Effect of zero-valent iron on the removal of Escherichia coli O157:H7 from water

In the past decade, outbreaks of Escherichia coli O157:H7 have been increasingly associated with leafy green vegetables. The ability of producers to deliver a fresh, safe, ready-to-eat product has come under intense scrutiny. Numerous studies have shown that current commercial sanitizers used at U.S. Food and Drug Administration-approved concentrations can only achieve a 1- to 2-log reduction in microbial populations and may also create by-products that are harmful to humans and the environment. In September 2006, one of the largest recorded outbreaks of E. coli O157:H7 occurred involving pre-bagged spinach grown in the Salinas Valley in California. Contaminated water was thought to be a possible source of contamination in this outbreak. Escherichia coli O157:H7 has been shown to survive for long periods in water and is known to be passed from the manure of infected animals such as cattle or pigs into lakes, rivers, and groundwater through runoff and leaching.;For many years, Zero-valent iron (ZVI) has been used for remediation of groundwater contaminated with chlorinated compounds, especially trichloroethene (TCE) and lesser amounts of degradation by-products such as dichloroethene (DCE). ZVI has also recently been shown to be successful in removing viruses from water, while nanoparticles of ZVI have been investigated and have demonstrated some positive results in the inactivation of generic E. coli strains.;A novel water filtration system using ZVI was investigated as a simple and inexpensive approach to reducing E. coli O157:H7 in water for both pre-and post-harvest processes. This study was initiated to determine the effectiveness of ZVI in the removal and inactivation of E. coli O157:H7 from water used for washing fresh-cut spinach. Breakthrough curves at pH values of 5.0, 7.5, and 9.0 showed reduction of 1-2 log cfu/ml of E. coli O157:H7 by the inclusion of ZVI compared to sand. After dissecting and investigating the columns, bacterial survival was observed to be 2.38 +/- 0.67 log cfu/ml less in ZVI than in the sand column at identical locations. These experiments were performed with feshly-packed columns, aged one month or less.;Mature columns (aged > 1 month) were found to be more effective in lowering survival of E. coli O157:H7 in water by ≥ 5 log cfu/ml, and were used in the following experiments. Dip and spot inoculations were performed using inoculated, ZVI-treated water on spinach. The ZVI-tread E. coli O157:H7 showed almost no survival as compared to E. coli O157:H7 recovered from spinach dipped in or spot inoculated with sand column-treated water. Levels of E. coli O157:H7 recovered from treated spinach leaves went from 3.59 +/- 0.27 log cfu/ml initial concentration down to 0.25 +/- 0.50 log cfu/ml and 3.69 +/- 1.98 log cfu/ml initial concentration down to 0.23 +/- 0.40 log cfu/ml for each method, respectively. As determined with a vero toxin assay, toxin was produced by ZVI-treated E. coli O157:H7 only after an enrichment and recovery period. ZVI-treated cells from mature columns were shown to produce less toxin than control cells. Storage tests showed survival of ZVI-treated E. coli O157:H7 for 7 days with cells detected only after enrichment.;ZVI is a simple, inexpensive, yet highly effective technology that has been shown to reduce E. coli O157:H7 in water. It has great potential for reducing water contamination and can be used to reduce or eliminate the risk of E. coli O157:H7 on produce such as lettuce, spinach, and other leafy greens which have been implicated in past outbreaks. The use of this technology has many far reaching applications including, but by no means limited to, decontaminating irrigation and wash water used for fresh produce.