Fate of Viable but Non-culturable Listeria monocytogenes in Pig Manure Microcosms

Neglected Drivers of Antibiotic Resistance: Survival of Extended-Spectrum β-Lactamase-Producing Pathogenic Escherichia coli from Livestock Waste through Dormancy and Release of Transformable Extracellular Antibiotic Resistance Genes under Heat Treatment

Extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae has caused a global pandemic with high prevalence in livestock and poultry, which could disseminate into the environment and humans. To curb this risk, heat-based harmless treatment of livestock waste was carried out. However, some risks of the bacterial persistence have not been thoroughly assessed. This study demonstrated that antibiotic-resistant bacteria (ARB) could survive at 55 °C through dormancy, and simultaneously transformable extracellular antibiotic resistance genes (eARGs) would be released. The ESBL-producing pathogenic Escherichia coli CM1 from chicken manure could enter a dormant state at 55 °C and reactivate at 37 °C. Dormant CM1 had stronger β-lactam resistance, which was associated with high expression of β-lactamase genes and low expression of outer membrane porin genes. Resuscitated CM1 maintained its virulence expression and multidrug resistance and even had stronger cephalosporin resistance, which might be due to the ultra-low expression of the porin genes. Besides, heat at 55 °C promoted the release of eARGs, some of which possessed a certain nuclease stability and heat persistence, and even maintained their transformability to an Acinetobacter baylyi strain. Therefore, dormant multidrug-resistant pathogens from livestock waste will still pose a direct health risk to humans, while the resuscitation of dormant ARB and the transformation of released eARGs will jointly promote the proliferation of ARGs and the spread of antibiotic resistance.

Viability and removal assessment of Escherichia coli and Salmonella spp. by real-time PCR with propidium monoazide in the hygienization of sewage sludge using three anaerobic processes

Sewage sludge should be stabilized for its beneficial use and pathogens, among other factors, should comply with environmental regulations. Three sludge stabilization process were compared to assess their suitability for producing Class A biosolids: MAD-AT (mesophilic (37 °C) anaerobic digestion (MAD) followed by an alkaline treatment (AT)); TAD (thermophilic (55 °C) anaerobic digester); and TP-TAD (mild thermal (80 °C, 1 h) pretreatment (TP) followed by a TAD). E. coli and Salmonella spp. were determined, differentiating three possible states: total cells (qPCR), viable cells using the propidium monoazide method (PMA-qPCR), and culturable cells (MPN). Culture techniques followed by the confirmative biochemical tests identified the presence of Salmonella spp. in PS and MAD samples, while the molecular methods (qPCR and PMA-qPCR) showed negative results in all samples. The TP + TAD arrangement reduced the concentration of total and viable E. coli cells in a greater extent than the TAD process. However, an increase of culturable E. coli was observed in the corresponding TAD step, indicating that the mild thermal pretreatment induced the viable but non-culturable state in E. coli. In addition, the PMA technique did not discriminate viable from non-viable bacteria in complex matrices. The three processes produced Class A biosolids (fecal coliforms < 1000 MPN/gTS and Salmonella spp, < 3 MPN/gTS) maintaining compliance after a 72 h storage period. It appears that the TP step favors the viable but not culturable state in E. coli cells, a finding that should be considered when adopting mild thermal treatment in sludge stabilization process arrangements.

Occurrence and Multidrug Resistance in Strains of Listeria monocytogenes Recovered from the Anaerobic Co-Digestion Sludge Contained in a Single Stage Steel Biodigester: Implications for Antimicrobial Stewardship

L. monocytogenes is a zoonotic foodborne pathogen with inherent adaptability to tolerate environmental and physiological stresses, thereby causing severe disease outbreaks. Antibiotic resistant foodborne pathogens are a challenge to the food industry. A total of 18 samples were pooled from a bio-digester co-digesting swine manure/pinewood sawdust, and evaluated for the occurrence of bacterium plus total viable counts using the spread plate method. The recovered bacterial isolates were presumptively identified by growth on selective medium and confirmed by biochemical characterisation, leading to the isolation of 43 L. monocytogenes. The isolates were characterized based on their susceptibility to antibiotics via the Kirby-Bauer disc diffusion technique against a panel of 14 antibiotics. Equally, the multiple antibiotic resistance (MAR) index was calculated, and MAR phenotypes generated. The bacterial counts were between 102 and104 cfu/mL. Complete susceptibility (100%) was demonstrated to ampicillin, gentamicin and sulfamethoxazole, which are the drugs of choice in the treatment of listeriosis. In addition, intermediate sensitivity occurred at 25.58% to cefotaxime, and the highest resistance (51.16%) was exhibited against nalidixic acid. The MAR index ranged from 0 to 0.71. Overall, 41.86% of the Listeria isolates displayed multidrug resistance, with 18 different MAR phenotypes, demonstrating CIP, E, C, TET, AUG, S, CTX, NA, AML, NI as the greatest MAR phenotype. It can be concluded that the isolates yielding MAR > 0.2 originated from the farm, where antibiotics had been in routine use. Therefore, strict monitoring of antibiotics use in the farm is crucial to mitigate further increase in antibiotic resistance amongst these bacterial isolates.

