Listeria monocytogenes from Food Products and Food Associated Environments: Antimicrobial Resistance, Genetic Clustering and Biofilm Insights

Abstract

Listeria monocytogenes, a foodborne pathogen, exhibits high adaptability to adverse environmental conditions and is common in the food industry, especially in ready-to-eat foods. L. monocytogenes strains pose food safety challenges due to their ability to form biofilms, increased resistance to disinfectants, and long-term persistence in the environment. The aim of this study was to evaluate the presence and genetic diversity of L. monocytogenes in food and related environmental products collected from 2014 to 2022 and assess antibiotic susceptibility and biofilm formation abilities. L. monocytogenes was identified in 13 out of the 227 (6%) of samples, 7 from food products (meat preparation, cheeses, and raw milk) and 6 from food-processing environments (slaughterhouse-floor and catering establishments). All isolates exhibited high biofilm-forming capacity and antibiotic susceptibility testing showed resistance to several classes of antibiotics, especially trimethoprim-sulfamethoxazole and erythromycin. Genotyping and core-genome clustering identified eight sequence types and a cluster of three very closely related ST3 isolates (all from food), suggesting a common contamination source. Whole-genome sequencing (WGS) analysis revealed resistance genes conferring resistance to fosfomycin (fosX), lincosamides (lin), fluoroquinolones (norB), and tetracycline (tetM). In addition, the qacJ gene was also detected, conferring resistance to disinfecting agents and antiseptics. Virulence gene profiling revealed the presence of 92 associated genes associated with pathogenicity, adherence, and persistence. These findings underscore the presence of L. monocytogenes strains in food products and food-associated environments, demonstrating a high virulence of these strains associated with resistance genes to antibiotics, but also to disinfectants and antiseptics. Moreover, they emphasize the need for continuous surveillance, effective risk assessment, and rigorous control measures to minimize the public health risks associated to severe infections, particularly listeriosis outbreaks. A better understanding of the complex dynamics of pathogens in food products and their associated environments can help improve overall food safety and develop more effective strategies to prevent severe health consequences and economic losses.

Keywords: Listeria monocytogenes; antimicrobial resistance; biofilm formation; food safety; genetic diversity.

Figure 1

Core-genome clustering analysis of L. monocytogenes (thirteen isolates). The Minimum Spanning Tree (MST) was constructed based on the cgMLST 1748-loci Pasteur schema [27]. Each circle (node) contains the sample code and represents a unique allelic profile, with numbers on the connecting lines representing allelic distances (ADs) between nodes. Cluster analysis was conducted with ReporTree v.2.0.3 [28] and data visualization was adapted from the GrapeTree (MSTreeV2 method) dashboard [29]. Straight and dotted lines reflect nodes linked with allelic distances (ADs) below and above a threshold of seven ADs, which can provide a proxy to the identification of genetic clusters with potential epidemiological concordance [30]. Nodes are colored according to the sequence type (ST), with clonal complex (CC) and lineage also presented. The surrounding orange shadow highlights a cluster supported by ≤7 ADs.

Figure 2

% Biofilm formation capacity (expressed as % in comparison to reference strain) of L. monocytogenes isolated from different food products and food-associated environments. Statistical significance was determined using Tukey’s multiple comparison test.