Comparative evaluation of irrigation waters on microbiological safety of spinach in field

The effect of a first flush rainwater harvesting and subsurface irrigation system on E. coli and pathogen concentrations in irrigation water, soil, and produce

Climate change is stressing irrigation water sources, necessitating the evaluation of alternative waters such as harvested rainwater to determine if they meet water quality and food safety standards. We collected water, soil, and produce samples between June and August 2019 from two vegetable rain garden (VRG) sites in Frederick, Maryland that harvest rainwater using a first flush system, and deliver this water to produce through subsurface irrigation. The raised VRG beds include layers of gravel, sand, and soil that act as filters. We recorded the average surface soil moisture in each bed as well as antecedent precipitation. All water (n = 29), soil (n = 55), and produce (n = 57) samples were tested for generic E. coli using standard membrane filtration, and water samples were also tested for Salmonella spp. and Listeria monocytogenes by selective enrichment. No Salmonella spp. or L. monocytogenes isolates were detected in any water samples throughout the study period. Average E. coli levels from all harvested rainwater samples at both sites (1.2 and 24.4 CFU/100 mL) were well below the Good Agricultural Practices food safety guidelines. Only 7 % (3/44) of produce samples from beds irrigated with harvested rainwater were positive for E. coli. E. coli levels in soil samples were positively associated with average surface soil moisture (r² = 0.13, p = 0.007). Additionally, E. coli presence in water samples was marginally associated with a shorter length of antecedent dry period (fewer days since preceding rainfall) (p = 0.058). Our results suggest that harvested rainwater collected through a first flush system and applied to produce through subsurface irrigation meets current food safety standards. Soil moisture monitoring could further reduce E. coli contamination risks from harvested rainwater-irrigated produce. First flush and subsurface irrigation systems could help mitigate climate change-related water challenges while protecting food safety and security.

Neutral electrolyzed oxidizing water is effective for pre-harvest decontamination of fresh produce

Pre-harvest sanitization of irrigation water has potential for reducing pathogen contamination of fresh produce. We compared the sanitizing effects of irrigation water containing neutral electrolyzed oxidizing water (EOW) or sodium hypochlorite (NaClO) on pre-harvest lettuce and baby spinach leaves artificially contaminated with a mixture of Escherichia coli, Salmonella Enteritidis and Listeria innocua (~1 × 108 colony-forming units/mL each resuspended in water containing 100 mg/L dissolved organic carbon, simulating a splash-back scenario from contaminated soil/manure). The microbial load and leaf quality were assessed over 7 days, and post-harvest shelf life evaluated for 10 days. Irrigation with water containing EOW or NaClO at 50 mg/L free chlorine significantly reduced the inoculated bacterial load by ≥ 1.5 log10, whereas tap water irrigation reduced the inoculated bacterial load by an average of 0.5 log10, when compared with untreated leaves. There were no visual effects of EOW or tap water irrigation on baby spinach or lettuce leaf surfaces pre- or post-harvest, whereas there were obvious negative effects of NaClO irrigation on leaf appearance for both plants, including severe necrotic zones and yellowing/browning of leaves. Therefore, EOW could serve as a viable alternative to chemical-based sanitizers for pre-harvest disinfection of minimally processed vegetables.

Persistence of Escherichia coli O157:H12 and Escherichia coli K12 as Non-pathogenic Surrogates for O157:H7 on Lettuce Cultivars Irrigated With Secondary-Treated Wastewater and Roof-Collected Rain Water in the Field

Treated wastewater (TW) and roof-collected rain water (RW) that meet the required microbial quality as per Food Safety Modernization Act (FSMA) regulation may serve as alternative irrigation water sources to decrease the pressure on the current water scarcity. Alternative water sources may have different water characteristics that influence the survival and transfer of microorganisms to the irrigated produce. Further, these water sources may contain pathogenic bacteria such as Shiga-toxigenic Escherichia coli. To evaluate the risk associated with TW and RW irrigation on the fresh produce safety, the effect of TW and RW irrigation on the transfer of two non-pathogenic E. coli strains as surrogates for E. coli O157:H7 to different lettuce cultivars grown in the field was investigated. Lettuce cultivars “Annapolis,” “Celinet,” and “Coastline” were grown in the field at the Fulton farm (Chambersburg, PA). Approximately 10 days before harvest, lettuce plants were spray-irrigated with groundwater (GW), TW, or RW containing 6 log CFU ml−1 of a mixture of nalidixic acid-resistant E. coli O157:H12 and chloramphenicol-resistant E. coli K12 in fecal slurry as non-pathogenic surrogates for E. coli O157:H7. On 0, 1, 3, 7, and 10 days post-irrigation, four replicate lettuce leaf samples (30 g per sample) from each group were collected and pummeled in 120 ml of buffered peptone water for 2 min, followed by spiral plating on MacConkey agars with antibiotics. Results showed that the recovery of E. coli O157:H12 was significantly greater than the populations of E. coli K12 recovered from the irrigated lettuce regardless of the water sources and the lettuce cultivars. The TW irrigation resulted in the lowest recovery of the E. coli surrogates on the lettuce compared to the populations of these bacteria recovered from the lettuce with RW and GW irrigation on day 0. The difference in leaf characteristics of lettuce cultivars significantly influenced the recovery of these surrogates on lettuce leaves. Populations of E. coli O157:H12 recovered from the RW-irrigated “Annapolis” lettuce were significantly lower than the recovery of this bacterium from the “Celinet” and “Coastline” lettuce (P < 0.05). Overall, the recovery of specific E. coli surrogates from the RW and TW irrigated lettuce was comparable to the lettuce with the GW irrigation, where GW served as a baseline water source. E. coli O157:H12 could be a more suitable surrogate compared to E. coli K12 because it is an environmental watershed isolate. The findings of this study provide critical information in risk assessment evaluation of RW and TW irrigation on lettuce in Mid-Atlantic area.