Distribution, adhesion, virulence and antibiotic resistance of persistent Listeria monocytogenes in a pig slaughterhouse in Brazil

Exploring the genetic variability, virulence factors, and antibiotic resistance of Listeria monocytogenes from fresh produce, ready-to-eat hummus, and food-processing environments

Listeria monocytogenes is ubiquitous in nature and persistent in food-processing facilities, farms, retail stores, and home and restaurant kitchens. Current research suggests ready-to-eat (RTE) products (including RTE hummus and fresh produce) to be of increasing interest and concern. These foods are typically stored at refrigeration temperatures suited to the survival of L. monocytogenes and are consumed without further processing. Since L. monocytogenes is ubiquitous in agricultural environments, the cultivation of fresh produce predisposes it to contamination. The contamination of RTE foods originates either from raw ingredients or, more commonly, from cross-contamination within food-processing facilities. Research on the food-processing environment has been recommended to reduce the incidence of L. monocytogenes in foods. The consumption of contaminated foods by immunocompromised individuals causes invasive listeriosis, with a 20% to 30% fatality rate despite treatment. The emergence of antibiotic-resistant strains has reduced the effectiveness of modern medicine and may increase morbidity and mortality. Without epidemiological surveillance and identifying trends in disease determinants, no action can be taken to improve food safety and mitigate the risk of such outbreaks.

Investigation of the seasonal prevalence, phenotypic, and genotypic characteristics of Listeria monocytogenes in slaughterhouses in Burdur

AIM

This study examined Listeria monocytogenes isolates from two slaughterhouses in Burdur province, southern Turkey, over four seasons for antibiotic resistance, serogroups, virulence genes, in vitro biofilm forming capacity, and genetic relatedness.

METHODS AND RESULTS

Carcass (540) and environment-equipment surface (180) samples were collected from two slaughterhouses (S1, S2) for 1 year (4 samplings). Of the 89 (12.4%) positive isolates, 48 (53.9%) were from animal carcasses, and 41 (46.1%) from the environment-equipment surfaces. Autumn was the peak season for Listeria monocytogenes compared to summer and spring (P < 0.05). In addition, the most common serotype between seasons was 1/2c. Except for plcA and luxS genes, all isolates (100%) harbored inlA, inlC, inlJ, hlyA, actA, iap, flaA genes. Listeria monocytogenes isolates were identified as belonging to IIc (1/2c-3c; 68.5%), IVb (4b-4d-4e; 29.2%), and IIa (1/2a-3a; 2.2%) in the screening using multiplex polymerase chain reaction-based serogrouping test. A total of 65 pulsotypes and 13 clusters with at least 80% homology were determined by using pulsed field gel electrophoresis on samples that had been digested with ApaI. Thirty-four (38.2%) of the isolates were not resistant to any of the 14 antibiotics tested. The antibiotic to which the isolates showed the most resistance was rifampicin (44.9%). Serotype 1/2c was the most resistant serotype to antibiotics. Despite having biofilm-associated genes (inlA, inlB, actA, flaA, and luxS), a minority (11%) of isolates formed weak biofilm.

CONCLUSION

This study revealed seasonal changes prevalence of Listeria monocytogenes, particularly higher in autumn, posing a greater risk of meat contamination. Notably, Serotype 1/2c showed significant prevalence and antibiotic resistance. Indistinguishable isolates indicated cross-contamination, underscoring the importance of prioritized training for slaughterhouse personnel in sanitation and hygiene protocols.

Effect of ginger essential oil and 6-gingerol on a multispecies biofilm of Listeria monocytogenes, Salmonella Typhimurium, and Pseudomonas aeruginosa

