The role of free-living pathogenic amoeba in the transmission of leprosy: a proof of principle

[[[ Objectives: ]]] Leprosy transmission remains poorly understood, though, prolonged skin contact and/or infection via nasal mucosa, are considered likely. Problematic in any transmission hypothesis is the fastidious nature of Mycobacterium leprae outside its host cell and the requirement for temporary survival in the environment, soil or water. Experiments were carried out to test the hypothesis that free living pathogenic amoeba might serve as host cells for M. leprae, protecting them from adverse environmental conditions. [[[ Design: ]]] In this study we employed cultures of Acanthamoeba castellanii, a free-living pathogenic soil amoeba, to determine whether these protozoa can ingest M. leprae and whether the intracellular bacilli remain viable. [[[ Results: ]]] More than 90% of cultured amoeba ingested M. leprae at a 20:1 multiplicity of infection while the infected amoebae thrived and multiplied normally. The ingested M. leprae were not degraded and remained viable for at least 72 hours as determined by their metabolic activity (radiorespirometry) and cell wall integrity (viability staining). M. leprae isolated from infected amoebae multiplied at the same rate as freshly harvested bacilli in the foot pads of nu/nu mice. [[[ Conclusions: ]]] These findings provide proof of principle that free-living pathogenic amoebae are capable of ingesting and supporting the viability of M. leprae expelled into the environment. Studies are underway to determine whether M. leprae-infected A. castellanii and other pathogenic amoebae may also play a role in transporting leprosy bacilli through broken skin or the nasal mucosa.