The adaptive response of bacterial food-borne pathogens in the environment, host and food: Implications for food safety

Salmonella Typhimurium and Vibrio cholerae can be transferred from plastic mulch to basil and spinach salad leaves

Plastic pollution is increasingly found in agricultural environments, where it contaminates soil and crops. Microbial biofilms rapidly colonise environmental plastics, such as the plastic mulches used in agricultural systems, which provide a unique environment for microbial plastisphere communities. Human pathogens can also persist in the plastisphere, and enter agricultural environments via flooding or irrigation with contaminated water. In this study we examined whether Salmonella Typhimurium and Vibrio cholerae can be transferred from the plastisphere on plastic mulch to the surface of ready-to-eat crop plants, and subsequently persist on the leaf surface. Both S. Typhimurium and V. cholerae were able to persist for 14 days on fragments of plastic mulch adhering to the surface of leaves of both basil and spinach. Importantly, within 24 h both pathogens were capable of dissociating from the surface of the plastic and were transferred onto the surface of both basil and spinach leaves. This poses a further risk to food safety and human health, as even removal of adhering plastics and washing of these ready-to-eat crops would not completely remove these pathogens. As the need for more intensive food production increases, so too does the use of plastic mulches in agronomic systems. Therefore, there is now an urgent need to understand the unquantified co-pollutant pathogen risk of contaminating agricultural and food production systems with plastic pollution.

Sequencing-based analysis of the microbiomes of Spanish food processing facilities reveals environment-specific variation in the dominant taxa and antibiotic resistance genes

In the last years, advances in high throughput sequencing technologies have opened the possibility to broaden environmental monitoring activities in facilities processing food, offering expanded opportunities for characterizing in an untargeted manner the microbiome and resistome of foods and food processing environments (FPE) with huge potential benefits in food safety management systems. Here the microbiome and resistome of FPE from slaughterhouses (n = 3), dairy (n = 12) and meat (n = 10) processing plants were assessed through whole metagenome sequencing of 2 composite samples for each facility, comprising 10 FPE swabs taken from food contact surfaces and 10 FPE samples from non-food contact surfaces, respectively. FPE from slaughterhouses had more diverse microbiomes and resistomes, while FPE from dairy processing plants showed the highest β-dispersion, consistent with a more heterogeneous microbiome and resistome composition. The predominant bacterial genera depended on the industry type, with Pseudomonas and Psychrobacter being highly dominant in surfaces from slaughterhouses and meat industries, while different lactic acid bacteria predominated in dairy industries. The most abundant antimicrobial resistance genes (ARG) found were associated with resistance to aminoglycosides, tetracyclines and quaternary ammonium compounds (QAC). ARGs relating to resistance to aminoglycosides and tetracyclines were significantly more prevalent in slaughterhouses than in food processing plants, while QAC resistance genes were particularly abundant in some food contact surfaces from dairy and meat processing plants, suggesting that daily sanitation under suboptimal conditions may be selecting for persistent microbiota tolerant to these biocides in some facilities. The taxonomic mapping of ARG pointed to specific bacterial genera, such as Escherichia, Bacillus, or Staphylococcus, as carriers of the most relevant resistance determinants. About 63% of all ARG reads were assigned to contigs classified as plasmid-associated, indicating that the resistome of FPE may be strongly shaped through the spread of mobile genetic elements. Overall, the relevance of FPE as reservoirs of ARG was confirmed and it was demonstrated that next generation sequencing technologies allowing a deep characterisation of sources and routes of spread of microorganisms and antimicrobial resistance determinants in food industry settings hold promise to be integrated in monitoring and food safety management programmes.

A comprehensive investigation of protein expression profiles in L. monocytogenes exposed to thermal abuse, mild acid, and salt stress conditions

Preventing L. monocytogenes infection is crucial for food safety, considering its widespread presence in the environment and its association with contaminated RTE foods. The pathogen's ability to persist under adverse conditions, for example, in food processing facilities, is linked to virulence and resistance mechanisms, including biofilm formation. In this study, the protein expression patterns of two L. monocytogenes 1/2a strains, grown under environmental stressors (mild acidic pH, thermal abuse, and high concentration of NaCl), were investigated. Protein identification and prediction were performed by nLC-ESI-MS/MS and nine different bioinformatic software programs, respectively. Gene enrichment analysis was carried out by STRING v11.05. A total of 1,215 proteins were identified, of which 335 were non-cytosolic proteins and 265 were immunogenic proteins. Proteomic analysis revealed differences in protein expression between L. monocytogenes strains in stressful conditions. The two strains exhibited unique protein expression profiles linked to stress response, virulence, and pathogenesis. Studying the proteomic profiles of such microorganisms provides information about adaptation and potential treatments, highlighting their genetic diversity and demonstrating the utility of bioinformatics and proteomics for a broader analysis of pathogens.

Biological control of foodborne pathogens by lactic acid bacteria: A focus on juice processing industries

The use of lactic acid bacteria (LAB) in foods as biocontrol agents against foodborne pathogens has become increasingly known. Under the premise that controlling the adhesion of microorganisms to food contact surfaces is an essential step for meeting the goals of food processing, the aim of this work was to investigate the inhibitory and anti-biofilm effectiveness of Lactobacillus rhamnosus GG (ATCC 53103) and Lactobacillus casei (ATCC 393) against Escherichia coli O157:H7, Salmonella enterica and Listeria monocytogenes. Lactobacillus strains (108CFU/ml) and pathogens (104CFU/ml) were evaluated to monitor LAB anti-adhesive and antibiofilm effect, in two main scenarios: (i) co-adhesion and (ii) pathogen incorporation to stainless steel surfaces with a protective biofilm of Lactobacillus cells. In (i) the predominant effect was observed in L. rhamnosus against S. enterica and L. monocytogenes, whereas in (ii) both LAB significantly reduced the number of pathogenic adherent cells. The effect of pre-established LAB biofilms was more successful in displacing the three pathogens than when they were evaluated under co-adhesion. These findings show that both LAB can be considered good candidates to prevent or inhibit the adhesion and colonization of L. monocytogenes, S. enterica and E. coli O157:H7 on surfaces and conditions of relevance for juice processing industries, offering alternatives for improving the safety and quality of fruit-based products.

