Leprosy

M. leprae is an unusual bacterium for several reasons. The bacterium divides very slowly; in some tests, researchers have noted a dividing time of once every twelve days. This differs from the dividing time of most bacteria, which is once every few hours. M. leprae cannot be grown on culture media, and is notoriously difficult to culture within living animals. Because of these culturing difficulties, researchers have not been able to investigate these bacteria as closely as they have other, more easily cultured, bacteria. Questions remain unanswered about M. leprae; for instance, researchers are still unclear about how the bacteria are transmitted from one person to another, and are not sure about the role an individual's genetic make up plays in the progression of the disease.

Because M. leprae almost exclusively infects humans, animal models for studying leprosy are few. Surprisingly, a few species of armadillo can also be infected with M. leprae. Recently, however, wild armadillos have been appearing with a naturally occurring form of leprosy. If the disease spreads in the armadillo population, researchers will not be able to use these animals for leprosy studies, since study animals must be completely free of the disease as well as the bacteria that cause it. Mice have also been used to study leprosy, but laboratory conditions, such as temperature, must be carefully controlled in order to sustain the infection in mice.

Scientists have been able to determine some facts about M. leprae from their experiments. M. leprae is temperature-sensitive; it favors temperatures slightly below normal human body temperature. Because of this predilection, M. leprae infects superficial body tissues such as the skin, bones, and cartilage, and does not usually penetrate to deeper organs and tissues. M. leprae is an intracellular pathogen; it crosses host cell membranes and lives within these cells. Once inside the host cell, the bacterium reproduces. The time required by these slow-growing bacteria to reproduce themselves inside host cells can be anywhere from a few weeks to as much as 40 years. Eventually, the bacteria lyse (burst open) the host cell, and new bacteria are released that can infect other host cells.

Researchers believe that the bacteria are transmitted via the respiratory tract. M. leprae exists in the nasal secretions and in the material secreted by skin lesions of infected individuals. M. leprae has also been found in the breast milk of infected nursing mothers. M. leprae may be transmitted by breathing in the bacteria, through breaks in the skin, or perhaps through breast-feeding.