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

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 m 2 ), a mixed agriculture farm (leafy green, peppers, beets, 4,500 m 2 ), and an apple orchard (114,000 m 2 ). The mean difference between paired samples collected with BPW or PBS ranged from −0.02 ± 0.09 to 0.26 ± 0.09 log 10 (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 log 10 (CFU/g) and TCs (mean difference 1.32 ± 0.96 to 5.32 ± 1.03 log 10 (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. IMPORTANCE 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.