Investigation of differences in susceptibility of Campylobacter jejuni strains to UV light-emitting diode (UV-LED) technology

Campylobacter jejuni remains a high priority in public health worldwide. Ultraviolet light emitting-diode technology (UV-LED) is currently being explored to reduce Campylobacter levels in foods. However, challenges such as differences in species and strain susceptibilities, effects of repeated UV-treatments on the bacterial genome and the potential to promote antimicrobial cross-protection or induce biofilm formation have arisen. We investigated the susceptibility of eight C. jejuni clinical and farm isolates to UV-LED exposure. UV light at 280 nm induced different inactivation kinetics among strains, of which three showed reductions greater than 1.62 log CFU/mL, while one strain was particularly resistant to UV light with a maximum reduction of 0.39 log CFU/mL. However, inactivation was reduced by 0.46-1.03 log CFU/mL in these three strains and increased to 1.20 log CFU/mL in the resistant isolate after two repeated-UV cycles. Genomic changes related to UV light exposure were analysed using WGS. C. jejuni strains with altered phenotypic responses following UV exposure were also found to have changes in biofilm formation and susceptibility to ethanol and surface cleaners.

Turning up the heat mimics allosteric signaling in imidazole-glycerol phosphate synthase

Allosteric drugs have the potential to revolutionize biomedicine due to their enhanced selectivity and protection against overdosage. However, we need to better understand allosteric mechanisms in order to fully harness their potential in drug discovery. In this study, molecular dynamics simulations and nuclear magnetic resonance spectroscopy are used to investigate how increases in temperature affect allostery in imidazole glycerol phosphate synthase. Results demonstrate that temperature increase triggers a cascade of local amino acid-to-amino acid dynamics that remarkably resembles the allosteric activation that takes place upon effector binding. The differences in the allosteric response elicited by temperature increase as opposed to effector binding are conditional to the alterations of collective motions induced by either mode of activation. This work provides an atomistic picture of temperature-dependent allostery, which could be harnessed to more precisely control enzyme function.

Susceptibility and transcriptomic response to plasma-activated water of Listeria monocytogenes planktonic and sessile cells

Plasma-Activated Water (PAW) was generated from tap water using a surface dielectric barrier discharge at different discharge power (26 and 36 W) and activation time (5 and 30 min). The inactivation of a three-strain Listeria monocytogenes cocktail in planktonic and biofilm state was evaluated. PAW generated at 36 W-30 min showed the lowest pH and the highest hydrogen peroxide, nitrates, nitrites contents and effectiveness against cells on planktonic state, resulting in 4.6 log reductions after a 15-min treatment. Although the antimicrobial activity in biofilms formed on stainless steel and on polystyrene was lower, increasing the exposure time to 30 min allowed an inactivation >4.5 log cycles. The mechanisms of action of PAW were investigated using chemical solutions that mimic its physico-chemical characteristics and also RNA-seq analysis. The main transcriptomic changes affected carbon metabolism, virulence and general stress response genes, with several overexpressed genes belonging to the cobalamin-dependent gene cluster.

Assessment of a plasmid conjugation procedure to monitor horizontal transfer of an extended-spectrum β-lactamase resistance gene under food chain scenarios

Plasmids are relevant reservoirs of antimicrobial resistance genes (ARGs) which confer adaptive advantages to their host and can be horizontally transferred. The aims of this study were to develop a conjugation procedure to monitor the horizontal transfer of a 193 kb plasmid containing the extended-spectrum β-lactamase production gene bla _CTX-M-14 between two Escherichia coli strains under a range of food chain-related scenarios, including temperature (20-37 °C), pH (5.0-9.0) or the presence of some biocidal agents (benzalkonium chloride, sodium hypochlorite or peracetic acid). The average conjugation rate in LB broth after 18 h at 37 °C was 2.09e-04 and similar rates were observed in a food matrix (cow's milk). The conjugation was reduced at temperatures below 37 °C, at alkaline pH (especially at pH 9.0) or in the presence of benzalkonium chloride. Peracetic acid and sodium hypochlorite slightly increased conjugation rates, which reached 5.59e-04 and 6.77e-03, respectively. The conjugation procedure described can be used to identify risk scenarios leading to an enhanced ARGs transmission via plasmid conjugation, as well as to identify novel intervention strategies impairing plasmid conjugation and tackling antimicrobial resistance.

An ultrasensitive fluorescence detection template of pathogenic bacteria based on dual catalytic hairpin DNA Walker@Gold nanoparticles enzyme-free amplification

Integrating the advantages of catalytic hairpin components and multi-foot DNA walker, we designed a 16S rRNA detection probe template for pathogen bacteria, which utilizes DNA ligation quencher and dual catalytic hairpin@DNA walker to induce signal recovery. The dual catalytic hairpin@DNA walker uses the walking position of the target on the AuNP as a foothold to promote the reaction, so that the biosensing of the low-abundance target sequence can induce signal recovery. During the entire experiment, no enzyme is required, which can avoid the limitation of enzyme degradation under unfavorable conditions and the inability to detect the target. Most importantly, the detection template has the advantages of high sensitivity, and its detection limit is significantly better than that of single hairpin DNA walker probe. As the detection system can sensitively and rapidly detect its targeted bacteria and not rely on any enzyme and sophisticated instrumentation, it has great potential for sensitive and specific pathogenic bacteria detection.

Genetic, physiological, and cellular heterogeneities of bacterial pathogens in food matrices: Consequences for food safety

In complex food systems, bacteria live in heterogeneous microstructures, and the population displays phenotypic heterogeneities at the single-cell level. This review provides an overview of spatiotemporal drivers of phenotypic heterogeneity of bacterial pathogens in food matrices at three levels. The first level is the genotypic heterogeneity due to the possibility for various strains of a given species to contaminate food, each of them having specific genetic features. Then, physiological heterogeneities are induced within the same strain, due to specific microenvironments and heterogeneous adaptative responses to the food microstructure. The third level of phenotypic heterogeneity is related to cellular heterogeneity of the same strain in a specific microenvironment. Finally, we consider how these phenotypic heterogeneities at the single-cell level could be implemented in mathematical models to predict bacterial behavior and help ensure microbiological food safety.

