Purpose: Factors that contribute to the presence of Salmonella including, soil moisture, use of poultry litter (PL), and protection from UV radiation by use of plastic mulch are evaluated.
Methods: Soil samples were collected from 12 field plots, half of which contained PL over 120 days from May to October. Two composite soil samples of 150 g were collected every seven days from each plot; under plastic (UP) or outside of plastic (NP). Samples (10 g) were analyzed for Salmonella using modified FDA BAM method, with incubation in Universal Preenrichment broth at 37°C for 24 hours, followed by tetrathionate (TT) broth or Rappaport-Vassiladis (RV) broth for 24 hours at 42°C, after which samples were plated on XLT-4 agar. Moisture content of each soil sample was determined in an oven at 105°C for 24 hours. Statistical analysis conducted using an unpaired t-test.
Results: Control plot samples on day zero were negative for Salmonella. Of the 348 analyzed soil samples, 309 where positive for Salmonella. Samples enriched using TT were significantly more likely to be positive for Salmonella compared to RV (P<0.00005). Soil samples collected from UP had significantly higher (P<0.0001) moisture than those from NP. Average moisture content for UP samples was 14.09±3.93% and average moisture content for NP was 9.76±5.52%. Moisture content of UP samples from soil with or without PL showed no obvious significant difference in moisture (P=0.35). Salmonellafrom UP samples ranged from 0.04 to 3.32 log CFU/g dry weight over the study.
Significance: While it may be intuitive to link soil moisture with Salmonella persistence, Salmonella survival in field plots is variable subject to other factors, like climate, wildlife, heat, and UV.