The objective of this study was to evaluate the potential antimicrobial and antibiofilm effect of ginger essential oil (GEO) and 6-gingerol on a multispecies biofilm formed by Listeria monocytogenes, Salmonella Typhimurium, and Pseudomonas aeruginosa on a polypropylene surface. The minimum inhibitory concentration concentrations obtained for GEO were 100 and 50 mg/mL and for 6-gingerol 1.25 mg/mL. Sessile cell counts ranged within 5.35-7.35 log CFU/cm2 in the control biofilm, with the highest sessile growth at 72 h. GEO treatments acted on the total population regardless of concentration at 1 and 48 h. L. monocytogenes behaved similarly to the total population, showing GEO action at 1 h and keeping the same pattern at 48, 72, and 96 h. Better action on S. Typhimurium was obtained at times of 1, 72, and 96 h. P. aeruginosa showed logarithmic reduction only when treated with GEO 50 mg at 24 h. As for 6-gingerol, in general, there was no significant action (p > 0.05) on the evaluated sessile cells. GEO showed antimicrobial activity against L. monocytogenes, S. Typhimurium, and P. aeruginosa, acting as an inhibitor of biofilm formation. As for 6-gingerol, it was considered a possible antimicrobial agent but without efficacy during biofilm formation.

Characterization of microbial ecology, Listeria monocytogenes, and Salmonella sp. on equipment and utensil surfaces in Brazilian poultry, pork, and dairy industries

This study aimed to evaluate the level of counting by indicator microorganisms, identify the microbial ecology, detect Listeria monocytogenes and Salmonella sp., and determine the presence of virulence genes and biofilm formation. A total of 480 samples were collected from the surfaces of the equipment and utensils using sterile swabs for the detection of L. monocytogenes and Salmonella sp. and counting mesophilic aerobes, Enterobacteriaceae, Escherichia coli, and Pseudomonas sp. The microbial ecology was evaluated by sequencing the 16S rRNA gene. Genes for virulence and biofilm formation were analyzed and adhesion capacity was evaluated for L. monocytogenes and Salmonella sp. The mesophilic aerobe count was the highest in the dairy processing facility, followed by the pork and poultry slaughterhouses. L. monocytogenes was detected in all facilities, with the highest detection in the pork slaughterhouse, followed by the poultry and dairy facilities. Salmonella sp. was only detected in the dairy. Isolates of L. monocytogenes and Salmonella sp. showed poor adhesion to polystyrene surfaces, virulence genes, and biofilm formation. The frequent contaminants in the slaughterhouses were Pseudomonas, Acinetobacter, and Aeromonas in poultry, Acinetobacter, Pseudomonas, and Brevundimonas in pork, and Pseudomonas, Kocuria, and Staphylococcus in dairy. Our results provide useful information to understand the microbiological risks associated with contamination.

Listeria monocytogenes Biofilms in Food-Associated Environments: A Persistent Enigma

Listeria monocytogenes (LM) is a bacterial pathogen responsible for listeriosis, a foodborne illness associated with high rates of mortality (20-30%) and hospitalisation. It is particularly dangerous among vulnerable groups, such as newborns, pregnant women and the elderly. The persistence of this organism in food-associated environments for months to years has been linked to several devastating listeriosis outbreaks. It may also result in significant costs to food businesses and economies. Currently, the mechanisms that facilitate LM persistence are poorly understood. Unravelling the enigma of what drives listerial persistence will be critical for developing more targeted control and prevention strategies. One prevailing hypothesis is that persistent strains exhibit stronger biofilm production on abiotic surfaces in food-associated environments. This review aims to (i) provide a comprehensive overview of the research on the relationship between listerial persistence and biofilm formation from phenotypic and whole-genome sequencing (WGS) studies; (ii) to highlight the ongoing challenges in determining the role biofilm development plays in persistence, if any; and (iii) to propose future research directions for overcoming these challenges.

Positive biofilms to guide surface microbial ecology in livestock buildings

The increase in human consumption of animal proteins implies changes in the management of meat production. This is followed by increasingly restrictive regulations on antimicrobial products such as chemical biocides and antibiotics, used in particular to control pathogens that can spread zoonotic diseases. Aligned with the One Health concept, alternative biological solutions are under development and are starting to be used in animal production. Beneficial bacteria able to form positive biofilms and guide surface microbial ecology to limit microbial pathogen settlement are promising tools that could complement existing biosecurity practices to maintain the hygiene of livestock buildings. Although the benefits of positive biofilms have already been documented, the associated fundamental mechanisms and the rationale of the microbial composition of these new products are still sparce. This review provides an overview of the envisioned modes of action of positive biofilms used on livestock building surfaces and the resulting criteria for the selection of the appropriate microorganisms for this specific application. Limits and advantages of this biosecurity approach are discussed as well as the impact of such practices along the food chain, from farm to fork.

