Enhanced inactivation of E. coli and MS-2 phage by silver ions combined with UV-A and visible light irradiation
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.
Section snippets
Reagents
AgNO3 and l–cysteine (cysteine) were obtained from Sigma-Aldrich Co. (USA) with purity higher than 99%. All chemicals for solutions were of reagent grade and were used without further purification. All stock solutions were prepared in distilled and deionized water (Barnstead NANO Pure, USA). Glassware was soaked in HNO3 solution overnight to remove the absorbed silver, washed with distilled water and autoclaved at 121 °C for 15 min prior to use. For the disinfection experiments, an aqueous stock
Inactivation of microorganisms with silver ions and light irradiation
Fig. 2(a) and (b) shows the inactivation curves of E. coli and MS-2 phage, respectively, by silver ions, UV-A irradiation and their combined application. As shown in Fig. 2(a), visible light and UV-A irradiation alone have negligible effects on E. coli inactivation, whereas silver ions alone resulted in approximately 1.5 log inactivation of E. coli in 30 min. A marked enhancement of the inactivation efficiency was observed by the combined use of silver ions with light irradiation. Approximately
Discussion
Two major explanations are available for the inactivation of microorganisms by silver ions. The first explanation involves the interaction of silver ions with the DNA of microorganisms. Fox and Modak (1974) pointed out that the antimicrobial activity of silver ions is likely due to cross-linking of the DNA helix, and Izatt et al. (1971) and Arakawa et al. (2001) reported that the silver ion preferentially binds the nitrogen atom of guanine in DNA, thus preventing cell replication.
As described
Conclusions
Major conclusions in this study can be summarized as follows:
- 1.
This study reported for the first time that the inactivation of E. coli and MS-2 phage by silver ions was significantly enhanced in the presence of UV-A and visible light irradiation.
- 2.
The photochemical destruction of the silver–cysteine complex and subsequent formation of monosulfide radicals were suggested to be responsible for the observed enhanced inactivation, and the spectrophotometric and MALDI-TOF mass spectroscopic studies of
Acknowledgment
This research was partially supported by the Brain Korea 21 Program (of the Ministry of Education). This support is greatly appreciated.
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