Probing antimicrobial resistance and sanitizer tolerance themes and their implications for the food industry through the Listeria monocytogenes lens

Environmental Monitoring of Nebraska Ready-to-eat Meat Processing Establishments Resulted in the Isolation of Listeria Alongside Pseudomonas Highly Resistant to Quaternary Ammonia Sanitizer

Robust environmental monitoring for Listeria monocytogenes often may not be feasible for small and very small meat processors in the United States due to the limitations in finances, staffing, or expertise. Three small/very small processors in Nebraska were sampled using sponge applicators in nonfood contact surface areas to determine if biofilm and sanitizer resistance behaviors of Pseudomonas could relate to the prevalence of L. monocytogenes and Listeria spp. in ready-to-eat meat processing environments. Samples were 3.3% (3/90) positive for L. monocytogenes, and 12.2% (11/90) of samples were positive for Listeria spp. Pseudomonas spp. were also isolated. When Listeria spp. and Pseudomonas spp. were assayed for biofilm production and resistance to a quaternary ammonia sanitizer, multiple isolates belonging to both genera capable of forming biofilms were identified. Four Pseudomonas spp. isolates resisted the 200 ppm manufacturer-recommended sanitizer concentration for food contact surface sanitation, and one Pseudomonas spp. isolated from a drain sample that was also positive for L. monocytogenes demonstrated a sanitizer minimum bactericidal concentration of 1000 ppm. These findings further support the need for monitoring of small and very small meat processors for L. monocytogenes as well as highlight the need to identify other bacteria in these processing environments, like Pseudomonas, that are resistant to environmental stressors.

Variability in cadmium tolerance of closely related Listeria monocytogenes isolates originating from dairy processing environments

Increased tolerance to cadmium in Listeria monocytogenes has been suggested to contribute to their persistence in natural and food production environments. This study investigated the phenotypic cadmium response of L. monocytogenes strains with efflux pump cadAC (variants 1-4) and related strains with cadA1C1. Growth of cadAC variant strains (n = 5) in 0 µM-120 µM cadmium salts (CdCl2, CdSO4) in Mueller-Hinton broth (MHB) was evaluated. Additionally, 88 L. monocytogenes strains from dairy processing facilities were exposed to 43.8 µM CdCl2 in MHB, and their lag phase duration (LPD) was measured. Strains with cadA1 through cadA3 showed similar growth trends in the presence of cadmium, while the cadA4 variant (Scott A) had the highest CdCl2 minimum inhibitory concentration (175 µM). Growth varied between the two salts, with CdSO4 significantly increasing LPD (P < 0.05) compared to CdCl2. In 43.8 µM CdCl2, cadA1 strains displayed LPDs ranging from 0.99 ± 0.14 h to 6.44 ± 0.08 h, with no clear genomic differences explaining this variability. Strains without cadA did not grow at 43.8 µM CdCl2 but exhibited low tolerance (10.9 µM CdCl2), potentially due to non-specific soft metal ATPases (626 aa; 737 aa) and soft metal resistance proteins encoded by czc genes (289 aa; 291 aa; 303 aa) within their chromosomes. These findings enhance our understanding of L. monocytogenes cadmium tolerance and underscore the need for further research to explore the genetic and physiological factors underlying these trends.

IMPORTANCE

Mobile genetic elements in Listeria monocytogenes contribute to its survival in natural and food processing environments. This study focused on how different genetic variants of the efflux pump gene cadAC and group of closely related cadA1C1 strains respond to cadmium exposure. When exposed to two cadmium salts, cadmium chloride and cadmium sulfate, we observed varying growth patterns, with a significantly longer lag phase in cadmium sulfate compared to cadmium chloride. Strains with cadA1 to cadA3 had similar growth trends, whereas a strain with the cadA4 variant had the highest minimum inhibitory concentration value. Among 88 strains from dairy processing facilities, significant phenotypic differences were observed despite core genome similarities, indicating other underlying genetic and physiological factors contribute to cadmium tolerance. Since cadmium tolerance studies in L. monocytogenes are limited, with rare phenotypic comparisons between closely related strains, our study makes an important observation and contribution to understanding of L. monocytogenes tolerance to cadmium by providing phenotypic comparisons between numerous strains within the same clonal group (<16 single nucleotide polymorphisms).