Antimicrobial Resistance of Listeria monocytogenes from Animal Foods to First- and Second-Line Drugs in the Treatment of Listeriosis from 2008 to 2021: A Systematic Review and Meta-Analysis

First-line drugs for the treatment of listeriosis are the same around the world, but particular conditions might reduce their efficacy, including antimicrobial resistance. Therefore, this study aimed to verify, based on a systematic review and meta-analysis, whether the prevalence of antimicrobial resistance in Listeria monocytogenes from animal foods is higher for first- or second-line antimicrobials. From the total of 302 identified studies, 16 met all the eligibility criteria from 2008 to 2021 and were included in this meta-analysis. They comprised a dataset of 1152 L. monocytogenes isolates, obtained from different animal food products, food processing environment, and live animals. The included studies were developed in South America (n = 5), Europe (n = 4), Asia (n = 3), Africa (n = 2), and North America (n = 2), testing a total of 35 different antimicrobials, 11 of them classified as first-line drugs. Complete lack of antimicrobial resistance across the studies (all L. monocytogenes isolates tested as susceptible) was only observed for linezolid, while widespread antimicrobial resistance (all L. monocytogenes isolates tested resistant) was described for amoxicillin, benzylpenicillin, cefoxitin, fusidic acid, imipenem, sulfamethoxazole, and vancomycin. Overall, the meta-analysis results indicated no evidence that antimicrobial resistance in L. monocytogenes isolated from animal-based food is higher for first-line antimicrobials compared to second-line compounds (p=0.37). A greater volume of publication, together with better characterization of the isolates, is still needed for a more precise estimate of the real prevalence of antimicrobial resistance in L. monocytogenes.

Molecular characterization and biofilm-formation analysis of Listeria monocytogenes, Salmonella spp., and Escherichia coli isolated from Brazilian swine slaughterhouses

This study aimed to verify the presence of Listeria monocytogenes, Salmonella spp., and Escherichia coli in two Brazilian swine slaughterhouses, as well as to perform antibiograms, detect virulence and antimicrobial resistance genes, and evaluate the in vitro biofilm-forming capability of bacterial isolates from these environments. One Salmonella Typhi isolate and 21 E. coli isolates were detected, while L. monocytogenes was not detected. S. Typhi was isolated from the carcass cooling chamber’s floor, resistant to several antimicrobials, including nalidixic acid, cefazolin, chloramphenicol, doxycycline, streptomycin, gentamicin, tetracycline, and sulfonamide, and contained resistance genes, such as tet(B), tet(C), tet(M), and ampC. It also showed moderate biofilm-forming capacity at 37°C after incubating for 72 h. The prevalence of the 21 E. coli isolates was also the highest on the carcass cooling chamber floor (three of the four samplings [75%]). The E. coli isolates were resistant to 12 of the 13 tested antimicrobials, and none showed sensitivity to chloramphenicol, an antimicrobial prohibited in animal feed since 2003 in Brazil. The resistance genes MCR-1, MCR-3, sul1, ampC, clmA, cat1, tet(A), tet(B), and blaSHV, as well as the virulence genes stx-1, hlyA, eae, tir α, tir β, tir γ, and saa were detected in the E. coli isolates. Moreover, 5 (23.8%) and 15 (71.4%) E. coli isolates presented strong and moderate biofilm-forming capacity, respectively. In general, the biofilm-forming capacity increased after incubating for 72 h at 10°C. The biofilm-forming capacity was the lowest after incubating for 24 h at 37°C. Due to the presence of resistance and virulence genes, multi-antimicrobial resistance, and biofilm-forming capacity, the results of this study suggest a risk to the public health as these pathogens are associated with foodborne diseases, which emphasizes the hazard of resistance gene propagation in the environment.