Microbiological Safety and Shelf-Life of Low-Salt Meat Products—A Review

Salt is widely employed in different foods, especially in meat products, due to its very diverse and extended functionality. However, the high intake of sodium chloride in human diet has been under consideration for the last years, because it is related to serious health problems. The meat-processing industry and research institutions are evaluating different strategies to overcome the elevated salt concentrations in products without a quality reduction. Several properties could be directly or indirectly affected by a sodium chloride decrease. Among them, microbial stability could be shifted towards pathogen growth, posing a serious public health threat. Nonetheless, the majority of the literature available focuses attention on the sensorial and technological challenges that salt reduction implies. Thereafter, the need to discuss the consequences for shelf-life and microbial safety should be considered. Hence, this review aims to merge all the available knowledge regarding salt reduction in meat products, providing an assessment on how to obtain low salt products that are sensorily accepted by the consumer, technologically feasible from the perspective of the industry, and, in particular, safe with respect to microbial stability.

Listeria monocytogenes Pathogenesis: The Role of Stress Adaptation

Adaptive stress tolerance responses are the driving force behind the survival ability of Listeria monocytogenes in different environmental niches, within foods, and ultimately, the ability to cause human infections. Although the bacterial stress adaptive responses are primarily a necessity for survival in foods and the environment, some aspects of the stress responses are linked to bacterial pathogenesis. Food stress-induced adaptive tolerance responses to acid and osmotic stresses can protect the pathogen against similar stresses in the gastrointestinal tract (GIT) and, thus, directly aid its virulence potential. Moreover, once in the GIT, the reprogramming of gene expression from the stress survival-related genes to virulence-related genes allows L. monocytogenes to switch from an avirulent to a virulent state. This transition is controlled by two overlapping and interlinked transcriptional networks for general stress response (regulated by Sigma factor B, (SigB)) and virulence (regulated by the positive regulatory factor A (PrfA)). This review explores the current knowledge on the molecular basis of the connection between stress tolerance responses and the pathogenesis of L. monocytogenes. The review gives a detailed background on the currently known mechanisms of pathogenesis and stress adaptation. Furthermore, the paper looks at the current literature and theories on the overlaps and connections between the regulatory networks for SigB and PrfA.

A Cruciform Petal-like (ZIF-8) with Bactericidal Activity against Foodborne Gram-Positive Bacteria for Antibacterial Food Packaging

Most antibacterial nanomaterials used in food packaging act by releasing reactive oxygen species (ROS), which cannot efficiently have an inhibitory effect by penetrating the cell wall of Gram-positive Staphylococcus aureus. In this work, we used the cruciform petal-like zeolite imidazole framework-8 (ZIF-8) synthesized in the water phase which can release active Zn compounds in aqueous solution and exert a stronger inhibitory effect on S. aureus. The experimental results demonstrated that the aqueous cruciform petal-like ZIF-8 has the same photocatalytic activity as traditional ZIF-8 and can be applied in photocatalytic bacterial inactivation. The cruciform petal-like ZIF-8 was also shown to release active Zn compounds in aqueous solution with a better antibacterial effect against S. aureus, reaching 95% inactivation efficiency. The antibacterial effect was therefore 70% higher than that of traditional ZIF-8. Based on its excellent antibacterial properties, we loaded petal-like ZIF-8, PDA and PVA onto ordinary fibers to prepare ZIF-8-Film. The results further showed that ZIF-8-Film has a high filtration capacity, which can be used in antibacterial packaging material with the required air permeability. Moreover, ZIF-8-Flim can clean the surface on its own and can maintain a sterile environment. It is different from other disposable materials on the market in that it can be reused and has a self-disinfection function.

Stress response modulation: the key to survival of pathogenic and spoilage bacteria during poultry processing

The control of bacterial contaminants on meat is a key area of interest in the food industry. Bacteria are exposed to a variety of stresses during broiler processing which challenge bacterial structures and metabolic pathways causing death or sublethal injury. To counter these stresses, bacteria possess robust response systems that can induce shifts in the transcriptome and proteome to enable survival. Effective adaptive responses, such as biofilm formation, shock protein production and metabolic flexibility, require rapid induction and implementation at a cellular and community level to facilitate bacterial survival in adverse conditions. This review aims to provide an overview of the scientific literature pertaining to the regulation of complex adaptive processes used by bacteria to survive the processing environment, with particular focus on species that impact the quality and safety of poultry products like Campylobacter spp., Salmonella enterica and Pseudomonas spp.

Multiplex PCR method for the detection of human norovirus, Salmonella spp., Shigella spp., and shiga toxin producing Escherichia coli in blackberry, coriander, lettuce and strawberry

A multiplex PCR method was developed for the simultaneous detection of murine norovirus (MNV-1) as a surrogate for human norovirus (HuNoV) GI and GII, Salmonella spp., Shigella spp., and Shiga toxin producing Escherichia coli (STEC) in fresh produce. The toxicity of the glycine buffer on bacterial pathogens viability was evaluated. The growth of each of the three pathogens (previously stressed) was evaluated at 35 and 41.5 °C in modified buffered peptone water (mBPW) and trypticase soy broth (TSB), supplemented with vancomycin, novobiocin and brilliant green at two concentration levels. The selected conditions for simultaneous enrichment were: 41.5 °C/mBPW/supplemented with 8 ppm vancomycin, 0.6 ppm novobiocin and 0.2 ppm brilliant green. The pathogens and aerobic plate count (APC) growth was evaluated in the enrichment of lettuce, coriander, strawberry and blackberry under the best enrichment conditions. Starting from 1 to 10 CFU/mL, Salmonella reached from 7.63 to 8.91, Shigella 6.81 to 7.76 and STEC 7.43 to 9.27 log CFU/mL. The population reached for the APC was 5.11-6.56 log CFU/mL. Simultaneous detection by PCR was done using designed primers targeting invA, ipaH, stx1 and stx2 genes, and MNV-1. The detection sensitivity was 10-100 PFU for the MNV-1 and 1-10 CFU for each pathogenic bacteria. This protocol takes 6 h for MNV-1 and 24 h for Salmonella spp., Shigella spp., and STEC detection from the same food portion. In total, 200 samples were analyzed from retail markets from Queretaro, Mexico. Two strawberry samples were positive for HuNoV GI and one lettuce sample was positive for STEC. In conclusion, the method developed in this study is capable of detecting HuNoV GI and GII, Salmonella spp., Shigella spp and STEC from the same fresh produce sample.

