Purpose: To assess the effect of sample size on the reliability of pathogen detection schemes in agricultural soils.
Methods: Soil samples were screened for Salmonella enterica and enterohemorrhagic Escherichia coli (EHEC), respectively. Thirteen fields from one ranch were monitored for persistence of a natural S. enterica contamination following application of inadequately managed compost. A single field affected by animal intrusion was screened for EHEC presence. Standard testing was performed using 10 g of soil, enrichment, and PCR screening. Soil samples from initially positive fields were comparatively analyzed using 10 g and 100 g directly- enriched and 100 g soil extraction with sodium phosphate supplemented with 0.1% Tween 20 (NPT). Selected negative fields, based on 10 g samples, were retested with 100 g from the same retained soil sample.
Results: Detection using standard 10 g was achieved in 45 of 252 samples, from 5 of 13 fields. S. enterica detection using 10 g and 100 g directly enriched had 29.2% and 62.5% positives, respectively. Contrary to other applications of NPT to soil, pre-enrichment extraction on 100 g resulted in 4.2% S. enterica positives. All 10 g positives tested at 100 g sample size increased the frequency of positives among replicates and previously negative lots had positive outcomes among the replicate samples. Detection of EHEC in a different soil using 10 g direct, 100 g NPT extraction, and 100 g direct-enrichment had 20%, 25% and 35% positives, respectively.
Significance: As part of a valid soil sampling plan, sample mass is a significant determinant for field risk assessment and pre-planting standards. Increasing the standard sample size to 100 grams may increase the chance of detecting low level pathogen contamination.