## Mycobacterium leprae: Microbiological Characteristics ### Culture and Growth Requirements **Key Point:** M. leprae is unique among mycobacteria because it CANNOT be cultured in vitro on standard mycobacterial media. This is a defining feature that distinguishes it from M. tuberculosis and other mycobacteria. **High-Yield:** M. leprae remains the only mycobacterium that cannot be reliably cultured on artificial media. Historically, it was cultivated only in: - Armadillo footpads (animal model of choice) - Nude mice - Murine footpad model This inability to culture in vitro has made diagnosis and drug susceptibility testing historically challenging, though PCR-based methods now aid diagnosis. ### Correct Features of M. leprae | Feature | Details | |---------|----------| | **Obligate intracellular pathogen** | Infects macrophages, Schwann cells, and fibroblasts | | **Doubling time** | 12–14 days (slowest of all mycobacteria; M. tuberculosis ~15–20 hours) | | **Acid-fast staining** | Ziehl-Neelsen, auramine-rhodamine, Fite-Faraco stains all positive | | **Culture capability** | CANNOT grow on standard media (Löwenstein-Jensen, MGIT, etc.) | | **Genome** | Reduced genome (~3.3 Mb) with high GC content; many pseudogenes | ### Clinical Pearl The inability to culture M. leprae in vitro is why: - Diagnosis relies on clinical features, slit-skin smear microscopy, and PCR - Drug susceptibility testing is not routinely performed - Lepromin test (intradermal antigen) assesses cell-mediated immunity rather than culture **Mnemonic:** **CANNOT Culture M. leprae** — Remember: M. leprae is the **exception** among mycobacteria; it refuses standard media.
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