.

Growth, reservoirs and genetics

The organism grows extremely slowly and cannot be grown on standard artificial culture media. Experimental propagation is possible in the mouse footpad and in certain wild reservoirs such as nine-banded armadillos in the Americas; summaries of culture and reservoir research are available at research reviews and zoonotic reports. Molecular studies show a reduced genome and adaptations to an obligate host-associated lifestyle, which helps explain its limited growth outside tissues.

Treatment, control and notable facts

Leprosy is curable with multidrug therapy (MDT), typically including rifampicin plus other agents such as dapsone and clofazimine. Early diagnosis and treatment prevent most complications; public-health programs have greatly reduced prevalence worldwide. Stigma and historical isolation remain social issues in some regions. For public-health policies and treatment protocols see treatment and policy.

  • Key points: acid-fast bacillus, causes chronic neuropathic skin disease, slow growth, limited culture, treatable with MDT.
  • Further reading: follow the linked resources above for taxonomy, diagnostics, clinical practice, history, research, and public-health guidance.