Purpose: The internalization potential of Escherichia coli O157:H7 (EHEC) into organic spinach roots and subsequent transfer to the edible portions of the plant was evaluated. Further, the effect of curli, spinach cultivar, and contamination level on EHEC internalization was studied.
Methods: 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 CFU/ml. Spinach leaves, stems, and roots were sampled on day 0, 7, 14, 21 and 35 and surface-disinfected by mercury chloride (0.1%). Incidence and populations of internalized bacteria were determined by spiral planting of tissue homogenate and 8-tube MPN enrichment procedure.
Results: EHEC internalized into hydroponically-grown spinach roots and dispersed to the stem and leaf level. A significantly (P ≤ 0.05) higher internalization incidence, 27.7% (n = 216) was observed in hydroponically-grown spinach inoculated with 7 log CFU/ml compared to that inoculated with 5 log CFU/ml (17.3%, n = 108). Internalization incidence was significantly greater (42.4%, n = 144) 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. Spinach cultivars did not influence EHEC internalization as evidenced from 49.1% (n = 216) and 50.9% (n = 216) internalization incidence for Space and Waitiki cultivars, respectively. Curli expression did not influence EHEC internalization into spinach.
Significance: 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.