Mycobacterium Leprae

 In 1873, Hansen discovered Mycobacterium leprae, the bacterium responsible for leprosy. While leprosy is rare in the United States, Brazil and the Indian subcontinent account for most global cases [1]. Notably, within the U.S., California and Hawaii report the most cases, predominantly among immigrants from Mexico, Asia, Africa, and the Pacific Islands [2].

Staining and Microbiologic features:

• M. leprae is an acid-fast organism because the abundance of lipids in its cell wall prevents decolorization by acid or alcohol during the staining process [3].
• It cannot be successfully cultured in artificial media [4].

Virulence: 

• M. lepare lacks motility and exhibits facultative intracellular growth [5].
• It has devised mechanisms to resist lysosomal enzyme action [6].
• It tests positive for catalase and phenolase [5]
• It exhibits optimum growth at low temperatures [5].

Transmission: 

Leprosy transmission occurs through person-to-person contact (via airborne respiratory droplets or skin contact with secretions from infected individuals) [2].

Diseases and Complications:

• Tuberculoid leprosy/Paucibacillary Hansen disease: It is a relatively milder form of the disease that occurs when the body elicits a robust cell-mediated immune response against the bacteria. The patient can present with a few well-demarcated, hairless, hypopigmented/erythematous skin lesions. 

File:Paucibacillary leprosy (PB).jpg by mayrabcm is licensed under CC BY-SA 4.0.

Nerve involvement leads to decreased sensation or complete loss of sensation in extremities and the enlargement of nerves. The patient is noninfectious, and lepromin skin testing will yield a strongly positive result. Immunoglobulins are within the normal range, erythema nodosum is absent, and histopathological examination of skin scrapings will not reveal acid-fast bacilli. [2,7,8]

• Lepromatous leprosy/Multibacillary Hansen disease: It is a severe form of the disease, occurring when the cell-mediated immune response against M. leprae is impaired, possibly due to defective T suppressor cells inhibiting the T helper cells’ response to the bacterium. Patients present with widespread lesions, and potential clinical manifestations include leonine facies, saddle nose deformity, and glove-and-stocking peripheral neuropathy. The patient is highly infectious, and lepromin skin testing will yield a negative result. Immunoglobulin levels are high, erythema nodosum may be present, and histopathological examination of skin scrapings will reveal numerous acid-fast bacilli. Testicular involvement can lead to infertility, ocular involvement can result in blindness, and the condition can be fatal if left untreated [8,9].

File:Lepromatous leprosy.jpg by Dr. Roshan Nasimudeen is licensed under CC BY-SA 3.0.

Diagnostic Testing: 

• Ziehl-Neelsen staining of the specimen [1]

Mycobacterium leprae in Ziehl-Neelsen stained smear of sputum by Ajay Kumar Chaurasiya is licensed under CC BY-SA 4.0.

• In lepromatous leprosy, acid-fast bacilli can be observed in biopsies of affected regions, whereas granulomas are present in the case of tuberculoid leprosy [10].

 References:

1. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 319)
2. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 225)
3. Jawetz, Melnick, & Adelberg’s Medical Microbiology Twenty-Seventh Edition (page no: 309)
4. CMMRS edition 6, 2016-17 (page no: 147)
5. CMMRS edition 6, 2016-17 (page no: 152)
6. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 141)
7. CMMRS edition 6, 2016-17 (page no: 149)
8. Medical Microbiology by Patrick R. Murray Ph.D., Ken Rosenthal Ph.D., Michael A. Pfaller MD, 8th edition (page no: 226)
9. CMMRS edition 6, 2016-17 (page no: 148)
10. CMMRS edition 6, 2016-17 (page no: 153)