Occurrence and Distribution of Antibiotic-Resistant Staphylococcus aureus in a Brazilian Pork Production Chain

ABSTRACT

Here we characterized the distribution and the antibiotic resistance of staphylococci from a Brazilian pork production chain. Samples (n = 1,114) from pig farms, pig lots, and slaughterhouses, located in two Brazilian states (Minas Gerais and Paraná), were subjected to coagulase-positive Staphylococcus enumeration. S. aureus isolates (n = 251) from this collection were further characterized for their resistance to oxacillin, cefoxitin, vancomycin, and tetracycline through phenotypic and molecular assays. Coagulase-positive Staphylococcus counts from pig farms were higher compared with other samples (P < 0.05). Other counts were relatively low but were present in all production stages. S. aureus isolates were commonly resistant to oxacillin and cefoxitin (54 of 73, 74.0%), qualifying them as methicillin-resistant S. aureus, but PCR assays indicated that few harbored the expected antimicrobial resistance genes (femB, mecA, and mecC). Lower frequencies of vancomycin and tetracycline resistance were found (6.8 to 37.0%). PCR sensitivity (34.5 to 86.7%) and specificity (26.6 to 85.0%) for detection of antibiotic resistance genes varied based on the assessed antibiotic. Antibiotic-resistant staphylococci are widely distributed in the Brazilian pork production chain, and methicillin-resistant S. aureus can become a potential health and economic impediment for the Brazilian pork industry.

HIGHLIGHTS

The prevalence of Listeria monocytogenes in meat products in Brazil: A systematic literature review and meta-analysis

Listeria monocytogenes, a foodborne pathogen that causes human listeriosis, is commonly found in meat products. This study aimed to estimate the prevalence of L. monocytogenes in a variety of Brazilian meat products, using a meta-analysis of data from the literature. A total of 29 publications from five databases, published between January 1, 2009, and December 31, 2019, were included in the study. Estimated by the random-effects model, the combined prevalence of L. monocytogenes was 13%, ranging from 0 to 59%. The combined prevalence of L. monocytogenes was 14% and 11% for raw meat and ready-to-eat (RTE) meat, respectively. The prevalence of L. monocytogenes was higher in the swine species' meat products and the Southeast region of Brazil. Regarding the type of establishment, it was the retail market that presented the highest combined prevalence rate (19%). The most prevalent serotypes of L. monocytogenes were 4b, 1/2a, 1/2b, and 1/2c. The knowledge of differences in the prevalence levels of L. monocytogenes in different meat products can guide in its efficient control by the competent authorities and by industry.

Molecular characterization of Salmonella spp. and Listeria monocytogenes strains from biofilms in cattle and poultry slaughterhouses located in the federal District and State of Goiás, Brazil

Listeria monocytogenes and Salmonella spp. are considered important foodborne pathogens that are commonly associated with foods of animal origin. The aim of this study was to perform molecular characterization of L. monocytogenes and Salmonella spp. isolated from biofilms of cattle and poultry slaughterhouses located in the Federal District and State of Goiás, Brazil. Fourteen L. monocytogenes isolates and one Salmonella sp. were detected in poultry slaughterhouses. No isolates were detected in cattle slaughterhouses. All L. monocytogenes isolates belonged to lineage II, and 11 different pulsotypes were detected. Pulsed-field gel electrophoresis analysis revealed the dissemination of two strains within one plant, in addition to the regional dissemination of one of them. The Salmonella isolate was identified via whole genome sequencing as Salmonella enterica serovar Minnesota ST548. In the sequence analysis, no premature stop codons were detected in the inlA gene of Listeria. All isolates demonstrated the ability to adhere to Caco-2 cells, while 50% were capable of invading them. Antimicrobial resistance was detected in 57.1% of the L. monocytogenes isolates, and resistance to sulfonamide was the most common feature. The tetC, ermB, and tetM genes were detected, and four isolates were classified as multidrug-resistant. Salmonella sp. was resistant to nine antimicrobials and was classified as multidrug-resistant. Resistance genes qnrB19, blaCMY-2, aac(6')-Iaa, sul2, and tetA, and a mutation in the parC gene were detected. The majority (78.5%) of the L. monocytogenes isolates were capable of forming biofilms after incubation at 37°C for 24 h, and 64.3% were capable of forming biofilms after incubation at 12°C for 168 h. There was no statistical difference in the biofilm-forming capacity under the different evaluated conditions. Salmonella sp. was capable of forming biofilms at both tested temperatures. Biofilm characterization was confirmed by collecting the samples consistently, at the same sampling points, and by assessing biofilm formation in vitro. These results highlight the potential risk of cross-contamination in poultry slaughterhouses and the importance of surveillance and pathogen control maintenance programs within the meat production industry.