Macrolide resistance outcomes after the Covid-19 pandemic: A one health approach investigation

During the Covid-19 pandemic period, the indiscriminate use of macrolide-class antibiotics was frequent among the Brazilian population due to the lack of knowledge and information with a scientific basis. Thus, the class of drugs that includes azithromycin, clarithromycin, and erythromycin, which alter metabolic reactions in the body and act on the immune system, was widely used without medical prescription. Samples of bacterial strains from hospital environments were obtained during the most extensive spread of Covid-19 and studied in the present article, emphasizing the investigation for macrolide resistance genes (erm and msr) and bacteria of the genus Staphylococcus isolated from urinary tract infections. In addition, the physiological, genetic, immunological, and socio-epidemiological aspects were highlighted with a focus on the One Health approach and implications on the gut-brain axis in this integrative research, revealing that the inappropriate use of antibiotics directly affects entire communities, representing a significant concern for public and environmental health.

Biofilm formation by Listeria monocytogenes from the meat processing industry environment and the use of different combinations of detergents, sanitizers, and UV-A radiation to control this microorganism in planktonic and sessile forms

This study aimed to evaluate the ability of biofilm formation by L. monocytogenes from the meat processing industry environment, as well as the use of different combinations of detergents, sanitizers, and UV-A radiation in the control of this microorganism in the planktonic and sessile forms. Four L. monocytogenes isolates were evaluated and showed moderate ability to form biofilm, as well as carried genes related to biofilm production (agrB, agrD, prfA, actA, cheA, cheY, flaA, sigB), and genes related to tolerance to sanitizers (lde and qacH). The biofilm-forming isolates of L. monocytogenes were susceptible to quaternary ammonium compound (QAC) and peracetic acid (PA) in planktonic form, with minimum inhibitory concentrations of 125 and 75 ppm, respectively, for contact times of 10 and 5 min. These concentrations are lower than those recommended by the manufacturers, which are at least 200 and 300 ppm for QAC and PA, respectively. Biofilms of L. monocytogenes formed from a pool of isolates on stainless steel and polyurethane coupons were subjected to 14 treatments involving acid and enzymatic detergents, QAC and PA sanitizers, and UV-A radiation at varying concentrations and contact times. All treatments reduced L. monocytogenes counts in the biofilm, indicating that the tested detergents, sanitizers, and UV-A radiation exhibited antimicrobial activity against biofilms on both surface types. Notably, the biofilm formed on polyurethane showed greater tolerance to the evaluated compounds than the biofilm on stainless steel, likely due to the material's surface facilitating faster microbial colonization and the development of a more complex structure, as observed by scanning electron microscopy. Listeria monocytogenes isolates from the meat processing industry carry genes associated with biofilm production and can form biofilms on both stainless steel and polyurethane surfaces, which may contribute to their persistence within meat processing lines. Despite carrying sanitizer tolerance genes, QAC and PA effectively controlled these microorganisms in their planktonic form. However, combinations of detergent (AC and ENZ) with sanitizers (QAC and PA) at minimum concentrations of 125 ppm and 300 ppm, respectively, were the most effective.

