Towards a Buruli Ulcer Rapid Diagnostic Test that Targets Mycolactone

We report the development of a prototype rapid diagnostic test for Buruli ulcer, an ulcerative necrotizing skin disease caused by M. ulcerans. The test was designed to detect mycolactone, a metabolite unique to M. ulcerans. The chief technical challenge was to develop a simple workflow to extract trace amounts of mycolactone from wound exudates collected with a swab and, after a concentration step, to visualize the mycolactone by means of a lateral flow assay. This was achieved by utilizing a mouse monoclonal antibody specific for mycolactone and magnetic gold nanoshells. The latter are a novel class of reporter particles consisting of a ferrite core, a silica gel middle layer that serves to decrease the overall density of the nanoparticle and facilitate its resuspension in aqueous media, and an outer layer of gold, which provides a dark coloration through plasmon resonance effects. These nanoparticles, once conjugated to the anti-mycolactone antibody, enable the immunomagnetic concentration of the targeted analyte and its detection by lateral flow assay. The test procedure can be conducted within 2 hours with a magnetic rack and no powered instrumentation is required. The test can detect as little as 3.5-7 ng of mycolactone collected on a swab. Author summaryBuruli ulcer is a neglected tropical disease that affects the poorest of the poorest in Africa. Even young people can harbor large ulcers, with raw flesh directly exposed. While antibiotic treatments exist, there are no simple methods to diagnose the disease. Here, we attempted to detect mycolactone, a small molecule produced by the bacteria that causes Buruli ulcer. One difficulty was the sample type to be used, as the open wounds are sampled with a swab. The question was how to extract mycolactone from the swab and then detect it by means of a rapid diagnostic test, without using sophisticated equipment. We tried to answer this question by combining monoclonal antibodies specific for mycolactone and novel magnetic nanoparticles to concentrate and visualize mycolactone with a rapid test.