A second complete genome sequence of the pathogen M. leprae, which causes leprosy, has been determined. These findings, as well as the deep re-sequencing of two other M. leprae strains, are reported online in this week's Nature Genetics.

M. leprae is the bacterial pathogen that causes leprosy, a disease that is characterized by disfiguring lesions on the skin and loss of sensation. Although leprosy is no longer a significant health problem in developed countries, pockets of high-risk areas remain in less-developed countries, such as those in Africa or South-East Asia. Currently, the worldwide burden of leprosy is under 250,000 cases a year.

The genome of an M. leprae strain from India was previously sequenced. Here, Stewart Cole and colleagues have assembled the complete genome of an M. leprae strain from Brazil and have also sequenced two more M. leprae strains from Thailand and the United States. Although the four M. leprae strains come from geographically distant locations, comparison of their four genomes shows that they are 99.995% identical. This surprising result implies that leprosy has arisen from a single clonal strain. Lack of genomic diversity in M. leprae is encouraging for treatment, as it suggests that drug effectiveness should be similar for most strains of M. leprae.