Low-Level Tolerance to Antibiotic Trimethoprim in QAC-Adapted Subpopulations of Listeria monocytogenes

Between January and July 2021, there were as many as 30 recalls in the U.S. due to potential Listeria monocytogenes contamination from a variety of food products including muffins, kimchi, chicken salad, ready-to-eat chicken, smoked fish, mushrooms, queso fresco cheese, ice cream, turkey sandwiches, squash, and other foods. A contaminated food chain can serve as a potential vehicle for transmitting antibiotic resistant bacteria since there is a slow emergence of multi-drug antibiotic resistance in L. monocytogenes. Biocides are essential for safe food processing, but they may also induce unintended selective pressure at sublethal doses for the expression of antibiotic resistance in L. monocytogenes. To better understand the sources of such slow emergence of antibiotic resistance through biocide residues present in the food environments, we are working on the role of sublethal doses of commonly used biocides in defined broth and water models for understanding L. monocytogenes adaptation. We recently published the development of low-level tolerance to fluoroquinolone antibiotic ciprofloxacin in quaternary ammonium compound (QAC) adapted subpopulations of L. monocytogenes (Microorganisms 9, 1052). Of the six different antibiotics tested to determine heterologous stress adaptation in eight strains of L. monocytogenes, trimethoprim was the second one that exhibited low-level tolerance development after continuous exposure (by three approaches) to sublethal concentrations of QAC against actively growing planktonic cells of L. monocytogenes. When adapted to daily cycles of fixed or gradually increasing sublethal concentrations of QAC, we observed three main findings in eight L. monocytogenes strains against trimethoprim: (a) 3 of the 8 strains exhibited significant increase in short-range minimum inhibitory concentration (MIC) of trimethoprim by 1.7 to 2.5 fold in QAC-adapted subpopulations compared to non-adapted cells (p < 0.05); (b) 2 of the 8 strains exhibited significant increase in growth rate in trimethoprim (optical density (OD) by 600 nm at 12 h) by 1.4 to 4.8 fold in QAC-adapted subpopulations compared to non-adapted cells (p < 0.05); and (c) 5 of the 8 strains yielded significantly higher survival by 1.3-to-3.1 log CFU/mL in trimethoprim in QAC-adapted subpopulations compared to the non-adapted control (p < 0.05). However, for 3/8 strains of L. monocytogenes, there was no increase in the survival of QAC-adapted subpopulations compared to non-adapted control in trimethoprim. These findings suggest the potential formation of low-level trimethoprim tolerant subpopulations in some L. monocytogenes strains where QAC may be used widely. These experimental models are useful in developing early detection methods for tracking the slow emergence of antibiotic tolerant strains through food chain. Also, these findings are useful in understanding the predisposing conditions leading to slow emergence of antibiotic resistant strains of L. monocytogenes in various food production and food processing environments.

Detection and Virulence Characterization of Listeria monocytogenes Strains in Ready-to-Eat Products

The public health risk posed by Listeria monocytogenes in ready-to-eat (RTE) foods depends on the effectiveness of its control at every stage of the production process and the strain involved. Analytical methods currently in use are limited to the identification/quantification of L. monocytogenes at the species level, without distinguishing virulent from hypovirulent strains. In these products, according to EU Regulation 2073/2005, L. monocytogenes is a mandatory criterion irrespective of strain virulence level. Indeed, this species encompasses a diversity of strains with various pathogenic potential, reflecting genetic heterogeneity of the species itself. Thus, the detection of specific L. monocytogenes virulence genes can be considered an important target in laboratory food analysis to assign different risk levels to foods contaminated by strains carrying different genes. In 2015-2016, a severe invasive listeriosis outbreak occurred in central Italy, leading to the intensification of routine surveillance and strain characterization for virulence genetic markers. A new multiplex real-time polymerase chain reaction targeting main virulence genes has been developed and validated against the enzyme-linked fluorescent assay (ELFA) culture-based method. Results of the improved surveillance program are now reported in this study.