Multi-species biofilms of environmental microbiota isolated from fruit packing facilities promoted tolerance of Listeria monocytogenes to benzalkonium chloride

Listeria monocytogenes may survive and persist in food processing environments due to formation of complex multi-species biofilms of environmental microbiota that co-exists in these environments. This study aimed to determine the effect of selected environmental microbiota on biofilm formation and tolerance of L. monocytogenes to benzalkonium chloride in formed biofilms. The studied microbiota included bacterial families previously shown to co-occur with L. monocytogenes in tree fruit packing facilities, including Pseudomonadaceae, Xanthomonadaceae, Microbacteriaceae, and Flavobacteriaceae. Biofilm formation ability and the effect of formed biofilms on the tolerance of L. monocytogenes to benzalkonium chloride was measured in single- and multi-family assemblages. Biofilms were grown statically on polystyrene pegs submerged in a R2A broth. Biofilm formation was quantified using a crystal violet assay, spread-plating, confocal laser scanning microscopy, and its composition was assessed using amplicon sequencing. The concentration of L. monocytogenes in biofilms was determined using the most probable number method. Biofilms were exposed to the sanitizer benzalkonium chloride, and the death kinetics of L. monocytogenes were quantified using a most probable number method. A total of 8, 8, 6, and 3 strains of Pseudomonadaceae, Xanthomonadaceae, Microbacteriaceae, and Flavobacteriaceae, respectively, were isolated from the environmental microbiota of tree fruit packing facilities and were used in this study. Biofilms formed by Pseudomonadaceae, Xanthomonadaceae, and all multi-family assemblages had significantly higher concentration of bacteria, as well as L. monocytogenes, compared to biofilms formed by L. monocytogenes alone. Furthermore, multi-family assemblage biofilms increased the tolerance of L. monocytogenes to benzalkonium chloride compared to L. monocytogenes mono-species biofilms and planktonic multi-family assemblages. These findings suggest that L. monocytogenes control strategies should focus not only on assessing the efficacy of sanitizers against L. monocytogenes, but also against biofilm-forming microorganisms that reside in the food processing built environment, such as Pseudomonadaceae or Xanthomonadaceae.

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

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.

Biocide Tolerance and Impact of Sanitizer Concentrations on the Antibiotic Resistance of Bacteria Originating from Cheese

In this study, we determined and identified the bacterial diversity of different types of artisanal and industrially produced cheese. The antibiotic (erythromycin, chloramphenicol, kanamycin, ampicillin, clindamycin, streptomycin, tetracycline, and gentamicin) and biocide (peracetic acid, sodium hypochlorite, and benzalkonium chloride) resistance of clinically relevant bacteria was determined as follows: Staphylococcus aureus, Macrococcus caseolyticus, Bacillus sp., Kocuria varians, Escherichia coli, Enterococcus faecalis, Citrobacter freundii, Citrobacter pasteurii, Klebsiella oxytoca, Klebsiella michiganensis, Enterobacter sp., Enterobacter cloacae, Enterobacter sichuanensis, Raoultella ornithinolytica, Shigella flexneri, and Salmonella enterica. Also, the effect of the sub-inhibitory concentration of three biocides on antibiotic resistance was determined. The microbiota of evaluated dairy products comprise diverse and heterogeneous groups of bacteria with respect to antibiotic and disinfectant tolerance. The results indicated that resistance was common in the case of ampicillin, chloramphenicol, erythromycin, and streptomycin. Bacillus sp. SCSSZT2/3, Enterococcus faecalis SRGT/1, E. coli SAT/1, Raoultella ornithinolytica MTT/5, and S. aureus SIJ/2 showed resistance to most antibiotics. The tested bacteria showed sensitivity to peracetic acid and a different level of tolerance to benzalkonium chloride and sodium hypochlorite. The inhibition zone diameter of antibiotics against Enterococcus faecalis SZT/2, S. aureus JS11, E. coli CSKO2, and Kocuria varians GRT/10 was affected only by the sub-inhibitory concentration of peracetic acid.

