The aim of this study was to investigate the basis of the putative persistence of Listeria monocytogenes in a new industrial facility dedicated to the processing of ready-to-eat (RTE) Iberian pork products. Quaternary ammonium compounds, which included benzalkonium chloride (BAC), were repeatedly used as surface disinfectants in the processing plant. Clean and disinfected surfaces were sampled to evaluate if resistance to disinfectants was associated with persistence. Of the 14 isolates obtained from product contact and non-product contact surfaces, only five different pulsed-field gel electrophoresis (PFGE) types were identified during the 27-month study period. Two of these PFGE types (S1 and S10-1) were previously identified to be persistent and BAC-resistant (BAC r ) strains in a geographically separate slaughterhouse belonging to the same company. The remaining three PFGE types, which were first identified in this study, were also BAC r . Whole-genome sequencing and in silico multilocus sequence typing (MLST) analysis of five BAC r isolates of the different PFGE types identified in this study showed that the isolate of the S1 PFGE type belonged to MLST sequence type 31 (ST31), a low-virulence type characterized by mutations in the inlA and prfA genes. The isolates of the remaining four PFGE types were found to belong to MLST ST121, a persistent type that has been isolated in several countries. The ST121 strains contained the BAC resistance transposon Tn6188. The disinfection-resistant L. monocytogenes population in this RTE pork product plant comprised two distinct genotypes with different multidrug resistance phenotypes. This work offers insight into the L. monocytogenes subtypes associated with persistence in food processing environments. Listeria monocytogenes is a Gram-positive bacterium of the phylum Firmicutes. Often found in raw foods, L. monocytogenes bacteria can cause the life-threatening disease listeriosis. Most cases of human listeriosis appear to be caused by ready-to-eat (RTE) foods, and the risk of illness increases with the number of cells ingested and with RTE foods that support the growth of L. monocytogenes (1). Processed foods can be contaminated by contact with equipment, by the handling of raw products by staff, or from postprocessing environmental niches in which L. monocytogenes can survive despite the routine use of thorough disinfection procedures (2, 3). Mechanisms that facilitate the survival of L. monocytogenes in food processing environments include biofilm formation (4, 5), acquisition of antimicrobial resistance (6-10), and stress resistance mechanisms (11,12).L. monocytogenes is capable of colonizing food production plants with certain subtypes that are found only in specific sections of the plants (13, 14). Furthermore, some of these subtypes may persist in food processing environments for years (15-17). Persistent strains have been identified to be major postprocessing contaminants of RTE foods, and in many cases, listeriosis outbreaks have been associated with cases of p...

The persistence of certain strains of Listeria monocytogenes, even after the food processing environment has been cleaned and disinfected, suggests that this may be related to phenomena that reduce the concentration of the disinfectants to subinhibitory levels. This includes (i) the existence of environmental niches or reservoirs that are difficult for disinfectants to reach, (ii) microorganisms that form biofilms and create microenvironments in which adequate concentrations of disinfectants cannot be attained, and (iii) the acquisition of resistance mechanisms in L. monocytogenes, including those that lead to a reduction in the intracellular concentration of the disinfectants. The only available data with regard to the resistance of L. monocytogenes to disinfectants applied in food production environments refer to genotypic resistance to quaternary ammonium compounds (QACs). Although there are several well-characterized efflux pumps that confer resistance to QACs, it is a low-level resistance that does not generate resistance to QACs at the concentrations applied in the food industry. However, dilution in the environment and biodegradation result in QAC concentration gradients. As a result, the microorganisms are frequently exposed to subinhibitory concentrations of QACs. Therefore, the low-level resistance to QACs in L. monocytogenes may contribute to its environmental adaptation and persistence. In fact, in certain cases, the relationship between low-level resistance and the environmental persistence of L. monocytogenes in different food production chains has been previously established. The resistant strains would have survival advantages in these environments over sensitive strains, such as the ability to form biofilms in the presence of increased biocide concentrations.

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