Low-Level Tolerance to Fluoroquinolone Antibiotic Ciprofloxacin in QAC-Adapted Subpopulations of Listeria monocytogenes

There was a development of low-level tolerance to fluoroquinolone antibiotic ciprofloxacin in Listeria monocytogenes after sublethal adaptation to quaternary ammonium compound (QAC). Using eight L. monocytogenes strains, we determined the changes in short-range MIC, growth rate, and survival for heterologous stress response to ciprofloxacin, after sublethal exposure to daily cycles of fixed or gradually increasing concentration of QAC. Three main findings were observed. (1) MIC increase—QAC-adapted subpopulations exhibited a significant increase in short-range MIC of ciprofloxacin, by 1.5 to 2.9 fold, as compared to non-adapted control for 4/8 strains (p < 0.05). (2) Growth rate increase—QAC-adapted subpopulations exhibited significant 2.1- to 6.8- fold increase in growth rate (OD₆₀₀ at 10 h) in ciprofloxacin-containing broth, as compared to non-adapted control for 5/8 strains (p < 0.05). (3) Survival increase—QAC-adapted subpopulations of L. monocytogenes yielded significantly higher survival in ciprofloxacin-containing agar by 2.2 to 4.3 log CFU/mL for 4/8 strains, as compared to non-adapted control (p ˂ 0.05). However, for other 4/8 strains of L. monocytogenes, there was no increase in survival of QAC-adapted subpopulations, as compared to non-adapted control in ciprofloxacin. These findings suggest the potential formation of low-level ciprofloxacin-tolerant subpopulations in some L. monocytogenes strains when exposed to residual QAC concentrations (where QAC might be used widely) and such cells if not inactivated might create food safety risk.

Genomic diversity and characterization of Listeria monocytogenes from dry-cured ham processing plants

Genomic diversity of Listeria monocytogenes isolates from the deboning and slicing areas of three dry-cured ham processing plants was analysed. L. monocytogenes was detected in 58 out of 491 samples from the environment and equipment surfaces, all from the deboning area, with differences in prevalence among facilities. The most frequent PCR-serogroup was IIa (74.1%) followed by IIb and IIc, and only one isolate was serogroup IVb. Twenty different pulsotypes and 11 sequence types (STs) grouped into 10 clonal complexes (CCs) were determined. ST121 (CC121) and ST9 (CC9) were the most abundant. Premature stop codons (PMSC6 and PMSC19) associated with attenuated virulence were found in the inlA sequence in 7 out of 12 selected strains. CC121 strains were strong biofilm formers and some harboured the transposon Tn6188, related with increased tolerance to quaternary ammonium compounds. L. monocytogenes clones considered hypovirulent resulted predominant in the deboning areas. The clonal structure and potential virulence of the isolates could help to establish adequate control measures and cleaning protocols for the comprehensive elimination of the pathogen in dry-cured ham processing environment.

Distribution, virulence, genotypic characteristics and antibiotic resistance of Listeria monocytogenes isolated over one-year monitoring from two pig slaughterhouses and processing plants and their fresh hams

