Toxicity of MTR for BSHP7014 strain. Strain BSHP7014 (mtnW::lacZ amyE::pxyl mtnXYZ) was grown on ED1 minimal medium plates in the presence of sulfate as sulfur source (panel A) or in the absence of any added sulfur source (agar as sole sulfur source, panel B). Xylose was added to the medium in order to trigger the expression of mtnXYZ from the pxyl promoter. 10 μl of methionine (met) or MTR was adsorbed on paper discs and plates were incubated over-night at 37°C. Methionine was used as a control for growth and/or toxicity of the sulfur source.