Mycobacterium leprae causes leprosy. M leprae strains collected worldwide have been genetically clonal, which poorly explains the varying severity and clinical features of the disease. We discovered a new Mycobacterium species from 2 patients who died of diffuse lepromatous leprosy (DLL). The Mycobacterium was purified from heavily infected, freshly frozen autopsy liver tissue followed by DNA extraction in 1 case. Paraffin-embedded skin tissue was used for DNA extraction in another case. Six genes of the organism were amplified by polymerase chain reaction, sequenced on cloning or from amplicons, and analyzed. Significant genetic differences with M leprae were found, including a 2.1% divergence of the 16S ribosomal RNA (rRNA) gene, a highly conserved marker of bacterial evolution, and 6% to 14% mismatches among 5 less conserved genes. Phylogenetic analyses of the genes of 16S rRNA, rpoB, and hsp65 indicated that the 2 most related organisms evolved from a common ancestor that had branched from other mycobacteria. These results and the unique clinicopathologic features of DLL led us to propose Mycobacterium lepromatosis sp nov. This species may account for some of the clinical and geographic variability of leprosy. This finding may have implications for the research and diagnosis of leprosy.

Author notes

Supported in part by a University Cancer Foundation grant from The University of Texas M.D. Anderson Cancer Center (MDACC) (to X.Y.H.); grant CA16672, for the MDACC DNA Core Facility, from the National Institutes of Health (NIH), Bethesda, MD; NIH contract N01 AI-25469 (to Colorado State University); and a College Research Council Award from Colorado State University (to J.S.S.).
Presented in part at the 17th International Leprosy Congress; January 30, 2008 to February 4, 2008; Hyderabad, India.
Dr Seo is currently with the Department of Laboratory Medicine, Gachon University Gil Medical Center, Inchon, Korea.