Listeria monocytogenes contamination in raw pork and ready to eat foods is an important food safety concern, also for the increasing detection of antimicrobial-resistant isolates. Data on L. monocytogenes occurrence, persistence, distribution and genetic characterization in two different plants, namely in continuum from slaughtered pigs, environment and unfinished products (fresh hams) were observed by one-year monitoring and were integrated with their antimicrobial resistance patterns. A total of 98 samples out of the overall 1131 (8.7%) were positive for L. monocytogenes, respectively 2.6% and 13.2% in plants A and B: only three serotypes were identified, 1/2c (50%), 1/2b (36.7%) and 1/2a (13.27%), and strains were classified in 35 pulsotypes and 16 clusters by PFGE; a unique P-type was highlighted according to the detection of virulence genes. The contamination flow of L. monocytogenes has a low occurrence in slaughterhouse (Plant A = 1.1%, Plant B: 3.1%; p > 0.05) and increased throughout the processing chain with trimming area as the most contaminated (Plant A: 25%, Plant B: 57%; (p < 0.05)), both in the environment and in unfinished products (80% in hams before trimming in plant B). The dominant role of environmental contamination in post-slaughter processing is confirmed to be a significant cause of meat contamination by L. monocytogenes. Very high levels of resistance were observed for clindamycin (57%) and high resistance levels (>20-50%) to ciprofloxacin, oxacillin, levofloxacin and daptomycin, confirming the L. monocytogenes resistance trend to a wide range of antimicrobial agents. A total of 11 L. monocytogenes isolates were multidrug resistant and 7 out of them were isolated from slaughtered pigs. An interesting significant (p < 0.05) statistical correlation has been found between resistance to some antimicrobial agents and lineage/serotypes. Microbiological sampling of food and environments after sanitization are commonly used as verification procedure for the absence of L. monocytogenes in food plants and to give assurance of food safety, but strains characterization is necessary for industries to target specific control measures, like the enforcement of the hygiene program and of the control of operator activities, at least for permanent strains. The only presence of L. monocytogenes could not be considered as the conclusive assessment of a potential risk for public health, also in terms of emerging and emerged antimicrobial resistances.

Assessment of the Prevalence and Drug Susceptibility of Listeria monocytogenes Strains Isolated from Various Types of Meat

Listeria monocytogenes are the etiological factor of listeriosis, and their main source for humans is food. The aim of the current study was to assess the contamination of various types of meat and the drug susceptibility of isolated L. monocytogenes. Between 2016–2018, 6000 swabs were taken (2000 annually) from the surface of pork, beef, and poultry. The analysis of intermediate and finished product samples was carried out in accordance with ISO 11290-1 (International Organization for Standardization). The genetic similarity assessment of the isolates obtained was based on the Pulsed Field Gel Electrophoresis (PFGE) method, and drug-sensitivity assessment using the disc-diffusion method. We found 2.1% of collected samples were L. monocytogenes positive. The level of meat contamination varied depending on its matrix. Most L. monocytogenes were isolated from poultry. It was shown that 39 (32.5%) strains were sensitive to all tested antibiotics and eight (6.7%) were resistant to all five tested antimicrobials. Most strains tested were resistant to cotrimoxazole (55; 45.8%) and meropenem (52; 43.3%), followed by erythromycin (48; 40.0%), penicillin (31; 25.8%), and ampicillin (21; 17.5%). High prevalence of this pathogen may be a serious problem, especially when linked with antibiotic resistance and high percentage of serotypes responsible for listeriosis outbreaks.

High-Hydrostatic-Pressure (HHP) Processing Technology as a Novel Control Method for Listeria monocytogenes Occurrence in Mediterranean-Style Dry-Fermented Sausages

Although conventional microbial control techniques are currently employed and largely successful, their major drawbacks are related to their effects on quality of processed food. In recent years, there has been a growing demand for high-quality foods that are microbially safe and retain most of their natural freshness. Therefore, several modern and innovative methods of microbial control in food processing have been developed. High-hydrostatic-pressure (HHP) processing technology has been mainly used to enhance the food safety of ready-to-eat (RTE) products as a new pre-/post-packaging, non-thermal purification method in the meat industry. Listeria monocytogenes is a pertinent target for microbiological safety and shelf-life; due to its capacity to multiply in a broad range of food environments, is extremely complicated to prevent in fermented-sausage-producing plants. The frequent detection of L. monocytogenes in final products emphasizes the necessity for the producers of fermented sausages to correctly overcome the hurdles of the technological process and to prevent the presence of L. monocytogenes by applying novel control techniques. This review discusses a collection of recent studies describing pressure-induced elimination of L. monocytogenes in fermented sausages produced in the Mediterranean area.