Mycobacterium lepromatosis

Mycobacterium lepromatosis is a bacterium that, along with "Mycobacterium leprae", causes leprosy (Hansen's disease). It was only recently discovered, in 2008. Analysis of its 16S rRNA gene confirms that the species is distinct from "Mycobacterium leprae".
"M. lepromatosis" is an acid-fast bacillus (AFB) which causes diffuse lepromatous leprosy (DLL). DLL is mainly found in Mexico and the Caribbean. DLL is a severe form of leprosy which manifests through nerve invasion and extensive skin ulcerations due to massive AFB burden in internal organs. "M. lepromatosis", like "M. leprae", has not been cultured in the laboratory because they both lack genes necessary to grow outside their hosts. These genes are believed to have been lost through reductive evolution.