Abstract
Bootie and drag swabs may collect more microbiologically representative aggregative soil samples than composite grabs from produce fields. Previous experimental field work has identified some practical wetting agents as potential alternatives to traditionally used skim milk. This study validates the two most promising wetting agents, phosphate-buffered saline (PBS) and buffered peptone water (BPW), by comparing swabs results from 100 m tracks through a melon farm (262,000 m2), a mixed agriculture farm (leafy green, peppers, beets, 4,500 m2), and an apple orchard (114,000 m2). The mean difference between paired samples collected with BPW or PBS ranged from −0.02 ± 0.09 to 0.26 ± 0.09 log10(CFU/g) for aerobic plate count (APC) and total coliforms (TCs). Bootie and drag swabs recovered greater APCs [mean difference 0.63 ± 0.38 to 1.83 ± 0.24 log10(CFU/g) and TCs (mean difference 1.32 ± 0.96 to 5.32 ± 1.03 log10(CFU/g)], and greater prevalence of Escherichia coli compared to soil grabs (90% versus 44% of samples positive by enrichment, P < 0.001). By 16S sequencing, samples collected with PBS had greater within-sample community richness (alpha diversity) than BPW (P-values 0.041 and 0.059) but similarly overlapping taxa. Soil samples had higher within-sample (alpha) diversity (P < 0.05), but lower between-sample (beta) diversity compared to booties and drags. Overall, there was no biologically meaningful difference between the performances of the two wetting agents for bootie and drag swabs, and compared to composite soil samples, these two swab methods recovered more indicator organisms from produce field soil representing five different commodities.
DOI
https://doi.org/10.1128/spectrum.01663-25