Enterobacteriaceae in food safety with an emphasis on raw milk and meat

There has been a growing interest in traditional dairy (such as raw milk cheeses) and meat products, in recent years. However, these products are suitable and nutrient medium and may be easily contaminated by microorganisms such as Enterobacteriaceae. Enterobacteriaceae are considered to be the indicator bacteria for microbiological quality of food and hygiene status of a production process. Additionally, the food contaminated by Enterobacteriaceae poses a microbiological risk for consumers. In fact, the contamination of raw milk and meat by Enterobacteriaceae amid manufacturing may easily occur from various environmental sources, and this group of bacteria is frequently detected in dairy and meat products. Therefore, monitoring the microbiological quality of the used raw material and maintaining high standards of hygiene in the production process are mandatory for a high quality of traditional products and the safety of the potential consumers. The goal of this review is to present the most recent survey on Enterobacteriaceae growth, number, and distribution in raw milk cheeses and meat, as well as to discuss the sources of contamination and methods of control. KEY POINTS: • Enterobacteriaceae: role and importance in milk and meat products, EU legal regulations • Dynamics, distribution, and survival of Enterobacteriaceae in milk and meat • Mechanisms of control of Enterobacteriaceae in dairy products.

Microbial colonization and resistome dynamics in food processing environments of a newly opened pork cutting industry during 1.5 years of activity

BACKGROUND

The microorganisms that inhabit food processing environments (FPE) can strongly influence the associated food quality and safety. In particular, the possibility that FPE may act as a reservoir of antibiotic-resistant microorganisms, and a hotspot for the transmission of antibiotic resistance genes (ARGs) is a concern in meat processing plants. Here, we monitor microbial succession and resistome dynamics relating to FPE through a detailed analysis of a newly opened pork cutting plant over 1.5 years of activity.

RESULTS

We identified a relatively restricted principal microbiota dominated by Pseudomonas during the first 2 months, while a higher taxonomic diversity, an increased representation of other taxa (e.g., Acinetobacter, Psychrobacter), and a certain degree of microbiome specialization on different surfaces was recorded later on. An increase in total abundance, alpha diversity, and β-dispersion of ARGs, which were predominantly assigned to Acinetobacter and associated with resistance to certain antimicrobials frequently used on pig farms of the region, was detected over time. Moreover, a sharp increase in the occurrence of extended-spectrum β-lactamase-producing Enterobacteriaceae and vancomycin-resistant Enterococcaceae was observed when cutting activities started. ARGs associated with resistance to β-lactams, tetracyclines, aminoglycosides, and sulphonamides frequently co-occurred, and mobile genetic elements (i.e., plasmids, integrons) and lateral gene transfer events were mainly detected at the later sampling times in drains.

CONCLUSIONS

The observations made suggest that pig carcasses were a source of resistant bacteria that then colonized FPE and that drains, together with some food-contact surfaces, such as equipment and table surfaces, represented a reservoir for the spread of ARGs in the meat processing facility. Video Abstract.

Mechanisms adopted by Salmonella to colonize plant hosts

Fruits and vegetables consumed fresh or as minimally-processed produce, have multiple benefits for our diet. Unfortunately, they bring a risk of food-borne diseases, for example salmonellosis. Interactions between Salmonella and crop plants are indeed a raising concern for the global health. Salmonella uses multiple strategies to manipulate the host defense system, including plant's defense responses. The main focus of this review are strategies used by this bacterium during the interaction with crop plants. Emphasis was put on how Salmonella avoids the plant defense responses and successfully colonizes plants. In addition, several factors were reviewed assessing their impact on Salmonella persistence and physiological adaptation to plants and plant-related environment. The understanding of those mechanisms, their regulation and use by the pathogen, while in contact with plants, has significant implication on the growth, harvest and processing steps in plant production system. Consequently, it requires both the authorities and science to advance and definite methods aiming at prevention of crop plants contamination. Thus, minimizing and/or eliminating the potential of human diseases.

A Diffusion Model to Quantify Membrane Repair Process in Listeria monocytogenes Exposed to High Pressure Processing Based on Fluorescence Microscopy Data

The effects of environmental stresses on microorganisms have been well-studied, and cellular responses to stresses such as heat, cold, acids, and salts have been extensively discussed. Although high pressure processing (HPP) is becoming more popular as a preservation method in the food industry, the characteristics of the cellular damage caused by high pressure are unclear, and the microbial response to this stress has not yet been well-explored. We exposed the pathogen Listeria monocytogenes to HPP (400 MPa, 8 min, 8°C) and found that the high pressure created plasma membrane pores. Using a common staining technique involving propidium iodide (PI) combined with high-frequency fluorescence microscopy, we monitored the rate of diffusion of PI molecules into hundreds of bacterial cells through these pores on days 0, 1, 2, 3, and 4 after pressurization. We also developed a mathematical dynamic model based on mass transfer and passive diffusion laws, calibrated using our microscopy experiments, to evaluate the response of bacteria to HPP. We found that the rate of diffusion of PI into the cells decreased over the 4 consecutive days after exposure to HPP, indicating repair of the pressure-created membrane pores. The model suggested a temporal change in the size of pores until closure. To the best of our knowledge, this is the first time that pressure-created membrane pores have been quantitatively described and shown to diminish with time. In addition, we found that the membrane repair rate in response to HPP was linear, and growth was temporarily arrested at the population level during the repair period. These results support the existence of a progressive repair process in some of the cells that take up PI, which can therefore be considered as being sub-lethally injured rather than dead. Hence, we showed that a subgroup of bacteria survived HPP and actively repaired their membrane pores.

