# Validating aggregative soil sampling using bootie and drag swabs hydrated with simple wetting agents in commercial produce fields

## TL;DR

Bootie and drag swabs with PBS or BPW collect more representative soil samples than traditional methods in commercial produce fields.

## Contribution

Validation of PBS and BPW as practical wetting agents for bootie and drag swabs in commercial agricultural settings.

## Key findings

- Bootie and drag swabs recovered more indicator organisms than composite soil grabs.
- PBS and BPW performed similarly in collecting microbiologically representative soil samples.
- Swab samples showed higher within-sample diversity but lower between-sample diversity compared to soil grabs.

## 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.

Bootie and drag swabs have emerged as a promising alternative to composite soil grabs for produce industry use for improved preharvest soil sampling for safety and quality purposes. This study builds on previous work in research field trials to identify practical wetting agents for the bootie and drag swabs because the skim milk powder classically used in animal operations is not practical for produce. Here, we validate the two most promising, practical wetting agents previously identified (phosphate-buffered saline and buffered peptone water), testing their use in commercial settings, sampling soil from melon, leafy green, beet, pepper, and apple production systems. By showing the wetting agents perform similarly, and that booties and drag swabs are at least as good or better than composite grabs at recovering indicator organisms, we have identified a viable method for improved agricultural soil sampling for microbiological profiling.