SUMMARY
Leprosy is best understood as two conjoined diseases. The first is a
chronic mycobacterial infection that elicits an extraordinary range of
cellular immune responses in humans. The second is a peripheral
neuropathy that is initiated by the infection and the accompanying
immunological events. The infection is curable but not preventable, and
leprosy remains a major global health problem, especially in the
developing world, publicity to the contrary notwithstanding.
Mycobacterium leprae remains noncultivable, and for over a
century leprosy has presented major challenges in the fields of
microbiology, pathology, immunology, and genetics; it continues to do
so today. This review focuses on recent advances in our understanding
of M. leprae and the host response to it, especially
concerning molecular identification of M. leprae, knowledge of
its genome, transcriptome, and proteome, its mechanisms of microbial
resistance, and recognition of strains by variable-number tandem repeat
analysis. Advances in experimental models include studies in gene
knockout mice and the development of molecular techniques to explore
the armadillo model. In clinical studies, notable progress has been
made concerning the immunology and immunopathology of leprosy, the
genetics of human resistance, mechanisms of nerve injury, and
chemotherapy. In nearly all of these areas, however, leprosy remains
poorly understood compared to other major bacterial
diseases.
Mycobacterium leprae is not cultivable in axenic media, and direct microscopic enumeration of the bacilli is complex, labor intensive, and suffers from limited sensitivity and specificity. We have developed a real-time PCR assay for quantifying M. leprae DNA in biological samples. Primers were identified to amplify a shared region of the multicopy repeat sequence (RLEP) specific to M. leprae and tested for sensitivity and specificity in the TaqMan format. The assay was specific for M. leprae and able to detect 10 fg of purified M. leprae DNA, or approximately 300 bacteria in infected tissues. We used the RLEP TaqMan PCR to assess the short and long-term growth results of M. leprae in foot pad tissues obtained from conventional mice, a gene knock-out mouse strain, athymic nude mice, as well as from reticuloendothelial tissues of M. leprae–infected nine-banded armadillos. We found excellent correlative results between estimates from RLEP TaqMan PCR and direct microscopic counting (combined r = 0.98). The RLEP TaqMan PCR permitted rapid analysis of batch samples with high reproducibility and is especially valuable for detection of low numbers of bacilli. Molecular enumeration is a rapid, objective and highly reproducible means to estimate the numbers of M. leprae in tissues, and application of the technique can facilitate work with this agent in many laboratories.
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