Lifestyle of Listeria monocytogenes and food safety: Emerging listericidal technologies in the food industry

Listeria monocytogenes, a causative agent of listeriosis, is a major foodborne pathogen. Among pathogens, L. monocytogenes stands out for its unique ecological and physiological characteristics. This distinct lifestyle of L. monocytogenes has a significant impact on food safety and public health, mainly through the ability of this pathogen to multiply at refrigeration temperature and to persist in the food processing environment. Due to a combination of these characteristics and emerging trends in consumer preference for ready-to-eat and minimally processed food, there is a need to develop effective and sustainable approaches to control contamination of food products with L. monocytogenes. Implementation of an efficient and reliable control strategy for L. monocytogenes must first address the problem of cross-contamination. Besides the preventive control strategies, cross-contamination may be addressed with the introduction of emerging post packaging non-thermal or thermal hurdles that can ensure delivery of a listericidal step in a packed product without interfering with the organoleptic characteristics of a food product. This review aims to present the most relevant findings underlying the distinct lifestyle of L. monocytogenes and its impact on food safety. We also discuss emerging food decontamination technologies that can be used to better control L. monocytogenes.

An Exploration of Listeria monocytogenes, Its Influence on the UK Food Industry and Future Public Health Strategies

Listeria monocytogenes is a Gram-positive intracellular pathogen that can cause listeriosis, an invasive disease affecting pregnant women, neonates, the elderly, and immunocompromised individuals. Principally foodborne, the pathogen is transmitted typically through contaminated foods. As a result, food manufacturers exert considerable efforts to eliminate L. monocytogenes from foodstuffs and the environment through food processing and disinfection. However, L. monocytogenes demonstrates a range of environmental stress tolerances, resulting in persistent colonies that act as reservoirs for the reintroduction of L. monocytogenes to food contact surfaces and food. Novel technologies for the rapid detection of L. monocytogenes and disinfection of food manufacturing industries have been developed to overcome these obstacles to minimise the risk of outbreaks and sporadic cases of listeriosis. This review is aimed at exploring L. monocytogenes in the UK, providing a summary of outbreaks, current routine microbiological testing and the increasing awareness of biocide tolerances. Recommendations for future research in the UK are made, pertaining to expanding the understanding of L. monocytogenes dissemination in the UK food industry and the continuation of novel technological developments for disinfection of food and the food manufacturing environment.

The Viable But Non-Culturable State of Listeria monocytogenes in the One-Health Continuum

Many bacterial species, including several pathogens, can enter a so-called “viable but non-culturable” (VBNC) state when subjected to stress. Bacteria in the VBNC state are metabolically active but have lost their ability to grow on standard culture media, which compromises their detection by conventional techniques based on bacterial division. Under certain conditions, VBNC bacteria can regain their growth capacity and, for pathogens, their virulence potential, through a process called resuscitation. Here, we review the current state of knowledge of the VBNC state of Listeria monocytogenes (Lm), a Gram-positive pathogenic bacterium responsible for listeriosis, one of the most dangerous foodborne zoonosis. After a brief summary of characteristics of VBNC bacteria, we highlight work on VBNC Lm in the environment and in agricultural and food industry settings, with particular emphasis on the impact of antimicrobial treatments. We subsequently discuss recent data suggesting that Lm can enter the VBNC state in the host, raising the possibility that VBNC forms contribute to the asymptomatic carriage of this pathogen in wildlife, livestock and even humans. We also consider the resuscitation and virulence potential of VBNC Lm and the danger posed by these bacteria to at-risk individuals, particularly pregnant women. Overall, we put forth the hypothesis that VBNC forms contribute to adaptation, persistence, and transmission of Lm between different ecological niches in the One-Health continuum, and suggest that screening for healthy carriers, using alternative techniques to culture-based enrichment methods, should better prevent listeriosis risks.

Occurrence and Diversity of Listeria monocytogenes Isolated from Two Pig Manure Treatment Plants in France

The presence of Listeria monocytogenes in piggery effluents intended for irrigation crops may be a source of bacterial dissemination in agriculture. The occurrence and diversity of L. monocytogenes in the farm environment were examined in two pig manure treatment systems (S1 and S2). Samples collected over the course of one year consisted of manure, the liquid fraction of treated manure (lagoon effluent), and soil surrounding the lagoon. L. monocytogenes was enumerated using the Most Probable Number (MPN) method, serotyped by PCR, genotyped by pulsed-field gel electrophoresis (PFGE), and sequenced for multilocus sequence typing (MLST). L. monocytogenes was detected in 92% of manure samples and in approximately 50% of lagoon effluent and soil samples. Concentrations ranged between 5 and 10³ MPN 100‍ ‍mL^-1. Serogroups IIa, IIb, and IVb were identified. Diversity was high with 44 PFGE profiles (252 isolates) and 17 clonal complexes (CCs) (96 isolates) with higher diversity in manure at site S1 supplied by four farms. Some PFGE profiles and CCs identified in manure or in pig feces from a previous study were also detected in lagoons and/or soil, reflecting pig L. monocytogenes circulation throughout the manure treatment and in the vicinity of the sampling sites. However, some PFGE profiles and CCs were only found in the lagoon and/or in soil, suggesting an origin other than pigs. The present study highlights the limited ability of biological treatments to eliminate L. monocytogenes from pig manure. The persistence of some PFGE profiles and CCs throughout the year in the lagoon and soil shows the ability of L. monocytogenes to survive in this type of environment.

Detection and Potential Virulence of Viable but Non-Culturable (VBNC) Listeria monocytogenes: A Review

The detection, enumeration, and virulence potential of viable but non-culturable (VBNC) pathogens continues to be a topic of discussion. While there is a lack of definitive evidence that VBNC Listeria monocytogenes (Lm) pose a public health risk, recent studies suggest that Lm in its VBNC state remains virulent. VBNC bacteria cannot be enumerated by traditional plating methods, so the results from routine Lm testing may not demonstrate a sample's true hazard to public health. We suggest that supplementing routine Lm testing methods with methods designed to enumerate VBNC cells may more accurately represent the true level of risk. This review summarizes five methods for enumerating VNBC Lm: Live/Dead BacLight^TM staining, ethidium monoazide and propidium monoazide-stained real-time polymerase chain reaction (EMA- and PMA-PCR), direct viable count (DVC), 5-cyano-2,3-ditolyl tetrazolium chloride-4',6-diamidino-2-phenylindole (CTC-DAPI) double staining, and carboxy-fluorescein diacetate (CDFA) staining. Of these five supplementary methods, the Live/Dead BacLight^TM staining and CFDA-DVC staining currently appear to be the most accurate for VBNC Lm enumeration. In addition, the impact of the VBNC state on the virulence of Lm is reviewed. Widespread use of these supplemental methods would provide supporting data to identify the conditions under which Lm can revert from its VBNC state into an actively multiplying state and help identify the environmental triggers that can cause Lm to become virulent. Highlights: Rationale for testing for all viable Listeria (Lm) is presented. Routine environmental sampling and plating methods may miss viable Lm cells. An overview and comparison of available VBNC testing methods is given. There is a need for resuscitation techniques to recover Lm from VBNC. A review of testing results for post VBNC virulence is compared.

Benzalkonium Chloride Induces a VBNC State in Listeria monocytogenes

The objective of our study was to investigate the effects of benzalkonium chloride (BC) adaptation of L. monocytogenes on the susceptibility to antimicrobial agents and on the viable but non culturable (VBNC) state of the bacterial cells. We adapted L. monocytogenes SLCC2540 to BC by applying BC below minimum inhibitory concentration (MIC) to above minimum bactericidal concentration (MBC). The culturable fractions and the susceptibility of adapted and parental cells to BC were assessed. In addition, cell membrane permeability and glucose uptake were analyzed by multi parametric flow cytometry using the fluorescent agents SYTO9, propidium iodide, and 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-D-glucose (2-NBDG). Adapted cells displayed a two-fold MIC increase of BC and reduced antibiotic susceptibility. At high BC concentrations, the decrease in the number of colony forming units was significantly lower in the population of adapted cells compared to parental cells. At the same time, the number of metabolically active cells with intact membranes was significantly higher than the number of culturable cells. Growth-independent viability assays revealed an adapted subpopulation after BC application that was not culturable, indicating increased abundance of viable but nonculturable (VBNC) cells. Moreover, adapted cells can outcompete non-adapted cells under sublethal concentrations of disinfectants, which may lead to novel public health risks.

