Leprosy causes

Overview
A bacillus, Mycobacterium leprae, that multiplies very slowly and mainly affects the skin, nerves, and mucous membranes. The organism has never been grown in bacteriologic media or cell culture, but has been grown in mouse foot pads.

Cause
Mycobacterium leprae is the causative agent of leprosy. An intracellular, acid-fast bacterium, M. leprae is aerobic, gram-positive, and rod-shaped, and is  surrounded by the waxy cell membrane coating characteristic of Mycobacterium species.

Due to extensive loss of genes necessary for independent growth, M. leprae is unculturable in the laboratory, a factor which leads to difficulty in definitively identifying the organism under a strict interpretation of Koch's postulates. The use of non-culture-based techniques such as molecular genetics has allowed for alternative establishment of causation.