Recent advances in understanding the effect of acid-adaptation on the cross-protection to food-related stress of common foodborne pathogens

Acid stress is one of the most common stresses that foodborne pathogens encounter. It could occur naturally in foods as a by-product of anaerobic respiration (fermentation), or with the addition of acids. However, foodborne pathogens have managed to survive to acid conditions and consequently develop cross-protection to subsequent stresses, challenging the efficacy of hurdle technologies. Here, we cover the studies describing the cross-protection response following acid-adaptation, and the possible molecular mechanisms for cross-protection. The current and future prospective of this research topic with the knowledge gaps in the literature are also discussed. Exposure to acid conditions (pH 3.5 - 5.5) could induce cross-protection for foodborne pathogens against subsequent stress or multiple stresses such as heat, cold, osmosis, antibiotic, disinfectant, and non-thermal technology. So far, the known molecular mechanisms that might be involved in cross-protection include sigma factors, glutamate decarboxylase (GAD) system, protection or repair of molecules, and alteration of cell membrane. Cross-protection could pose a serious threat to food safety, as many hurdle technologies are believed to be effective in controlling foodborne pathogens. Thus, the exact mechanisms underlying cross-protection in a diversity of bacterial species, stress conditions, and food matrixes should be further studied to reduce potential food safety risks. HighlightsFoodborne pathogens have managed to survive to acid stress, which may provide protection to subsequent stresses, known as cross-protection.Acid-stress may induce cross-protection to many stresses such as heat, cold, osmotic, antibiotic, disinfectant, and non-thermal technology stress.At the molecular level, foodborne pathogens use different cross-protection mechanisms, which may correlate with each other.

Survival of Selected Pathogenic Bacteria during PDO Pecorino Romano Cheese Ripening

This study was conducted to assess, for the first time, the survival of the pathogenic bacteria Listeria monocytogenes, Salmonella spp., Escherichia coli O157:H7, and Staphylococcus aureus during the ripening of protected designation of origin (PDO) Pecorino Romano cheese. A total of twenty-four cheese-making trials (twelve from raw milk and twelve from thermized milk) were performed under the protocol specified by PDO requirements. Sheep cheese milk was first inoculated before processing with approximately 106 colony-forming unit (CFU) mL−1 of each considered pathogen and the experiment was repeated six times for each selected pathogen. Cheese composition and pathogens count were then evaluated in inoculated raw milk, thermized milk, and cheese after 1, 90, and 150 days of ripening. pH, moisture, water activity, and salt content of cheese were within the range of the commercial PDO Pecorino Romano cheese. All the cheeses made from raw and thermized milk were microbiologically safe after 90 days and 1 day from their production, respectively. In conclusion, when Pecorino Romano cheese is produced under PDO specifications, from raw or thermized milk, a combination of factors including the speed and extent of curd acidification in the first phase of the production, together with an intense salting and a long ripening time, preclude the possibility of growth and survival of L. monocytogenes, Salmonella spp., and E. coli O157:H7. Only S. aureus can be still detectable at such low levels that it does not pose a risk to consumers.

Sublethal concentrations of undissociated acetic acid may not always stimulate acid resistance in Salmonella enterica sub. enterica serovar Enteritidis Phage Type 4: Implications of challenge substrate associated factors

Acid adaptation enhances survival of foodborne pathogens under lethal acid conditions that prevail in several food-related ecosystems. In the present study, the role of undissociated acetic acid in inducing acid resistance of Salmonella Enteritidis Phage Type 4 both in laboratory media and in an acid food matrix was investigated. Several combinations of acetic acid (0, 15, 25, 35 and 45 mM) and pH values (4.0, 4.5, 5.0, 5.5, 6.0) were screened for their ability to activate acid resistance mechanisms of pathogen exposed to pH 2.5 (screening assay). Increased survival was observed when increasing undissociated acetic acid within a range of sublethal concentrations (1.9–5.4 mM), but only at pH 5.5 and 6.0. No effect was observed at lower pH values, regardless of the undissociated acetic acid levels. Three combinations (15mM/pH5.0, 35mM/pH5.5, 45mM/pH6.0) were selected and further used for adaptation prior to inoculation in commercial tarama (fish roe) salad, i.e., an acid spread (pH 4.35 ± 0.02), stored at 5°C. Surprisingly and contrary to the results of the screening assay, none of the acid adaptation treatments enhanced survival of Salmonella Enteritidis in the food matrix, as compared to non-adapted cells (control). Further examination of the food pH value, acidulant and storage (challenge) temperature on the responses of the pathogen adapted to 15mM/pH5.0, 35mM/pH5.5 and 45mM/pH6.0 was performed in culture media. Cells adapted to 35mM/pH5.5 were unable to induce acid resistance when exposed to pH 4.35 (tarama salad pH value) at 37°C and 5°C, whereas incubation under refrigeration (5°C) at pH 4.35 sensitized 45mM/pH6.0 adapted cells against the subsequent acid and cold stress. In conclusion, pre-exposure to undissociated acetic acid affected the adaptive responses of Salmonella Enteritidis Phage Type 4 in a concentration- and pH-dependent manner, with regard to conditions prevailing during acid challenge.

