Jory Scott 12E Mycobacterium leprae: the leprosy pathogen

Jory Scott 12E Mycobacterium leprae, along with the less pathogenic M. lepromatosis, causes leprosy in humans and some other mammals. Leprosy affecting the fourth digit of the hand.

Jory Scott 12E M. leprae belongs to the same genus as M. tuberculosis, another pathogenic agent that causes tuberculosis. This genus has a distinctive waxy coat that gives it its name. The cell walls are rich in lipids which cause a negative Gram stain result, but Mycobacteria can be stained using an acid-fast stain, in which they show up as red. Though the bacteria don't take a Gram stain, they are said to be acid-fast Gram- positive anyway because they lack outer cell membranes. A diagram showing the unique thick, waxy cell wall structure of Mycobacterium

Jory Scott 12E Mycobacteria in general are difficult to culture for several reasons. Though they are aerobic and readily adapt to different nutrients, some species, including M. leprae, grow and divide extremely slowly; M. leprae has the longest doubling time of any bacteria known to man. Further, M. leprae is an obligate intracellular parasite, like a virus, and does not survive outside a host. For this reason mice were used for diagnostic culturing, and then nine- banded armadillos, a species that like humans can contract leprosy.

Jory Scott 12E M. leprae are non-motile bacilli bacteria, possessing no cilia or flagella. This bacterium is well equipped to withstand the hardships of being an obligate parasite. It is resistant to both alkaline and acidic environments, and cannot be lysed by complement proteins, because of its tough hydrophobic cell wall. A microscope slide image of M. leprae showing the rod-like (bacillus) shape and approximate length (3-4µm)

Jory Scott 12E It's still uncertain how M. leprae spreads, but anecdotal accounts have led to the hypothesis that sustained close contact and respiratory droplets are the most likely modes of transmission. It's possible that M. leprae can enter through the skin, even when it's unbroken, though more likely that transmission occurs through the respiratory system.

Jory Scott 12E M. leprae can exist in a host without becoming pathogenic. There is believed to be certain genetic factors involved in the development of leprosy as a result of an M. leprae infection. It's suggested that humans are normally immune to leprosy and only develop symptoms as an allergic reaction.

Jory Scott 12E Even when it does become pathogenic, M. leprae's slow growth causes leprosy to have a slow onset. Sometimes the incubation period is decades long. The normal incubation period is about three years according to most sources. Leprosy on the foot of a nine-banded armadillo. Besides humans, armadillos are the only animals that can naturally contract leprosy. Contrary to popular belief, leprosy does not cause infected digits or limbs to drop off, but amputation may be necessary in the case of necrosis.

Jory Scott 12E The primary symptom of leprosy is widespread cell death, particularly affecting the skin and nerves of peripheral areas of the body. The disease caused by M. leprae that has no skin involvement, ie., affects the nervous system only, is called Hansen's disease. The damage to the nervous system causes weakeness and loss of sensation. M. leprae prefers temperatures cooler than the core body temperature and is therefore closer to the skin in areas like the hands and feet, which is why skin lesions occur more in those areas.

Jory Scott 12E M. leprae is believed to normally overcome the first line of defence by entering the respiratory system. Its waxy coating protects it from the enzymes in mucus and if it avoids being trapped by the cilia lining the bronchial tubes and coughed up again, it can make its way into other parts of the body. M. leprae grows well in macrophages, the very cells that the body uses to attack non-self particles and organisms.

Jory Scott 12E Because of its highly resistant cell wall, M. leprae can't be destroyed by nonspecific immune response. M. leprae activates cell death procedures in cells it infects. Granulated and cytotoxic immune cells attack the infected tissue. When lysis of an infected cell occurs, M. leprae, like a virion, is able to infect a new host cell and thus continue the response.

Jory Scott 12E Treatment with a drug regimen called MDT is very effective, targeting and killing M. leprae. Depending on the extent of the damage, symptoms usually disappear very rapidly and nerve regrowth will occur. An important part of recovery is clearing diseased and dead tissue. One particular antibiotic called rifampicin is particularly effective in preventing the development of leprosy and also curing an infection that has already begun, because of its lipophilic nature.

Jory Scott 12E Repeated infections after treatment are rare because of the body's improved ability to identify M. leprae as non-self and attack it before it spreads in the body, even when exposure to the bacterium continues. A general graph of acquired or adaptive immune response as compared to innate, nonspecific immune response

Jory Scott 12E References Wikipedia: emedicine: MedicineNet: World Health Organisation: MedlinePlus: The Leprosy Mission Canada: American Leprosy Mission: