Regulation of Mycolactone, the Mycobacterium ulcerans Toxin, Depends on Nutrient Source

Abstract

Background Mycobacterium ulcerans, a slow-growing environmental bacterium, is the etiologic agent of Buruli ulcer, a necrotic skin disease. Skin lesions are caused by mycolactone, the main virulence factor of M. ulcerans, with dermonecrotic (destruction of the skin and soft tissues) and immunosuppressive activities. This toxin is secreted in vesicles that enhance its biological activities. Nowadays, it is well established that the main reservoir of the bacilli is localized in the aquatic environment where the bacillus may be able to colonize different niches. Here we report that plant polysaccharides stimulate M. ulcerans growth and are implicated in toxin synthesis regulation.Methodology/Principal FindingsIn this study, by selecting various algal components, we have identified plant-specific carbohydrates, particularly glucose polymers, capable of stimulating M. ulcerans growth in vitro. Furthermore, we underscored for the first time culture conditions under which the polyketide toxin mycolactone, the sole virulence factor of M. ulcerans identified to date, is down-regulated. Using a quantitative proteomic approach and analyzing transcript levels by RT-qPCR, we demonstrated that its regulation is not at the transcriptional or translational levels but must involve another type of regulation. M. ulcerans produces membrane vesicles, as other mycobacterial species, in which are the mycolactone is concentrated. By transmission electron microscopy, we observed that the production of vesicles is independent from the toxin production. Concomitant with this observed decrease in mycolactone production, the production of mycobacterial siderophores known as mycobactins was enhanced.Conclusions/SignificanceThis work is the first step in the identification of the mechanisms involved in mycolactone regulation and paves the way for the discovery of putative new drug targets in the future.

FVB/N Mice Spontaneously Heal Ulcerative Lesions Induced by Mycobacterium ulcerans and Switch M. ulcerans into a Low Mycolactone Producer

“…We did not observe that mycolactone production genes were modulated in tail tissues of FVB/N mice (Supplemental Fig. 2), as already suggested in in vitro studies (25,30). This paradox could be explained by different process.…”

“…Specific gene expression was calculated using the DDCt method (using GAPDH as calibrator). Dormancy of the bacteria was estimated by quantifying the expression of devR, devS, and icl (27)(28)(29), and mycolactone biosynthesis gene expression, that is, mup053, mup045, mup038, mlsA1 (30)(31)(32), and KR (33), was also estimated. Analysis was performed utilizing the DDCt method with the ppk gene as housekeeping gene (34).…”

An Optimized Method for Extracting Bacterial RNA from Mouse Skin Tissue Colonized by Mycobacterium ulcerans

“…Surprisingly, transcriptomic studies based on RT-qPCR showed that the bacteria in these tissues were not dormant. Paradoxically, transcription levels for the principal genes involved directly in toxin synthesis were unaffected, suggesting that mycolactone synthesis was regulated upstream, as already shown in vitro (Deshayes et al, 2013). Our previous findings suggest that RNA-seq is the most appropriate approach for deciphering the regulation of mycolactone synthesis in vivo .…”

Nontuberculous Mycobacteria

“…Because of the immunosuppressive properties of the mycolactone toxin, the disease can progress with minimal to no pain or fever. 51 Of note, patients with HIV have an increased risk for all other NTM except for M ulcerans. 52 Clinical presentation M ulcerans infection typically begins as a painless, small nodule less than 5 cm in diameter.…”