Evaluation of larvicidal potential against larvae of Aedes aegypti (Linnaeus, 1762) and of the antimicrobial activity of essential oil obtained from the leaves of Origanum majorana L

This study evaluated the larvicidal activity of Origanum majorana Linnaeus essential oil, identified the chemical composition, evaluated the antimicrobial, cytotoxic and antioxidant potential. The larvicidal activity was evaluated against larvae of the third stage of Aedes aegypti Linaeus, whereas the chemical composition was identified by gas chromatography coupled to mass spectrometer, the antimicrobial activity was carried out against the bacteria Pseudomonas aeruginosa, Escherichia coli and Staphylococcus auereus, the antioxidant activity was evaluated from of 2.2-diphenyl-1-picryl-hydrazila sequestration and Artemia salina Leach cytotoxicity. Regarding to the results, the larvicidal activity showed that O. majorana L. essential oil caused high mortality in A. aegypti L. larvae. In the chromatographic analysis, the main component found in O. majorana L. essential oil was pulegone (57.05%), followed by the other components verbenone (16.92%), trans-p-menthan-2-one (8.57%), iso-menthone (5.58%), piperitone (2.83%), 3-octanol (2.35%) and isopulegol (1.47%). The antimicrobial activity showed that E. coli and P. aeruginosa bacteria were more sensitive to oil than S. aureus, which was resistant at all concentrations. Essential oil did not present antioxidant activity, but it has high cytotoxic activity against A. salina L.

Selection for Antimicrobial Resistance in Foodborne Pathogens through Exposure to UV Light and Nonthermal Atmospheric Plasma Decontamination Techniques

This study was aimed at assessing whether the repeated exposure of 12 strains of Salmonella spp., Escherichia coli, and Listeria monocytogenes to alternative nonthermal decontamination techniques with UV light (UV-C) and nonthermal atmospheric plasma (NTAP) may cause the emergence of variants showing increased resistance to clinically relevant antibiotics (ampicillin, cefotaxime, ciprofloxacin, gentamicin, streptomycin, tetracycline, erythromycin, vancomycin, and colistin). UV-C and NTAP treatments were applied on the surface of inoculated brain heart infusion (BHI) agar plates. Survivors were recovered and after 24 h of growth in BHI broth were again subjected to the decontamination treatment; this was repeated for 10 consecutive cycles. A total of 174 strain/decontamination technique/antibiotic combinations were tested, and 12 variant strains with increased resistance to one of the antibiotics studied were identified, with the increases in the MICs in Mueller-Hinton broth ranging from 2- to 256-fold. The variant strains of Salmonella spp. isolated were further characterized through phenotypic screenings and whole-genome sequencing (WGS) analyses. Most changes in susceptibility were observed for antibiotics that act at the level of protein synthesis (aminoglycosides, tetracyclines, and glycylcyclines) or DNA replication (fluoroquinolones), as well as for polymyxins. No changes in resistance to β-lactams were detected. WGS analyses showed the occurrence of sequence alterations in some antibiotic cellular targets (e.g., gyrA for ciprofloxacin-resistant variants, rpsL for a streptomycin-resistant variant), accompanied by variations in stress response regulators and membrane transporters likely involved in the nonselective efflux of antibiotics, which altogether resulted in a low- to medium-level increase in microbial resistance to several antibiotics.IMPORTANCE The emergence and spread of antibiotic resistance along the food chain can be influenced by the different antimicrobial strategies used from farm to fork. This study evidences that two novel, not yet widely used, nonthermal microbial decontamination techniques, UV light and nonthermal atmospheric plasma, can select variants with increased resistance to various clinically relevant antibiotics, such as ciprofloxacin, streptomycin, tetracycline, and erythromycin. Whole-genome analysis of the resistant variants obtained for Salmonella spp. allowed identification of the genetic changes responsible for the observed phenotypes and suggested that some antimicrobial classes are more susceptible to the cross-resistance phenomena observed. This information is relevant, since these novel decontamination techniques are being proposed as possible alternative green techniques for the decontamination of environments and equipment in food and clinical settings.

Stress Response of Vibrio parahaemolyticus and Listeria monocytogenes Biofilms to Different Modified Atmospheres

The sessile biofilms of Vibrio parahaemolyticus and Listeria monocytogenes have increasingly become a critical threat in seafood safety. This study aimed to evaluate the effects of modified atmospheres on the formation ability of V. parahaemolyticus and L. monocytogenes biofilms. The stress responses of bacterial biofilm formation to modified atmospheres including anaerobiosis (20% carbon dioxide, 80% nitrogen), micro-aerobiosis (20% oxygen, 80% nitrogen), and aerobiosis (60% oxygen, 40% nitrogen) were illuminated by determining the live cells, chemical composition analysis, textural parameter changes, expression of regulatory genes, etc. Results showed that the biofilm formation ability of V. parahaemolyticus was efficiently decreased, supported by the fact that the modified atmospheres significantly reduced the key chemical composition [extracellular DNA (eDNA) and extracellular proteins] of the extracellular polymeric substance (EPS) and negatively altered the textural parameters (biovolume, thickness, and bio-roughness) of biofilms during the physiological conversion from anaerobiosis to aerobiosis, while the modified atmosphere treatment increased the key chemical composition of EPS and the textural parameters of L. monocytogenes biofilms from anaerobiosis to aerobiosis. Meanwhile, the expression of biofilm formation genes (luxS, aphA, mshA, oxyR, and opaR), EPS production genes (cpsA, cpsC, and cpsR), and virulence genes (vopS, vopD1, vcrD1, vopP2β, and vcrD2β) of V. parahaemolyticus was downregulated. For the L. monocytogenes cells, the expression of biofilm formation genes (flgA, flgU, and degU), EPS production genes (Imo2554, Imo2504, inlA, rmlB), and virulence genes (vopS, vopD1, vcrD1, vopP2β, and vcrD2β) was upregulated during the physiological conversion. All these results indicated that the modified atmospheres possessed significantly different regulation on the biofilm formation of Gram-negative V. parahaemolyticus and Gram-positive L. monocytogenes, which will provide a novel insight to unlock the efficient control of Gram-negative and Gram-positive bacteria in modified-atmosphere packaged food.

