The invention relates to an antimicrobial agent where a cysteine compound is covalently bound to a substrate, in particular by binding through an S--S bridge via a spacer molecule to the substrate. The spacer comprises a carbon chain, optionally interrupted by one or more heteroatoms, e.g. O, S, N, P and Si; the chain is optionally substituted with one or more alkyl groups, preferably lower alkyl groups with 1-5 carbon atoms, hydroxyl groups or alkoxy groups. Also, the invention refers to a substrate that is coated with the antimicrobial agent of the invention. The agent has excellent antimicrobial properties and can be used to coat surfaces and substrates of various devices, such as medical devices or devices used in food handling, in order to prevent or inhibit accumulation and/or growth and/or proliferation and/or the viability of microorganisms and/or formation of biofilm.