High Disinfectant Tolerance in Pseudomonas spp. Biofilm Aids the Survival of Listeria monocytogenes

Pseudomonas spp. are the most commonly found bacteria in food-processing environments due to properties such as a high growth rate at low temperatures, a high tolerance of antimicrobial agents, and biofilm formation. In this study, a set of Pseudomonas isolates originating from cleaned and disinfected surfaces in a salmon processing facility were screened for biofilm formation at 12 °C. A high variation in biofilm formation between the isolates was observed. Selected isolates, in both planktonic and biofilm states, were tested for resistance/tolerance to a commonly used disinfectant (peracetic acid-based) and antibiotic florfenicol. Most isolates showed a much higher tolerance in the biofilm state than in the planktonic state. In a multi-species biofilm experiment with five Pseudomonas strains with and without a Listeria monocytogenes strain, the Pseudomonas biofilm appeared to aid the survival of L. monocytogenes cells after disinfection, underscoring the importance of controlling the bacterial load in food-processing environments.

A Comprehensive Virulence and Resistance Characteristics of Listeria monocytogenes Isolated from Fish and the Fish Industry Environment

Listeria monocytogenes is an important pathogen, often associated with fish, that can adapt and survive in products and food processing plants, where it can persist for many years. It is a species characterized by diverse genotypic and phenotypic characteristics. Therefore, in this study, a total of 17 L. monocytogenes strains from fish and fish-processing environments in Poland were characterized for their relatedness, virulence profiles, and resistance genes. The Core Genome Multilocus Sequence Typing (cgMLST) analysis revealed that the most frequent serogroups were IIa and IIb; sequence types (ST) were ST6 and ST121; and clonal complexes (CC) were CC6 and CC121. Core genome multilocus sequence typing (cgMLST) analysis was applied to compare the present isolates with the publicly available genomes of L. monocytogenes strains recovered in Europe from humans with listeriosis. Despite differential genotypic subtypes, most strains had similar antimicrobial resistance profiles; however, some of genes were located on mobile genetic elements that could be transferred to commensal or pathogenic bacteria. The results of this study showed that molecular clones of tested strains were characteristic for L. monocytogenes isolated from similar sources. Nevertheless, it is worth emphasizing that they could present a major public health risk due to their close relation with strains isolated from human listeriosis.

Integrative Assessment of Reduced Listeria monocytogenes Susceptibility to Benzalkonium Chloride in Produce Processing Environments

For decades, quaternary ammonium compounds (QAC)-based sanitizers have been broadly used in food processing environments to control foodborne pathogens such as Listeria monocytogenes. Still, there is a lack of consensus on the likelihood and implication of reduced Listeria susceptibility to benzalkonium chloride (BC) that may emerge due to sublethal exposure to the sanitizers in food processing environments. With a focus on fresh produce processing, we attempted to fill multiple data and evidence gaps surrounding the debate. We determined a strong correlation between tolerance phenotypes and known genetic determinants of BC tolerance with an extensive set of fresh produce isolates. We assessed BC selection on L. monocytogenes through a large-scale and source-structured genomic survey of 25,083 publicly available L. monocytogenes genomes from diverse sources in the United States. With the consideration of processing environment constraints, we monitored the temporal onset and duration of adaptive BC tolerance in both tolerant and sensitive isolates. Finally, we examined residual BC concentrations throughout a fresh produce processing facility at different time points during daily operation. While genomic evidence supports elevated BC selection and the recommendation for sanitizer rotation in the general context of food processing environments, it also suggests a marked variation in the occurrence and potential impact of the selection among different commodities and sectors. For the processing of fresh fruits and vegetables, we conclude that properly sanitized and cleaned facilities are less affected by BC selection and unlikely to provide conditions that are conducive for the emergence of adaptive BC tolerance in L. monocytogenes. IMPORTANCE Our study demonstrates an integrative approach to improve food safety assessment and control strategies in food processing environments through the collective leveraging of genomic surveys, laboratory assays, and processing facility sampling. In the example of assessing reduced Listeria susceptibility to a widely used sanitizer, this approach yielded multifaceted evidence that incorporates population genetic signals, experimental findings, and real-world constraints to help address a lasting debate of policy and practical importance.

