Rapid large-volume concentration for increased detection of Escherichia coli O157:H7 and Listeria monocytogenes in lettuce wash water generated at commercial facilities

Abstract

In pilot-scale testing, dead-end ultrafiltration concentration (DEUF-C) sampling increased detection of pathogens in lettuce wash water. This study investigated DEUF-C performance when scaled up to commercial processing levels. Two sets of experiments were done to: 1) compare pathogen detection probability in lettuce wash water generated from commercial pilot-scale processing using DEUF-C versus standard grab sampling, and 2) evaluate DEUF-C to concentrate pathogens inoculated into commercially-generated flume and centrifuge lettuce wash water. In pilot-scale runs, one lettuce head inoculated to contain 2–8 × 104 CFU of Escherichia coli O157:H7 and 3–10 × 105 CFU of Listeria monocytogenes was processed along with 907 kg of uninoculated lettuce using sanitizer-free tap water. Two to four 40-L volumes of the wash water were concentrated to 400 mL by DEUF-C, and pathogen detection probability in these samples was compared to standard grab samples after 24 h–48 h of enrichment using qPCR. In the second set of trials, chlorine in flume and centrifuge water from a local commercial processing facility was neutralized with sodium thiosulfate (100 mg/L), spiked with both E. coli O157:H7 and L. monocytogenes at 102 CFU/mL, and processed by DEUF-C. Total filterable volume was determined for DEUF-C samples, and detection (qPCR) probabilities in unenriched DEUF-C and grab samples were determined. E. coli O157:H7 and L. monocytogenes were detected in 100% of the DEUF-C samples from pilot-scale water samples. Comparable grab samples yielded 6.7% detection for E. coli O157:H7, and 20 and 60% for 24 h- and 48 h-enriched L. monocytogenes samples, respectively. The total filterable volume in commercially-generated waters was significantly higher for flume than for centrifuge water. All unenriched DEUF-C samples yielded E. coli O157:H7 and L. monocytogenes compared to 11.1 and 22.2%, respectively, for the unenriched grab samples. These findings indicate DEUF-C can improve the probability of detecting E. coli and Listeria in commercial wash water. However, filtration of commercial lettuce wash water remains challenging, with more work needed to ensure DEUF-C is feasible for commercial use.