Enhanced inactivation of E. coli and MS-2 phage by silver ions combined with UV-A and visible light irradiation

Silver ions have been widely used as an effective water disinfectant or antimicrobial material for many decades. In addition, the application of silver ions in combination with other biocides, especially UV(254) (UV-C) irradiation, was reported to be effective in enhancing its germicidal activity. However, it is not yet known how UV-A (300-400 nm) or visible light irradiation, which have little or no antimicrobial activities, affect microorganism inactivation by silver ions. This study newly reports that the inactivation efficiencies of Escherichia coli and MS-2 phage by silver ions were significantly enhanced by UV-A or visible light irradiation. UV-A irradiation enhanced the inactivation of E. coli and MS-2 phage by 3.0 and 2.5 log/30 min, respectively, as compared with the simple summated value of individual applications of silver ions and UV-A. A similar trend was observed with visible light irradiation (>400 nm) although the level of enhancement was lessened. The photochemical reaction of silver-cysteine complex was suggested as a possible mechanism for this enhancement. Spectrophotometric and MALDI-TOF mass analyses support the fact that silver ions coupled with light irradiation causes critical cell damage through the complexation of silver ions with thiol (-SH) groups in structural or enzymatic proteins of the microorganisms and their subsequent photochemical destruction.