Analysis of Sanitizer Rotation on the Susceptibility, Biofilm Forming Ability and Caco-2 Cell Adhesion and Invasion of Listeria

The purpose of this study was to determine the effect of sanitizer use conditions on the susceptibility, biofilm forming ability and pathogenicity of Listeria monocytogenes. Two different strains of L. monocytogenes and a non-pathogenic L. innocua were exposed to sodium hypochlorite, benzalkonium chloride and peroxyacetic acid at different concentrations (4 to 512 ppm) and treatment times (30 s to 5 min), respectively. Under the tested conditions, no significant difference (p > 0.05) in reduction was observed among the three tested sanitizers. A reduction of 1 to 8 log CFU/mL was observed depending upon the sanitizer concentration and treatment times. The survived cells at the highest sublethal concentration and treatment time of a particular sanitizer upon re-exposure to the same or different sanitizer showed either no change or increased susceptibility when compared to parent strains. Upon repeated exposure to sanitizers at progressively increasing concentrations from 1 to 128 ppm, L. innocua was able to survive concentrations of up to 32 ppm benzalkonium chloride and 64 ppm peroxyacetic acid treatments, respectively. At the tested sub-lethal concentrations, no significant difference (p > 0.05) in biofilm formation was observed among the tested strains. Caco-2 interaction with L. innocua showed a reduction in invasion ability with sublethal concentrations of sanitizers.

Nonsynonymous Mutations in fepR Are Associated with Adaptation of Listeria monocytogenes and Other Listeria spp. to Low Concentrations of Benzalkonium Chloride but Do Not Increase Survival of L. monocytogenes and Other Listeria spp. after Exposure to Benzalkonium Chloride Concentrations Recommended for Use in Food Processing Environments

Selection for Listeria monocytogenes strains that are tolerant to quaternary ammonium compounds (such as benzalkonium chloride [BC]) is a concern across the food industry, including in fresh produce processing environments. This study evaluated the ability of 67 strains of produce-associated L. monocytogenes and other Listeria spp. (“parent strains”) to show enhanced BC tolerance after serial passaging in increasing BC concentrations and to maintain this tolerance after substreaking in the absence of BC. After serial passaging in BC, 62/67 “BC passaged cultures” showed higher MICs (4 to 20 mg/L) than parent strains (2 to 6 mg/L). After the substreaking of two isolates from BC passaged cultures for each parent strain, 105/134 “adapted isolates” maintained MICs (4 to 6 mg/L) higher than parent strain MICs. These results suggested that adapted isolates acquired heritable adaptations that confer BC tolerance. Whole-genome sequencing and Sanger sequencing of fepR, a local repressor of the MATE family efflux pump FepA, identified nonsynonymous fepR mutations in 48/67 adapted isolates. The mean inactivation of adapted isolates after exposure to use-level concentrations of BC (300 mg/L) was 4.48 log, which was not significantly different from inactivation observed in parent strains. Serial passaging of cocultures of L. monocytogenes strains containing bcrABC or qacH did not yield adapted isolates that showed enhanced BC tolerance in comparison to that of monocultures. These results suggest that horizontal gene transfer either did not occur or did not yield isolates with enhanced BC tolerance. Overall, this study provides new insights into selection of BC tolerance among L. monocytogenes and other Listeria spp.

IMPORTANCEListeria monocytogenes tolerance to quaternary ammonium compounds has been raised as a concern with regard to L. monocytogenes persistence in food processing environments, including in fresh produce packing and processing environments. Persistence of L. monocytogenes can increase the risk of product contamination, food recalls, and foodborne illness outbreaks. Our study shows that strains of L. monocytogenes and other Listeria spp. can acquire heritable adaptations that confer enhanced tolerance to low concentrations of benzalkonium chloride, but these adaptations do not increase survival of L. monocytogenes and other Listeria spp. when exposed to concentrations of benzalkonium chloride used for food contact surface sanitation (300 mg/L). Overall, these findings suggest that the emergence of benzalkonium chloride-tolerant Listeria strains in food processing environments is of limited concern, as even strains adapted to gain higher MICs in vitro maintain full sensitivity to the concentrations of benzalkonium chloride used for food contact surface sanitation.