www.thelancet.com/infection Vol 9 November 2009 699 Review Introduction Buruli ulcer, caused by Mycobacterium ulcerans, is an emerging disease that at present is the third most common human mycobacteriosis worldwide after tuberculosis and leprosy. 1 Buruli ulcer is found mostly in humid tropical areas of Asia, Latin America, and Africa where its incidence has been increasing, surpassing tuberculosis and leprosy in some regions. 1 Buruli ulcer is a necrotising disease of the skin (mainly the subcutaneous tissue), that mostly affects children, producing massive, disfiguring ulcers and permanent disabling scars. 2,3 There is no vaccine against this disease, and treatment is difficult and generally requires surgery, usually accompanied by skin grafting and prolonged courses of antibiotics. Despite its huge social effect Buruli ulcer remains a largely neglected disease. 3,4 Buruli ulcer is also of great scientific interest because of the unique characteristics of its pathogenicity that have been the source of contradictory and controversial interpretations, and remain largely enigmatic. An extreme among mycobacteria M ulcerans is genetically very close to the typical intracellular parasites Mycobacterium tuberculosis and Mycobacterium marinum, 5 and the greater than 98% nucleotide sequence identity between M ulcerans and M marinum provides evidence that M ulcerans evolved from M marinum. 5–7 However, over the course of its evolution M ulcerans acquired a giant virulence plasmid, pMUM001, responsible for the synthesis of the exotoxin mycolactone. 8 Acquisition of this plasmid has been deemed to be the main driver for Buruli ulcer emergence in people. 9 Additionally, by comparison with M marinum, M ulcerans has undergone extensive gene loss, 10,11 as is the case in the obligate intracellular parasite Mycobacterium leprae, although to a lesser degree. 12 M ulcerans grows very slowly in vivo 13 and, like most M marinum strains, has an optimum growth temperature of about 32°C, 14 explaining the predilection of M marinum for the skin and its restricted systemic dissemination. 15–19 However, bones can be infected by M ulcerans because of contiguous spreading or by lymphatic or haematogenous dissemination. 2,20 Since M ulcerans strains isolated from human bone infections do not grow at 37°C, 14 osteomyelitis is one of the still enigmatic features of Buruli ulcer. Progression of the infection leads to coagulative necrosis of the dermis and subcutis, resulting in varied non- ulcerative clinical forms that can progress to ulceration following necrosis of the epidermis. 21 Extensive necrosis is a hallmark of the histopathology of Buruli ulcer and has been associated with the high toxigenicity of M ulcerans. This characteristic of M ulcerans, 18,19,22 is unique among human pathogenic mycobacteria, and is due to the production of a family of cytotoxic exotoxins, the mycolactones. 23,24 Therefore, three features of M ulcerans emerge as relevant for its pathogenicity: the first is the low optimum growth temperature that makes the skin its almost exclusive territory, the second is the slow growth rate that translates into slowly progressing lesions, and the third is the mycolactone-associated high cytotoxicity that adds to its mycobacterial nature with consequences for pathology and immunity. The importance of mycolactone for pathogenesis Mycolactones are unusual among bacterial exotoxins because they are poorly immunogenic polyketide-derived macrolides. 23,25 Mycolactone A/B is the most active and widespread, and it is characteristic of M ulcerans strains from Africa. 26 Experiments with externally added mycolactone A/B show that it has intense cytotoxic activity in vitro, affecting monocytes, macrophages, neutrophils, Pathogenetic mechanisms of the intracellular parasite Mycobacterium ulcerans leading to Buruli ulcer Manuel T Silva, Françoise Portaels, Jorge Pedrosa The necrotising skin infection Buruli ulcer is at present the third most common human mycobacteriosis worldwide, after tuberculosis and leprosy. Buruli ulcer is an emergent disease that is predominantly found in humid tropical regions. There is no vaccine against Buruli ulcer and its treatment is difficult. In addition to the huge social effect, Buruli ulcer is of great scientific interest because of the unique characteristics of its causative organism, Mycobacterium ulcerans. This pathogen is genetically very close to the typical intracellular parasites Mycobacterium marinum and Mycobacterium tuberculosis. We review data supporting the interpretation that M ulcerans has the essential hallmarks of an intracellular parasite, producing infections associated with immunologically relevant inflammatory responses, cell-mediated immunity, and delayed-type hypersensitivity. This interpretation judges that whereas M ulcerans behaves like the other pathogenic mycobacteria, it represents an extreme in the biodiversity of this family of pathogens because of its higher cytotoxicity due to the secretion of the exotoxin mycolactone. The acceptance of the interpretation that Buruli ulcer is caused by an intracellular parasite has relevant prophylactic and therapeutic implications, rather than representing the mere attribution of a label with academic interest, because it prompts the development of vaccines that boost cell- mediated immunity and the use of chemotherapeutic protocols that include intracellularly active antibiotics. Lancet Infect Dis 2009; 9: 699–710 IBMC-Instituto de Biologia Molecular e Celular, Universidade do Porto, Rua do Campo Alegre 823, Porto 4150–180, Portugal (M T Silva MD); Mycobacteriology Unit, Department of Microbiology, Institute of Tropical Medicine, Antwerp, Belgium (Prof F Portaels PhD); and Life and Health Sciences Research Institute (ICVS), School of Health Sciences, University of Minho, Braga, Portugal (Prof J Pedrosa PhD) Correspondence to: Françoise Portaels, Mycobacteriology Unit, Department of Microbiology, Institute of Tropical Medicine, Nationalestraat 155, B-2000 Antwerp, Belgium portaels@itg.be For more on M ulcerans infection see http://www.afip.org/hot-topics/ bu/