Molecular regulatory mechanisms of Escherichia coli O157:H7 in response to ultrasonic stress revealed by proteomic analysis

The antimicrobial effects of ultrasonic filed have been studied for years at the phenotypic level, but there is little research to reveal the molecular regulatory mechanisms underlying the phenotypes. In this study, isobaric tag for relative and absolute quantification (iTRAQ) proteome was applied to analyze the regulatory networks of Escherichia coli O157:H7 in response to ultrasonic stress in whole-genome scale. A total of 1856 differentially expressed proteins were identified, of which 1141 were significant up-regulated and 715 down-regulated compared with live control cells. The comprehensive proteome coverage analysis showed that ultrasonic filed influenced various metabolic pathways in Escherichia coli O157:H7 cells. The ultrasound-induced up-regulation of global stress response regulator RpoS, bacterial mechanosensitive channels and SOS response protein RecA were described from the molecular level for the first time. In addition, we proposed a possible action mechanism that the free radicals produced by acoustic cavitation might enter into cells via the activated mechanosensitive channels, leading to the elevated intracellular ROS level and subsequent cell death. Last but not the least, we illustrated the all-or-nothing phenomenon of power ultrasound might due to the destruction of crucial cell defensive systems, including heat shock proteins and oxidative response regulators. These new findings can let us understand the ultrasonic effects more deeply and will contribute to this area.

Food-to-Humans Bacterial Transmission

Microorganisms vehiculated by food might benefit health, cause minimal change within the equilibrium of the host microbial community or be associated with foodborne diseases. In this chapter we will focus on human pathogenic bacteria for which food is conclusively demonstrated as their transmission mode to human. We will describe the impact of foodborne diseases in public health, the reservoirs of foodborne pathogens (the environment, human and animals), the main bacterial pathogens and food vehicles causing human diseases, and the drivers for the transmission of foodborne diseases related to the food-chain, host or bacteria features. The implication of food-chain (foodborne pathogens and commensals) in the transmission of resistance to antibiotics relevant to the treatment of human infections is also evidenced. The multiplicity and interplay of drivers related to intensification, diversification and globalization of food production, consumer health status, preferences, lifestyles or behaviors, and bacteria adaptation to different challenges (stress tolerance and antimicrobial resistance) from farm to human, make the prevention of bacteria-food-human transmission a modern and continuous challenge. A global One Health approach is mandatory to better understand and minimize the transmission pathways of human pathogens, including multidrug-resistant pathogens and commensals, through food-chain.

Influence of the norepinephrine and medium acidification in the growth and adhesion of Salmonella Heidelberg isolated from poultry

Salmonella spp. are among the leading pathogens responsible for foodborne illnesses worldwide. Bacterial communities use a quorum sensing (QS) system to control biofilm formation. QS is a cell-to-cell signaling mechanism involving compounds called auto-inducers (AI). Norepinephrine utilizes the same bacterial signaling of AI-3 and serves as a signal of QS. Acid stress is a challenge encountered by microorganisms in food processing environments and in the gastrointestinal tracts of hosts. Thus, adaptation to acidic environments may increase the pathogenicity of the strain. The aim of this study was to evaluate the influence of two concentrations of norepinephrine (100 μM and 250 μM) and acidification (pH 3.0) of the medium on the growth and adhesion of Salmonella Heidelberg strains isolated from poultry sources at 12 °C and 25 °C. Furthermore, three genes associated with the biofilm formation process were detected (adrA, csgD, and sidA). Norepinephrine stimulation did not influence the growth or adhesion of Salmonella Heidelberg strains, regardless of the catecholamine concentration and temperature. On the other hand, the use of acidified medium (pH 3.0) resulted in a significant reduction of growth and a significant increase of S. Heidelberg adhesion at both temperatures, indicating that the acidified medium favors the biofilm formation process. The adrA and sidA genes showed higher detection frequencies than csgD. Experiments analyzing the biofilm production process by S. Heidelberg strains are not common, and further studies are necessary to understand this complex process.

Comparative stress response to food preservation conditions of ST19 and ST213 genotypes of Salmonella enterica serotype Typhimurium

The replacement of the most prevalent Salmonella enterica genotypes has been documented worldwide. Here we tested the hypothesis that the current prevalent sequence type ST213 of serotype Typhimurium in Mexico has a higher resistance to stressful food preservation conditions than the displaced sequence ST19. ST19 showed higher cell viability percentages than ST213 in osmotic (685 mM NaCl) and acidic (pH 3.5) stress conditions and in combination with refrigeration (4 °C) and ambient (≈22 °C) temperatures. Both genotypes showed the same poststress recovery growth. ST213 formed biofilm and filamentous cells (FCs) under stress, whereas ST19 did not. ST213 cells also showed higher motility. The capacity of ST213 to form FCs may explain its lower viability percentages when compared with ST19, i.e., ST213 cells divided less under stress conditions, but FCs had the same recovery capacity of ST19 cells. ST213 presented a higher unsaturated/saturated fatty acids ratio (0.5-0.6) than ST19 (0.2-0.5), which indicates higher membrane fluidity. The transcript levels of the rpoS gene were similar between genotypes under the experimental conditions employed. Biofilm formation, the generation of FCs, cell motility and membrane modification seem to make ST213 more resistant than ST19 to food preservation environments.

Salmonella enterica Growth Conditions Influence Lettuce Leaf Internalization

Human pathogens on plants (HPOP) have evolved complex interactions with their plant host. Stomatal internalization is one such mode of interaction, where bacteria are attracted to stomata and penetrate into the substomatal cavity by a process mediated by chemotaxis. Internalization enables HPOP to evade the hostile environment of the leaf surface and find a protected, nutrient-rich niche within the leaf. Numerous studies have documented attachment and entry of the foodborne pathogens, Salmonella enterica and Escherichia coli into stomata. Internalization, however, varies considerably among different pathogens and in different plants, and both bacterial and plant’s factors were reported to influence HPOP attachment and internalization. Here we have studied the effect of laboratory growth conditions, on the internalization of Salmonella enterica serovar Typhimurium (STm) into lettuce leaf. We have further tested the potential involvement of universal stress-proteins in leaf internalization. We found that STm grown in Luria Bertani broth devoid of NaCl (LBNS), or in diluted LB (0.5×LB) internalized lettuce leaf better (62 ± 5% and 59 ± 7%, respectively) compared to bacteria grown in LB (15 ± 7%). Growth under non-aerated conditions also enhanced STm internalization compared to growth under aerated conditions. Growth temperature of 25 and 37°C did not affect STm internalization, however, growth at 42°C, significantly augmented leaf internalization. Since, the tested growth conditions represent moderate stresses, we further investigated the involvement of five universal-stress genes in STm leaf internalization following growth in LBNS medium. Knockout mutations in ydaA, yecG, ybdQ, and uspAB, but not in ynaF, significantly reduced STm internalization compared to the wild-type (wt) strain, without affecting bacterial attachment and motility. Transduction of the mutations back to the parent strain confirmed the linkage between the mutations and the internalization phenotype. These findings support a specific role of the universal-stress genes in leaf internalization. The present study highlights the complexity of bacterial internalization process and may provide partial explanation for the variable, sometimes-contrasting results reported in the literature regarding stomatal internalization by HPOP. Characterization of the regulatory networks that mediate the involvement of usp genes and the tested growth factors in STm internalization should contribute to our understanding of human pathogens-plant interactions.

