E coli
Protect your home and family from bacteria E coli
Antibacterial Activity and Mechanism of Action of the Silver Ion in Staphylococcus aureus and Escherichia coli......
The antibacterial effect and mechanism of action of a silver ion solution that was electrically generated were investigated for Staphylococcus aureus and Escherichia coli by analyzing the growth, morphology, and ultrastructure of the bacterial cells following treatment with the silver ion solution. Bacteria were exposed to the silver ion solution for various lengths of time, and the antibacterial effect of the solution was tested using the conventional plate count method and flow cytometric (FC) analysis. Reductions of more than 5 log10 CFU/ml of both S. aureus and E. coli bacteria were confirmed after 90 min of treatment with the silver ion solution. Significant reduction of S. aureus and E. coli cells was also observed by FC analysis; however, the reduction rate determined by FC analysis was less than that determined by the conventional plate count method. These differences may be attributed to the presence of bacteria in an active but nonculturable (ABNC) state after treatment with the silver ion solution. Transmission electron microscopy showed considerable changes in the bacterial cell membranes upon silver ion treatment, which might be the cause or consequence of cell death. In conclusion, the results of the present study suggest that silver ions may cause S. aureus and E. coli bacteria to reach an ABNC state and eventually die.
Since ancient times, the silver ion has been known to be effective against a broad range of microorganisms. Today, silver ions are used to control bacterial growth in a variety of medical applications, including dental work, catheters, and the healing of burn wounds (17, 30, 31). Silver ions are also used for a number of nonmedical purposes, such as in electrical appliances (14, 36). The slow-release “nanosilver” linings of laundry machines, dishwashers, refrigerators, and toilet seats are also marketed and advertised. It is clear that we are exposed to a wide range of mostly unfamiliar uses of silver-containing products intended to function as antimicrobial biocides. Therefore, it is necessary to elucidate the antimicrobial activity of the silver ion, which is widely used in these products.
The mechanism of the antimicrobial action of silver ions is closely related to their interaction with thiol (sulfhydryl) groups (1, 5, 9, 10), although other target sites remain a possibility (27, 34). Amino acids, such as cysteine, and other compounds containing thiol groups, such as sodium thioglycolate, neutralized the activity of silver against bacteria (18). By contrast, disulfide bond-containing amino acids, non-sulfur-containing amino acids, and sulfur-containing compounds, such as cystathione, cysteic acid, l-methionine, taurine, sodium bisulfate, and sodium thiosulfate, were all unable to neutralize the activity of silver ions. These and other findings imply that the interaction of silver ions with thiol groups in enzymes and proteins plays an essential role in its antimicrobial action, although other cellular components, like hydrogen bonding, may also be involved (10). Silver was also proposed to act by binding to key functional groups of enzymes. Silver ions cause the release of K+ ions from bacteria; thus, the bacterial plasma or cytoplasmic membrane, which is associated with many important enzymes, is an important target site for silver ions (9, 22, 25, 29).
Effect of the silver ions on the bacterial reduction rate.
The antibacterial effects of the silver ion solution at different concentrations of silver ions against S. aureus and E. coli bacteria as determined by the conventional plate count technique are shown in Fig. 2. The total number of S. aureus bacteria was reduced by over 5 log10 CFU/ml after treatment with the original silver ion solution (0.2 ppm) for 90 min, demonstrating that the antibacterial activity of the silver ion solution was significantly greater than that of PBS treatment (P < 0.05). The E. coli bacterial count was reduced from the inoculum size (105 CFU/ml) to the limit of detection (<20 CFU/ml) within 30 min at a silver ion concentration of 0.2 ppm. All of the tested silver ion solutions (0.2, 0.1, and 0.05 ppm) significantly eliminated E. coli cells in comparison to PBS treatment (P < 0.05).
FC analysis in conjunction with a BacLight kit was also performed to examine the antibacterial effect of the original silver ion solution (0.2 ppm) against S. aureus and E. coli bacteria in terms of damage to the cell membrane, shown in different colors (green in live cells and red in damaged or dead cells). In addition, CFDA staining was used for the enumeration of esterase-active bacteria because CFDA is cell permeant and undergoes hydrolysis of the diacetate groups into fluorescent carboxyfluorescein by intracellular nonspecific esterases. Based on the side light scatter and green (FL1) fluorescence, the R1 and R2 gates were used to identify live and damaged or dead cells, respectively. The proportions of damaged or dead cells (both S. aureus and E. coli) in the silver ion solution-treated groups were significantly greater (P < 0.05) at 30 min, 1 h, 1.5 h, and 2 h of treatment than with the control (PBS) groups (Fig. 3 and 4). Longer treatment times (from 30 min to 2 h) had a positive effect on the antibacterial effect of the silver ion solution (P < 0.05); however, there were no significant differences in the proportions of live or dead cells when both S. aureus and E. coli cells were treated with the silver ion solution for 2 or 3 h (P > 0.05).
The antibacterial-efficacy results determined by conventional plate count and FC analyses are compared in Fig. 5. For the PBS-treated control group and silver ion-treated experimental groups tested, both the BacLight kit and the CFDA assay gave similar antibacterial efficacies (P > 0.05). The number of physiologically active bacteria enumerated by FC analysis in conjunction with the BacLight kit or the CFDA assay was relatively higher than the bacterial count determined by conventional plate counting (P < 0.05), except for E. coli bacteria after 2 and 3 h of treatment. This difference appeared to be nonlinear across different treatment times, suggesting that the difference in antibacterial efficacy determined by the two analyses decreased as the silver ion treatment times approached 2 and 3.
Acknowledgments
We thank Young Hwan Paik for his technical assistance.
This study was supported by Korea Research Foundation grants (KRF-2006-005-J02903 and KRF-2007-331-E00254), a grant from the Technology Development Program for Agriculture and Forestry provided by the Ministry of Agriculture and Forestry (grant no. 305003-3), and the Korea Bio-Hub Program of the Korea Ministry of Commerce, Industry Energy (2005-B0000002). Additional support was provided by the Research Institute of Veterinary Science, Department of Veterinary Microbiology, College of Veterinary Medicine, and the BK21 Program for Veterinary Science, Seoul National University.
