Sanitizer Resistance and Persistence of Listeria monocytogenes Isolates in Tree Fruit Packing Facilities

The foodborne pathogen Listeria monocytogenes can persist in produce processing environments, which increases the risk for food contamination. Increased resistance to antimicrobials commonly used in cleaning and sanitizing procedures may contribute to L. monocytogenes' persistence in these environments. This study aimed to evaluate sanitizer resistance in L. monocytogenes isolates collected from three tree fruit packing facilities (F1, F2, and F3) during packing seasons 2020-2021 (Y1) and 2021-2022 (Y2), and to assess evidence of persistence based on the genomic similarity of isolates to historical isolates collected in previous years. L. monocytogenes isolates collected in 2020-2022 (n = 44) were tested for resistance to peroxyacetic acid (PAA) and a proprietary biofilm-removing agent using a broth microdilution assay. Further, L. monocytogenes isolates were whole genome sequenced and screened for the presence of antimicrobial resistance and virulence genes, as well as to assess the genomic similarity of isolates using the CFSAN SNP bioinformatic pipeline. Over half (57%) of the tested isolates had a PAA minimum inhibitory concentration (MIC) of 250 ppm, which was similar to the applied concentration of the PAA sanitizer in the three facilities (230 ppm). In contrast, 80% of tested isolates had a biofilm remover MIC of 0.13 ppm, which was substantially below the concentration applied in the facilities (137 ppm). Genomes of all tested isolates carried antimicrobial resistance (fosX, lin, mdrL, mprF, and norB) and virulence (inlA, inlB, plcA, plcB, prfA, hly, mpl, and iap) genes. L. monocytogenes isolates collected between 2020 and 2022 belonged to three distinct lineages, with 22 multilocus sequence types (MLSTs) belonging to 22 different clonal complexes. Genomic similarity analysis with historical isolates collected from the same facilities in 2016-2017 demonstrated a 5-year persistence of the genotypes ST 1003 and ST 554 in F2, which were no longer detected in 2022. Overall, our results highlight the need to re-evaluate sanitizer concentrations to effectively control persistent L. monocytogenes strains in tree fruit packing facilities.