Chlorine dioxide is a broad-spectrum disinfectant against Shiga toxin-producing Escherichia coli and Listeria monocytogenes in agricultural water

Abstract

Agricultural water is commonly treated with chlorine-based disinfectants, which are impacted by water quality. Understanding how water quality influences disinfectants such as chlorine dioxide (ClO₂) against pathogenic bacteria is important for creating efficacious sanitation regimens. In this study, the minimum inhibitory concentration (MIC) of ClO₂ needed to achieve a 3-Log reduction against Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes was compared across agricultural water samples. Sterile ddH₂O served as a control to compare with environmental samples from Salinas Valley, CA, and laboratory standards. To test different dosages and water qualities, stock ClO₂ was diluted in 24-well plates with target concentrations of 10, 5, 2.5, and 1.25 mg/L. Well plates were inoculated with pathogens and treated with sanitizer for 5 min. Following treatment, surviving pathogens were enumerated using viable cell counts. The results demonstrate that groundwater samples had the highest water quality of the environmental samples and required the lowest concentration of disinfectant to achieve 3-Log reduction against both bacteria, with MIC between 1.4 and 2.0 mg/L. Open-source samples had lower water quality and required a higher concentration of ClO₂ for 3-Log reduction, with MIC between 2.8 and 5.8 mg/L for both pathogens. There was no correlation between pH, turbidity, or conductivity/TDS and reduction for either STEC or L. monocytogenes, suggesting no individual water metric was driving reduction. A lower dosage was required to achieve 3-Log reduction against STEC, while L. monocytogenes required greater concentrations to achieve the same level of reduction. Overall, these results help guide growers in using ClO₂ as a broad-spectrum disinfectant and demonstrate its efficacy in reaching 3-Log reduction across agricultural water samples.

Keywords: Escherichia coli; Listeria monocytogenes; agricultural water; chlorine dioxide; minimum inhibitory concentration; water treatment.

FIGURE 1

STEC reduction (log CFU/ml ± SD) after a 5-min exposure to a gradient of chlorine dioxide dosages in seven water samples. Dosage (mg/L) is shown on the left of each panel at 10, 5, 2.5, and 1.25. Black dots represent the individual data points. Means (N = 3) with similar letter designations represent similar reductions across water samples as determined by two-way ANOVA and Tukey post hoc tests (α = 0.05). AW, agricultural well; DW, domestic well; EPA_6.5/EPA_8.4, laboratory standards with adjusted pH; ENV₁/ENV₂, open source environmental samples; ddH₂0, double distilled water.

FIGURE 2

Listeria monocytogenes reduction (log CFU/ml ± SD) following 5-min exposure to a gradient of chlorine dioxide dosages in seven water samples. Dosage (mg/L) is shown on the left of each panel at 10, 5, 2.5, and 1.25. Black dots represent individual data points. Means (N = 3) with similar letter designations represent similar reductions across water samples as determined by two-way ANOVA and Tukey post hoc tests (α = 0.05). AW, agricultural well; DW, domestic well; EPA_6.5/EPA_8.4, laboratory standards with adjusted pH; ENV₁/ENV₂, open source environmental samples; ddH₂0, double distilled water.

FIGURE 3

Water metrics (mean ± SD, N = 2) compared between summer (dark) and winter (light) seasons using students t-test. Summer sampling occurred between June and August, and winter sampling was conducted between November and January. Water was sampled from the Salinas River near Gonzales, CA, and tested on-site for pH, temperature, conductivity, chlorine, turbidity, and total dissolved solids—sampling site based on Gorski et al. (2022). *Represent significant differences (p < 0.05) between seasons.

FIGURE 4

Water metrics (mean ± SD, N = 2) compared between summer (dark) and winter (light) seasons using students t-test. Summer sampling occurred between June and August, and winter sampling was conducted between November and January. Water samples were collected from San Jon Rd. canal near Salinas, CA, and tested on-site for pH, temperature, conductivity, chlorine, turbidity, and total dissolved solids. Sampling sites were based on Gorski et al. (2022). *Represent significant differences (p < 0.05) between seasons.

FIGURE 5

Comparing Escherichia coli Most Probable Number (MPN) (mean ± SD) between seasons at open-source sites E1 and E2 in Salinas Valley, CA (N = 2) using a student’s t-test. Summer (dark) season sampling occurred between June and August. Winter (light) season sampling occurred between November and January. *Represent significant differences (p < 0.05) between seasons.