Title: Role of curli and plant cultivation conditions on Escherichia coli O157:H7 internalization into organic spinach grown on hydroponics and in soil Authors
Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2013
Publication Date: January 1, 2014
Citation: Macarisin, D., Patel, J.R., Sharma, V.K. 2014. Role of curli and plant cultivation conditions on Escherichia coli O157:H7 internalization into organic spinach grown on hydroponics and in soil. International Journal of Food Microbiology. 173:68-53. Interpretive Summary: Shiga-toxigenic Escherichia coli O157:H7 (EHEC) outbreaks have been linked to consumption of fresh produce. E. coli O157:H7 may be internalized into organic leafy greens via root uptake. The internalization potential of E. coli O157:H7 into organic spinach roots and subsequent transfer to the edible portions of the plant was evaluated. Further, the effect of curli, (a surface appendage),spinach cultivar, and contamination level on EHEC internalization was studied. Spinach cultivars Space and Waitiki were grown in soil and hydroponically under controlled conditions. After emergence of four true leaves, soil and hydroponics solution were inoculated with curli-expressing or curli-deficient EHEC mutants to obtain 5 or 7 log Colony Forming Units(CFU)/ml. Spinach leaves, stems, and roots were sampled on day 0, 7, 14, 21 and 35 and surface-disinfected. Incidence and populations of internalized bacteria were determined by spiral planting of tissue homogenate and 8-tube Most Probable Number (MPN) enrichment procedure. EHEC internalized into hydroponically-grown spinach roots and dispersed to the stem and leaf level. The internalization incidence was significantly affected by contamination level but not by curli expression or spinach cultivar. Further, internalization incidence was significantly greater (42.4%) in soil-grown spinach (5 log CFU/g inoculation) than in hydroponically grown spinach; probably due to extensive root damage in plants grown in soil. Current study demonstrates that internalization is influenced by the contamination level and farming practices, necessitating the pre-harvest interventions for controlling pathogens in composted manure and irrigation water. This information should be useful to other scientists, the produce industry and government action agencies.
Technical Abstract: Contamination of organic fresh produce is an important public health concern because no terminal disinfection treatment is applied during harvest or at the packing facility to kill pathogens. In addition, once contaminated, pathogens may internalize into produce and be protected from disinfectants during the postharvest processing step. The objective of the current study was to determine the potential internalization of Escherichia coli O157:H7 into organic spinach roots and subsequent transfer to the edible parts. Because curli are involved in the biofilm formation, we investigated whether it influences the internalization of E. coli O157:H7 into spinach. Further, the effect of the spinach cultivar on E. coli O157:H7 internalization was evaluated. Spinach plants were grown in contaminated soil as well as hydroponically to prevent mechanical wounding of the roots and inadvertent transfer of pathogens from the contamination source to the non-exposed plant surfaces. Results showed that E. coli O157:H7 could internalize into hydroponically grown intact spinach plants through the root system and move to the stem and even leaf level. The incidence of internalization was higher in hydroponically grown plants when roots were exposed to 7 log CFU/ml compared to those exposed to 5 log CFU/ml, indicating that contamination level was directly proportional to the incidence of the internalization. Wounding of the root system in hydroponically grown spinach increased the incidence of E. coli O157:H7 internalization and translocation to the edible portions of the plant. Experimental contamination of the plants grown in soil resulted in a greater number of internalization events then in those grown hydroponically, suggesting that E. coli O157:H7 internalization is dependent on root damage, which is more likely to occur when plants are grown in soil. Curli expression by E. coli O157:H7 did not affect its root uptake by spinach plants.