Mycobacterium leprae: Properties of the Bacteria

Mycobacterium leprae

M. leprae was discovered in 1873 by Arinuer Hansen in Italy.Leprosy was not initially thought to be an infectious disease, despite discovery of M. leprae. Tremendous advancement has been made on the knowledge of pathogenesis, cause, treatment, and prevention of leprosy during the last 25 years. Nevertheless, the lesions were responsible for the social stigma attached to the disease.

Properties of the Bacteria

◗ Morphology

leprae shows the following morphological features:

·           M. leprae is less acid fast than Mycobacterium tuberculosis.Hence 5% sulphuric acid, instead of 20%, is used for deco-lourization after staining with carbol fuchsin.

·            It is Gram positive and stains more readily than M. tuberculosis.

·           It is a straight or slightly curved rod with parallel sides and rounded ends. It measures 1–8 m in length and 0.2–0.5 m in diameter, showing considerable morphological variations. The bacteria exhibit cubical, lateral, or even branching forms.

·           M. leprae is an obligate intracellular bacterium that multi-plies preferentially in tissues at cooler temperature.

·           M. leprae are nonmotile and nonsporing.

The bacilli are seen singly and in groups intracellularly as well as extracellularly lying free outside the cell. The bacilli inside the cell are usually present in parallel bundles of 50 or more. Acid-fast bacteria are bound together by a lipid-like substance known as glia. These masses of bacteria are known as globi. The parallel rows of bacilli in the globi present a “cigar bundle” appearance. These are present inside the large undifferentiated histiocytes, which have a foamy appearance. These are known as Virchow’s lepra cells or foamy cells.

◗ Culture

So far, M. leprae has not been cultivated in vitro either in bacte-riological media or in tissue culture. The ICRC (Indian Cancer Research Center) bacillus is an example of an acid-fast bacillus and was first isolated from a leprosy patient employing human fetal spinal ganglion cell culture. This bacillus was reported in 1962 from the Indian Cancer Research Center, Mumbai, and has been adapted for growth on Lowenstein–Jensen (LJ) medium. However, its relation to M. leprae is uncertain, and many studies have suggested that ICRC bacillus is not M. leprae but may be a variant of Mycobacterium, belonging to Mycobacterium avium intra-cellulare group.

Experimental animal models: Many attempts have beenmade to develop a suitable experimental animal model for M. leprae. These include the following:

Footpads of mice: Shepard, in 1960, was the first to culturelepra bacilli in the footpads of mice kept at a low temperature of 20°C. The mouse footpad inoculation method is now being used as a standard procedure for experimental works by using M. leprae. Intradermal inoculation of lepra bacilli into the foot-pads of mice results in development of granuloma at the site of inoculation in 1–6 months. The mouse footpad model has been used to test (a) the maximum required concentration of antileprosy drugs and (b) sensitivity of the bacilli to new ant-ileprosy drugs.

Thymectomized mouse: A thymectomized mouse is an experi-mental animal model, in which cell-mediated immunity (CMI) is suppressed by thymectomy on administration of antilymphocytic serum. In such mice, inoculation of M. leprae produces a generalized infection similar to that of leproma-tous leprosy. Thymectomized irradiated mice model has also been used to detect small numbers of live lepra bacilli and is also used to detect persistent disease following treatment by chemotherapy.

Nine-banded armadillo: The armadillo (Dasypus novemcinctus) isanother animal highly susceptible to infection with M. leprae. In armadillos, M. leprae cause a generalized infection with exten-sive multiplication of the bacteria. M. leprae in these animals produces a lesion typical of lepromatous leprosy and survives for about 400 days in infected armadillos. This animal is now being used as the most important source of lepra bacilli for genetic studies including development of vaccine. Natural infections by Mycobacteriumorganisms resembling M. leprae have also been observed in some wild armadillos held in captiv-ity in Texas and Mexico.

Other animals: Other animals that have been used for experi-mental infection by M. leprae include slender loris, Indian pangolin, and Korean chipmunks. M. leprae shows the longest generation time among all bacteria requiring 12–13 days to double in experimental infected mice as compared to about 14 hours in case of M. tuberculosis and about 20 minutes in case of coliform bacilli.

◗ Other properties

Susceptibility to physical and chemical agents: M. lepraeremains viable in warm humid environment for 9–16 days and in moist soil for 46 days. They also remain viable on exposure to ultraviolet light for 30 minutes and to direct sunlight for 2 hours.