Mycolactone-independent pathogenicity of Mycobacterium ulcerans: an experimental study in plants.

Mycobacterium ulcerans, the etiologic agent of Buruli ulcer in humans and animals, secretes macrolide exotoxins mycolactones which damage tissues after a cascade of cellular effects. M. ulcerans, an environmental organism with still elusive reservoirs and sources has been detected in soil and water in endemic areas where it could be in contact with plants. Symptom observations, microscopy and molecular biology were used to investigate M. ulcerans contact with plants in an experimental model mimicking the known pathology of Buruli ulcer in humans. Solanum lycopereum (tomato) plants with scarified or intact roots were transplanted into pots containing contaminated soil with M. ulcerans or a mixture of mycolactones A/B and C in the presence of negative control groups. Whereas plants with intact roots remained asymptomatic, M. ulcerans-infected plants with scarified roots had significantly more diseased leaves than controls (p = 0.004). Optic microscopy examination showed significantly more mycobacteria in the secondary and main roots than in controls (p=0.0008). Real-time PCRs detected M. ulcerans DNA in 7/12 (58%) of infected root samples versus none in the control plants (p = 0.04). Further study of plants with mycolactones A/B and C yielded no significant difference with negative controls. These results suggest that in this model, M. ulcerans exhibits a mycolactone-independent pathogenicity whose mechanism remains to be elucidated.