Note: Apurba Sastry's Essentials of Medical Microbiology is not available in the digital library. The content below is compiled from equivalent standard microbiology textbooks (Medical Microbiology 9e, Sherris & Ryan's Medical Microbiology 8e, Harrison's Principles of Internal Medicine 22e, Quick Compendium of Clinical Pathology 5e) and organized in the same chapter structure that Apurba's text follows.

Leprosy (Hansen's Disease) - Microbiology

1. Organism

• Kingdom: Bacteria; Phylum: Actinobacteria; Order: Corynebacteriales; Family: Mycobacteriaceae
• A new species, Mycobacterium lepromatosis, was described in 2008 - causes diffuse lepromatous leprosy (Lucio's leprosy), mainly in Mexico and Central America; clinically and microbiologically very similar to M. leprae, responds to the same regimens

2. Morphology and Staining

Viability Index

• Morphological Index (MI): % of uniformly stained solid (viable) bacilli among total bacilli counted on slit-skin smear under oil-immersion
• Bacteriological Index (BI): Logarithmic-scaled measure of density of all bacilli (viable + dead) in dermis on slit-skin smear. Ranges from 0 to 6+ (tuberculoid to lepromatous end). Falls ~1 log unit/year with MDT

3. Cultural Characteristics

• Cannot be cultured on any artificial/laboratory media - this is a hallmark feature
• Reason: Underwent reductive evolution - genome has many inactive/deleted genes compared to M. tuberculosis, explaining the inability to grow ex vivo
• Animal models used:
  • Mouse footpad (gold standard for drug susceptibility testing and viability assessment)
  • Nine-banded armadillo (Dasypus novemcinctus) - best animal model; naturally susceptible; used for antigen and drug studies
  • Athymic (nude) mice and gene-knockout mice
• Generation time: Extremely slow - ~12-14 days (compared to 15-20 hrs for E. coli or 20 days for M. tuberculosis)
• Optimal growth temperature: 27-30°C - this explains the predilection for cooler body parts: skin, peripheral nerves, testes, nasal mucosa, upper airways; spares deeper viscera

4. Genome

• Circular genome
• Molecular mass: 2.2 × 10⁹ Da
• 3,268,203 base pairs; G+C content: 57.8%
• Much smaller than M. tuberculosis genome due to reductive evolution
• Four distinct strains originated in East Africa or Central Asia; spread to Europe, then West Africa and the Americas

5. Cell Wall and Antigenic Structure

1. Capsule - contains lipids, especially:
   • Phthiocerol dimycocerosate (PDIM)
   • Phenolic glycolipid-1 (PGL-1) - has a species-specific trisaccharide moiety unique to M. leprae; used in serologic diagnosis; also facilitates invasion of Schwann cells and binding to peripheral nerve basal lamina
2. Cell wall - peptidoglycans bound to arabinogalactans + mycolic acids + lipoarabinomannan (LAM)
3. Plasma membrane - lipid bilayer with protein surface antigens
4. Cytoplasm

• PGL-1 (species-specific; trisaccharide detected by serology)
• Polysaccharides → mainly humoral response
• Proteins (18, 28, 36, 65, 70 kDa HSPs) → both humoral and cell-mediated response
• Two groups: cytoplasmic antigens and cell-wall antigens

6. Pathogenesis

• Primarily via nasal droplets (aerosols from florid leprosy patients)
• Minor skin abrasions/traumatic inoculation (tattooing)
• Environmental sources and possibly biting insects

• Schwann cells of the peripheral nervous system (only bacterium known to infect Schwann cells)
• Macrophages (in lepromatous form)

1. PGL-1 and laminin-binding protein facilitate invasion of Schwann cells + binding to basal lamina of peripheral nerve axon units
2. This causes demyelination of peripheral sensory nerves → local anesthesia
3. Demyelination precedes but is enhanced by the delayed-type hypersensitivity (DTH) immune response
4. Sensory loss → trauma → secondary infections → deformity

