Studies aimed at understanding Escherichia coli O157:H7 soil survival dynamics are paramount due to their inevitable introduction into the organic vegetable production systems via animal manure-based fertilizer. Therefore, a greenhouse study was conducted to determine the survival of E. coli O157:H7 in highly controlled soil matrices subjected to two variable environmental stressors: (1) soil volumetric water content (25 or 45 % VWC), and (2) the growth of clover (planted or unplanted). During the 7-week study, molecular-based qPCR analyses revealed that E. coli O157:H7 survival was significantly lower in soils maintained at either near water-holding capacity (45 % VWC) or under clover growth. The significant reduction under clover growth was only observed when E. coli populations were determined relative to all bacteria, indicating the need to further study the competition between E. coli O157:H7 and the total bacterial community in organic soils. Given the significant effect of clover on E. coli O157:H7 survival under different moisture condi- tions in this greenhouse-based study, this work highlights the antimicrobial potential of clover exudates in arable soils, and future work should concentrate on their specific mechanisms of inhibition; ultimately leading to the devel- opment of crop rotations/production systems to improve pre-harvest food safety and security in minimally pro- cessed, ready-to-eat and organic production systems.
According to the results of this study, qPCR-based estimates are highly correlated to the direct plating ‘‘gold standard’’, which is important for determining the efficacy of using these higher-throughput, faster molecular-based assays in future soil-based studies on the survival of E. coli O157:H7. Two important findings resulted from this con- trolled greenhouse study, with potential broader implications that need to be studied in field-based systems. First, looking at E. coli O157:H7 estimates in relation to the survival of the entire bacterial community (EC/TB) should be further investigated as a method to assess E. coli survival in soils. As was observed in the lower VWC treatments, E. coli O157:H7 can become a significantly larger portion of the overall bacterial community even if their numbers overall are significantly decreased, indirectly concentrating E. coli O157:H7 within the soil and potentially resulting in a greater human health risk. Lastly, given the significant effect of clover on E. coli O157:H7 survival, future work should focus on the biochemical makeup of clover exudates in agricultural soils and their specific mechanisms of inhibition within the soil. Future testing of these concepts under field-scale conditions could potentially lead to the development of crop rotations/best management practices to accentuate the antibacterial effects of clover towards not only E. coli O157:H7, but other important soil-associated zoonotic human pathogens.