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

Abstract

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 UV₂₅₄ (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.

Introduction

The antimicrobial action of silver ions has been widely reported against a broad spectrum of microorganisms (Chambers et al., 1962; Thurman and Gerba, 1989; Yahya et al., 1992; Yamanaka et al., 2005) along with a lack of negative effects such as taste, odor and color (Tilton and Rosenberg, 1978). For these reasons, various silver compounds have been developed and used as an effective water disinfectant or antimicrobial materials for many decades. In addition, the combination of silver ions with other biocides such as hydrogen peroxide, copper and chlorine has been shown to be more reactive as a system than silver ions alone (Landeen et al., 1989; Lin et al., 1996; Pedahzur et al., 1995; Yahya et al., 1992; Yu-sen et al., 1996).

In particular, a synergistic effect of silver ions in the presence of UV₂₅₄ irradiation has been reported. Rahn et al. (1973) reported that silver ions can accelerate the photo-dimerization of DNA using the DNA virus Haemophilus influenzae phage HPlcl (H. influenzae phage). They observed that the silver ion forms a complex with a variety of polynucleotides under UV₂₅₄ irradiation (UV-C) and then proceeds through the subsequent photochemical transformation via the triplet state intermediate. The photo-dimerization of DNA accelerated in the presence of silver ions led to the enhancement of H. influenzae phage inactivation.

Recently, Butkus et al. (2004) reported the enhanced inactivation of MS-2 phage, which is a surrogate of pathogenic RNA viruses, as silver ions were applied in combination with UV₂₅₄ irradiation. This synergism is explained by the same mechanism described previously for inactivation of the H. influenzae phage. They further examined the possibility that the combined silver–UV system could be an effective technique for drinking water disinfection by exploring the influences of various water quality parameters on MS-2 phage inactivation (Butkus et al., 2005).

However, the previous studies on the silver–UV synergism have focused only on the use of UV₂₅₄ irradiation, restraining the wide application of this technology to various environments. This is especially important since the UV₂₅₄ of solar energy consists of less than 5% of total sunlight, besides which UV₂₅₄ is already effectively antimicrobial. Meanwhile, it is not yet known whether or how UV-A (300–400 nm) and visible light irradiation, which have little or no antimicrobial activities, affect microorganism inactivation by silver ions. In addition, the behavior and mechanism of silver–UV synergism in the case of UV-A (300–400 nm) or visible light irradiation, if it exists, cannot be explained by the photo-dimerization of DNA of silver–UV₂₅₄ synergism since UV-A or visible light is scarcely absorbed by DNA.

The objectives of this study are to investigate whether the antimicrobial effect of silver ions can be enhanced in the presence of UV-A and visible light irradiation and to explore possible mechanistic interpretations if the synergistic effect is observed. Escherichia coli and MS-2 phage are chosen as surrogate microorganisms for bacteria and viruses, respectively.