Structural and Proteomic Changes in Viable but Non-culturable Vibrio cholerae

Aquatic environments are reservoirs of the human pathogen Vibrio cholerae O1, which causes the acute diarrheal disease cholera. Upon low temperature or limited nutrient availability, the cells enter a viable but non-culturable (VBNC) state. Characteristic of this state are an altered morphology, low metabolic activity, and lack of growth under standard laboratory conditions. Here, for the first time, the cellular ultrastructure of V. cholerae VBNC cells raised in natural waters was investigated using electron cryo-tomography. This was complemented by a comparison of the proteomes and the peptidoglycan composition of V. cholerae from LB overnight cultures and VBNC cells. The extensive remodeling of the VBNC cells was most obvious in the passive dehiscence of the cell envelope, resulting in improper embedment of flagella and pili. Only minor changes of the peptidoglycan and osmoregulated periplasmic glucans were observed. Active changes in VBNC cells included the production of cluster I chemosensory arrays and change of abundance of cluster II array proteins. Components involved in iron acquisition and storage, peptide import and arginine biosynthesis were overrepresented in VBNC cells, while enzymes of the central carbon metabolism were found at lower levels. Finally, several pathogenicity factors of V. cholerae were less abundant in the VBNC state, potentially limiting their infectious potential. This study gives unprecedented insight into the physiology of VBNC cells and the drastically altered presence of their metabolic and structural proteins.

Toward rational selection criteria for selection of probiotics in pigs

An accurate understanding of properties of probiotics is a prerequisite for selecting probiotic organisms for use in swine production. This review aims to review selection criteria for probiotic organism in swine. The systematically investigated ecological history rather than the source of isolates should be regarded as the natural origin of probiotic strains, which helps to correct the inconsistencies arising from incorrect identification of the source. Moreover, in vivo studies are suggested as follow-up assessment to validate the characteristics of probiotic predicted by in vitro experiments. In addition, the intended probiotic effect depends on the age of the animal and disease prevention in young animals may require different probiotic strains when compared to growth promotion in older animals. With adequate selection criteria, the inclusion of probiotic in feed supplementation is a promising way to exert positive effects on sows, newborns, weanling animals and grower-finisher pigs. Both host-adapted probiotics and nomadic probiotics can be applied for pathogen inhibition but host adapted organisms appear to have a different mode of action. Host-adapted probiotic strains are likely to be associated with exclusive colonization while the nomadic or environmental strain exert better immune stimulating functions. Strains with potent enzymatic activity are fitter for grower pigs favoring feed digestion and enhancing growth performance.

Effect of Cellulose Nanocrystals from Different Lignocellulosic Residues to Chitosan/Glycerol Films

Interest in nanocellulose obtained from natural resources has grown, mainly due to the characteristics that these materials provide when incorporated in biodegradable films as an alternative for the improvement of the properties of nanocomposites. The main purpose of this work was to investigate the effect of the incorporation of nanocellulose obtained from different fibers (corncob, corn husk, coconut shell, and wheat bran) into the chitosan/glycerol films. The nanocellulose were obtained through acid hydrolysis. The properties of the different nanobiocomposites were comparatively evaluated, including their barrier and mechanical properties. The nanocrystals obtained for coconut shell (CS), corn husk (CH), and corncob (CC) presented a length/diameter ratio of 40.18, 40.86, and 32.19, respectively. Wheat bran (WB) was not considered an interesting source of nanocrystals, which may be justified due to the low percentage of cellulose. Significant differences were observed in the properties of the films studied. The water activity varied from 0.601 (WB Film) to 0.658 (CH Film) and the moisture content from 15.13 (CS Film) to 20.86 (WB Film). The highest values for tensile strength were presented for CC (11.43 MPa) and CS (11.38 MPa) films, and this propriety was significantly increased by nanocellulose addition. The results showed that the source of the nanocrystal determined the properties of the chitosan/glycerol films.

Identification of Bacterial Composition in Freeze-Dried Agaricus bisporus During Storage and the Resultant Odor Deterioration

Moisture absorption and bacterial growth are critical factors for quality deterioration of freeze-dried Agaricus bisporus. In order to explore the bacterial composition and the resultant odor changes in freeze-dried A. bisporus during storage under three typical conditions (RT: 25°C, 55% RH; HT: 37°C, 85% RH; AT: ambient temperature), bacterial diversity and communities were analyzed using metagenomics. Moreover, volatile compounds were determined using SPME-GC-MS. The results demonstrated that the bacterial composition in freeze-dried A. bisporus was dominated by Pseudomonas, followed by Rhizobium and Pedobacter. In addition, Mucilaginibacter, Flavobacterium, and Thermus were a few other genera more dominant in HT samples, Chryseobacterium was the other genera more dominant in AT samples, while, Sphingobacterium and Chryseobacterium were a few other genera more dominant in RT samples. Furthermore, the increase of benzaldehyde content in HT samples may have been induced by the growth of Pseudomonads and the esters production in RT and AT samples might have been induced by Chryseobacterium. This study provided comprehensive information on exogenous bacterial composition and the resultant odor in freeze-dried A. bisporus. These results may be a theoretical basis for quality control and quick quality detection based on volatiles of freeze-dried A. bisporus.