Microbiologically it was demonstrated that amino acids, e.g. cysteine (CySH), and other compounds, e.g. sodium thioglycollate, containing thiol groups neutralized the activity of silver nitrate against Pseudomonas aeruginosa PAO1. Amino acids with disulphide bonds were inactive, with the exception of L-cystine dimethyl ester, as were all amino acids with no sulphur groups. Iodoacetamide reacted with CySH to produce a CyS-acetamide complex that was unable to quench the activity of Ag+. Chemical analyses using cyclic voltammetry demonstrated that high coordination numbers (3.1) were obtained with thiol-containing amino acids and low numbers (0.28-0.4) with other amino acids. Both microbiologically and chemically, the results imply that interaction of Ag+ with thiol groups plays an essential role in bacterial inactivation.