Glutathione serves as the major thiol-redox buffer in the defense against Reactive Oxygen Species (ROS) in eukaryotes. Firmicutes bacteria utilize as thiol redox buffer bacillithiol (Cys-GlcN-Mal, BSH) and Actinomycetes produce the related redox buffer mycothiol (AcCys-GlcN-Ins, MSH). In eukaryotes, proteins are post-translational modified to S-glutathionylated proteins in response to oxidative stress. S-glutathionylation has emerged as major redox-regulatory mechanism and protects cysteine residues against overoxidation to sulfonic acids. Using thiol-redox proteomics and mass spectrometry (MS) we have recently discovered protein S-bacillithiolations as mixed BSH protein disulfides in response to oxidative stress in Firmicutes bacteria. Protein S-bacillithiolation controls the activity of the redox-sensing OhrR repressor and protects active site cysteine residues of metabolic enzymes, antioxidant function proteins and translation factors. However, it is unknown if ROS and infection conditions cause protein S-mycothiolations and affect the cellular MSH redox potential in pathogenic Mycobacteria and Corynebacteria. Here we aim to explore the comprehensive mycothiolome in the major respiratory pathogen Corynebacterium diphtheriae.We apply gel-based and novel MS-based thiol-redox proteomic approaches for the quantitative analysis of the S-mycothiolome in C. diphtheriae under oxidative stress conditions (e.g. NEM-Biotin-Switch-Assay). Novel genetically encoded redox biosensors (Mrx1-roGFP2 and roGFP2-Orp1) will be developed for real-time imaging of the MSH redox potential and ROS production during infections in C. diphtheriae. The role of S-mycothiolated proteins for redox regulation, fitness, stress resistance and virulence mechanisms will be investigated. Our studies provide leads to understand the physiological role of thiol-redox switches in the defense against the host immune system and in the regulation of virulence mechanisms in Gram-positive pathogens.