7. Immunity

• Cell-mediated immunity (CMI) is the key - Th1 response is protective
• Patients with tuberculoid leprosy have strong Th1 responses (IL-2, IFN-γ)
• Lepromatous patients have a specific defect in M. leprae CMI (not a global immunodeficiency)
• Lepromin test (Mitsuda reaction): intradermal injection of heat-killed M. leprae antigen
  • Positive in tuberculoid, negative in lepromatous
  • Tests cell-mediated immunity; not a diagnostic test (positive even in healthy uninfected individuals)
  • No longer commercially available

8. Clinical Types (Ridley-Jopling Classification)

TT -------- BT -------- BB -------- BL -------- LL
(Tuberculoid)    (Borderline)          (Lepromatous)

• TT (Tuberculoid): Localized; 1-5 well-defined macules/plaques with raised erythematous edges, dry-pale-hairless centers, anesthetic; few bacilli; non-infectious
• LL (Lepromatous): Diffuse, bilateral symmetric infiltrative lesions; "leonine facies"; loss of eyebrows (madarosis); nasal collapse; claw fingers; testicular atrophy; highly infectious
• Borderline forms (BT, BB, BL): Intermediate features

• Paucibacillary (PB): <5 skin lesions (TT, BT)
• Multibacillary (MB): ≥6 skin lesions (BB, BL, LL)

1. Hypopigmented/erythematous skin lesion with loss of sensation
2. Thickened peripheral nerves
3. AFB in slit-skin smear

9. Laboratory Diagnosis

A. Slit-Skin Smear (SSS)

• Sites: ear lobes, forehead, chin, active edge of lesion
• Stain with Fite stain (modified ZN; milder decolorizer - 1:1 xylene-peanut oil instead of acid-alcohol)
• Lepromatous: abundant AFB arranged in globi within macrophages
• Tuberculoid: very few or no AFB

B. Skin Biopsy (Full-thickness)

• Histology: noncaseating granulomas (tuberculoid) vs. foamy macrophages with many bacilli (lepromatous)
• Nerve involvement confirmed by biopsy

C. Nasal Smear

• Useful in lepromatous leprosy (large numbers of bacilli in nasal secretions)

D. Culture

• Not useful - cannot be cultured on artificial media

E. PCR

• Increasingly used as mainstay along with skin biopsy
• Detects M. leprae-specific DNA sequences
• Useful in paucibacillary disease where AFB are absent on smear

F. Serology

• Anti-PGL-1 IgM antibody: positive in ~95% of lepromatous, only ~30% of tuberculoid cases (due to poor humoral response in TT)
• Not useful for diagnosis alone

G. Lepromin Test (Mitsuda test)

• NOT a diagnostic test
• Used to classify disease (indicates CMI status) and determine prognosis
• Read at 48-72 hrs (Fernandez reaction - DTH) and 3-4 weeks (Mitsuda reaction - granuloma)

10. Treatment (WHO MDT Regimen)

• Dapsone mechanism: blocks para-aminobenzoic acid (PABA) metabolism in M. leprae
• Rifampicin is the most bactericidal drug against M. leprae

11. Key Points Summary (Apurba-style mnemonics)

• M. leprae = Obligate intracellular, Non-culturable, Acid-fast, targets Schwann cells
• Only AFB that cannot be cultured on artificial media
• Fite stain for tissue (not standard ZN)
• Mouse footpad for drug susceptibility
• Armadillo = best animal model and natural reservoir outside humans
• PGL-1 = species-specific antigen (trisaccharide); used in serology
• Temperature optimum = 27-30°C → explains skin/nerve/testes involvement
• Lepromin test = tests CMI, NOT a diagnostic test
• Lepromatous = Multibacillary = Poor CMI = Infectious
• Tuberculoid = Paucibacillary = Good CMI = Non-infectious

Fite-stained tissue section showing M. leprae (red AFB) within a peripheral nerve bundle - Quick Compendium of Clinical Pathology 5e

Lepromatous leprosy: diffuse cutaneous infiltrates and loss of eyebrows. Slit-skin smears from this patient would show numerous AFB (globi). - Sherris & Ryan's Medical Microbiology, 8e