Acid-fast cause of .

Among M. leprae is relatively unique as it displays a low , thereby replicating particularly in cooler extremities. It also is an organism that cannot be grown using but instead must be grown in animals, most notably the but also in food pads. Even there, the bacterium grows extremely slowly. See also simply Mycobacterium as well as .

Why does M. leprae grow at such low temperatures? Here is one :

One might wonder at the example of a human pathogen with a temperature optimum of 27 to 30 C, since so many human pathogens have optimal ranges that include 37 C, the normal . Certainly in , in which is frequent, bacilli are repeatedly offered the opportunity of multiplying in tissue at core temperature. However, in lepromatous leprosy, which is the important infectious form of the disease, involvement is very frequent, and in most cases at least 10⁵ to 10⁷ leprosy bacilli are excreted from the daily (Shepard, 1962) . Although discharging bacilli may also be found at other sites, non-nasal ulcers are much less frequent. Thus, the pertinent temperature feature of the life history of M. leprae may be that the bacilli being excreted into the environment, where they may reach new hosts, have usually multiplied in the nasal . This could lead to selection for maximal growth at nasal temperatures. Temperatures of nasal mucosa at of 20 to 30 C have been found to range several degrees above and below 30 C (unpublished data).