, 2004; Vo et al., 2006). In 2007, approximately 50 individuals living

in Norway, Denmark and Finland became infected with S. Weltevreden due to the consumption of alfalfa sprouts (Emberland et al., 2007). Seeds contaminated with S. Weltevreden bought from producers in infested countries were identified as the source of the outbreak, indicating that this bacterial strain is able to survive on plant seeds for prolonged periods. As S. Weltevreden 2007-60-3289-1 appears to RG-7204 have great potential as a food safety hazard, this strain was selected for evaluation of its capability to persist and survive in soil and spread onto spinach plant roots and leaves. The S. enterica ssp. enterica serovar Weltevreden strain 2007-60-3289-1, isolated from Danish alfalfa sprouts in 2007 (Emberland et al., 2007), was provided by Dr Annette Nygaard Jensen (DTU-FOOD, Denmark) and used in the current experiments. Salmonella enterica serovar Weltevreden was grown in Luria–Bertani medium (1 L: 10 g tryptone, 5 g yeast extract, 5 g NaCl) and incubated at 37 °C overnight until an OD600 nm of approximately 0.9 (early exponential phase) was reached. For inoculation of slurry and soil, bacteria were harvested, washed three times with 0.9% NaCl and resuspended in

0.9% NaCl. Cattle manure slurry (Table 1) was collected at an organic farm in Sandviken, Sweden, and stored at 4 °C for 4 weeks until use. Clay loam soil (Table 1) was collected at a biodynamic CAL-101 mouse farm in Järna, Sweden, and stored at 4 °C for 4 weeks until use. Soil was collected from a 1 × 1 m square at a depth of approximately 0–20 cm, sieved (2 mm) and mixed before use. Chemical analyses were performed by Eurofins Lab (Kristianstad, Sweden). Two separate experiments were performed (A and B). In Experiment A, S. Weltevreden was inoculated into cattle slurry at three different concentrations corresponding

to 104, 105 and 106 cells g−1 soil before addition to soil that was subsequently planted with spinach seeds. A 220-mL aliquot of cattle slurry was mixed with a 22-mL bacterial suspension or 0.9% NaCl and added to 3 kg of soil. Each pot received 130 g of the mixture, and six organically Farnesyltransferase produced spinach seeds (Spinacia oleracea variety Gamma) were sown at a depth of approximately 2 cm. In Experiment B, S. Weltevreden was washed and resuspended in 0.9% NaCl and inoculated directly into the soil, 14 days after sowing at a bacterial density of 106 cells g−1 soil. Similar proportions of soil and slurry as in Experiment A were mixed, but all samples received 0.9% NaCl solution, and spinach seeds were sown in the soil/manure/saline mixture. Fourteen days after sowing, each pot in Experiment B received a 10-mL suspension of S. Weltevreden in 0.9% NaCl to obtain an approximate bacterial concentration of 106 cells g−1 soil. The suspensions were carefully added to soil around the plant and the lowest 2 cm of the stems. Both experiments included a nonbacterial control with 0.