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Mycobacterium leprae

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Mycobacterium leprae

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A 34-year-old man comes to the clinic because of skin changes and numbness in his hands and feet for the past few months. For the past three years, he has worked as a zookeeper, spending most of his time with the armadillos. He says that his skin, especially on his face, has gradually begun to develop folds and feel tough. His temperature is 38.3°C (101.5°F), pulse is 82/min, respirations are 14/min, and blood pressure is 132/84 mm Hg. Physical examination shows leonine facies, diffuse skin changes, and loss of sensation in the distal upper and lower extremities. A skin biopsy is performed and shows acid-fast bacilli invading a cutaneous nerve. Which of the following is the most likely cause of this condition?

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Mycobacterium leprae is a rod-shaped bacteria which was first discovered in 1873 by Hansen.

Mycobacterium leprae is a non tuberculous mycobacteria and it causes a disease called leprosy, or Hansen disease. In US, the animal reservoir for Mycobacterium leprae are armadillos.

Now, Mycobacterium leprae it’s an acid-fast bacillus which means it’s resistant to decolorization by acids and it has a high content of mycolic acid in its cell wall, which makes it waxy, hydrophobic and impermeable to routine stain such as Gram stain.

So, it needs special staining methods to be visualized such as Ziehl-Neelsen staining which uses carbol fuchsin combined with phenol which is able to penetrate the waxy mycobacterial cell wall.

So, the stain binds to the mycolic acid in the mycobacterial cell wall and after staining, an acid decolorizing solution is applied which removes the red dye from the background cells, tissue fibres, and any organisms in the smear except Mycobacteria, which retain the dye.

So Mycobacterium leprae appears bright red on a blue background. Other staining methods can be used such as Kinyoun staining, in which the bacteria appear bright red on a green background and fluorescence microscopy using specific fluorescent dyes such as auramine-rhodamine stain.

Now, Mycobacterium leprae is an obligate intracellular microorganism, which means it can survive only inside cells, and it’s an obligate aerobe which means it can survive only in the presence of oxygen.

Finally, Mycobacterium leprae grows best at cool temperatures, between 27 to 33 degrees Celsius, and it proliferates slowly and it cannot be cultivated in vitro.

Instead, it can be inoculated in nine-banded armadillos, which have a much lower body temperature than most mammals and, like humans, are susceptible to leprosy.

Now, Mycobacterium leprae can enter the body through the lungs or broken skin.

Once inside the body, it goes for regions in which the temperature is lower than the rest of the body such as skin, peripheral nerves and mucosa of the upper respiratory tract.

So, the bacteria goes for the Schwann cells of peripheral nerves.

These cells wrap their plasma membrane around peripheral nerve axons forming the myelin sheath.

This is possible because of a virulence factor, called phenolic glycolipid 1, or PGL-1 for short, which attaches to a protein called laminin-2 which is found on the Schwann cells.

Binding to Schwann cells induces demyelination, affecting transmission of the electrical impulses through the nerve axon, and causing nerve injury.

Additionally, Mycobacterium leprae can also infect skin macrophages.

So, the bacteria is ingested by macrophages and wrapped up in a vesicle called a phagosome, which would normally merge with another intracellular organelle called a lysosome.

Inside the phagolysosome, the bacteria would normally be destroyed.

But, Mycobacterium leprae has the ability to inhibit the phagolysosomal fusion, which allows the bacteria to survive inside macrophages and replicate there.

Now, the host responds to leprosy through cell mediated immunity via T-helper cells.

These cells help the activity of other immune cells by releasing T cell cytokines, and there are two types of T-helper cells - Th1 and Th2.

Depending on which T-helper cells are involved in the immune response, there are two major forms of leprosy - lepromatous and tuberculoid.

Now, in the lepromatous form, the infection induces a Th2 cell response, and Th2 cells secrete IL-4, IL-5 and IL-10.

These molecules are cytokines, which means that they signal B cells to make antibodies against Mycobacterium leprae, so this is a type of humoral, antibody-based response.

Now, this response is not effective in killing intracellular pathogens like Mycobacterium leprae, that can escape humoral immune mechanisms and replicate inside macrophages.

So, the inefficient immune response seen in the lepromatous form is sometimes also called a low cell mediated immunity response.

This leads to extensive skin involvement and symmetric nerve involvement, and the lepromatous form is also called multibacillary leprosy because of the large number of bacteria found in the lesions.

On the other hand, with is tuberculoid form, the infection induces a Th1 response, and Th1 cells secrete IL-2 and interferon-gamma.

Summary
Mycobacterium leprae is an aerobic bacillus that causes leprosy (Hansen's disease). It is an intracellular, pleomorphic, acid-fast, pathogenic bacterium. M. leprae is an aerobic bacillus surrounded by the characteristic waxy coating unique to mycobacteria. M. leprae stains with a carbol fuchsin rather than with the traditional gram stain. The culture takes several weeks to mature. It replicates intracellularly inside histiocytes and nerve cells and has two forms. One form is tuberculoid, which induces a cell-mediated response that limits its growth. The second form of leprosy is the lepromatous form. This form of the microbe proliferates within the macrophages at the site of entry. It also grows within the epithelial tissues of the face and ear lobes.