Spiking a Silty-Sand Reference Soil with Bacterial DNA: Limits and Pitfalls in the Discrimination of Live and Dead Cells When Applying Ethidium Monoazide (EMA) Treatment

In the present study, EMA (ethidium monoazide) treatment was applied to a silty-sand reference soil prior to DNA extraction to enable a differentiation between dead and living cells. For this purpose, a reference soil was spiked with Listeria monocytogenes cells or cell equivalents, respectively. With the purpose of evaluating optimum treatment conditions, different EMA concentrations have been tested. However, the results remained largely inconclusive. Furthermore, varied dark incubation periods allowing EMA to penetrate dead cells did not allow the selective removal of DNA from membrane-compromised cells in downstream analyses. In contrast to undiluted soil, an effect of EMA treatment during DNA extraction could be observed when using a 1:10 dilution of the reference soil; however, the effect has not been sufficiently selective to act on heat-treated cells only. Although the application of EMA to soil requires further evaluation, the procedure harbors future potential for improving DNA-based approaches in microbial ecology studies.

Salmonella enterica in Soils Amended with Heat-Treated Poultry Pellets Survived Longer than Bacteria in Unamended Soils and More Readily Transferred to and Persisted on Spinach

Untreated biological soil amendments of animal origin (BSAAO) are commonly used as biological fertilizers but can harbor foodborne pathogens like Salmonella enterica, leading to potential transfer from soils to fruits and vegetables intended for human consumption. Heat-treated poultry pellets (HTPP) can provide produce growers with a slow-release fertilizer with a minimized risk of pathogen contamination. Little is known about the impact of HTPP-amended soil on the survival of Salmonella enterica The contributions of RpoS and formation of viable but nonculturable cells to Salmonella survival in soils are also inadequately understood. We quantified the survival of Salmonella enterica subsp. enterica serovar Newport wild-type (WT) and rpoS-deficient (ΔrpoS mutant) strains in HTPP-amended and unamended soil with or without spinach plants over 91 days using culture and quantitative PCR methods with propidium monoazide (PMA-qPCR). Simulated "splash" transfer of S. Newport from soil to spinach was evaluated at 35 and 63 days postinoculation (dpi). The S. Newport WT and ΔrpoS mutant reached the limit of detection, 1.0 log CFU/g (dry weight), in unamended soil after 35 days, whereas 2 to 4 log CFU/g (dry weight) was observed for both WT and ΔrpoS mutant strains at 91 dpi in HTPP-amended soil. S. Newport levels in soils determined by PMA-qPCR and plate count methods were similar (P > 0.05). HTPP-amended soils supported higher levels of S. Newport transfer to and survival on spinach leaves for longer periods of time than did unamended soils (P < 0.05). Salmonella Newport introduced to HTPP-amended soils survived for longer periods and was more likely to transfer to and persist on spinach plants than was S. Newport introduced to unamended soils.IMPORTANCE Heat-treated poultry pellets (HTPP) often are used by fruit and vegetable growers as a slow-release fertilizer. However, contamination of soil on farms may occur through contaminated irrigation water or scat from wild animals. Here, we show that the presence of HTPP in soil led to increased S. Newport survival in soil and to greater likelihood of its transfer to and survival on spinach plants. There were no significant differences in survival durations of WT and ΔrpoS mutant isolates of S. Newport. The statistically similar populations recovered by plate count and estimated by PMA-qPCR for both strains in the amended and unamended soils in this study indicate that all viable populations of S. Newport in soils were culturable.

Sources and survival of Listeria monocytogenes on fresh, leafy produce

Listeria monocytogenes is an intracellular human pathogen which enters the body through contaminated food stuffs and is known to contaminate fresh leafy produce such as spinach, lettuce and rocket. Routinely, fresh leafy produce is grown and processed on a large scale before reaching the consumer through various products such as sandwiches and prepared salads. From farm to fork, the fresh leafy produce supply chain (FLPSC) is complex and contains a diverse range of environments where L. monocytogenes is sporadically detected during routine sampling of produce and processing areas. This review describes sources of the bacteria in the FLPSC and outlines the physiological and molecular mechanisms behind its survival in the different environments associated with growing and processing fresh produce. Finally, current methods of source tracking the bacteria in the context of the food supply chain are discussed with emphasis on how these methods can provide additional, valuable information on the risk that L. monocytogenes isolates pose to the consumer.

Transcriptomic Analysis of the Adaptation of Listeria monocytogenes to Lagoon and Soil Matrices Associated with a Piggery Environment: Comparison of Expression Profiles

Understanding how Listeria monocytogenes, the causative agent of listeriosis, adapts to the environment is crucial. Adaptation to new matrices requires regulation of gene expression. To determine how the pathogen adapts to lagoon effluent and soil, two matrices where L. monocytogenes has been isolated, we compared the transcriptomes of L. monocytogenes CIP 110868 20 min and 24 h after its transfer to effluent and soil extract. Results showed major variations in the transcriptome of L. monocytogenes in the lagoon effluent but only minor modifications in the soil. In both the lagoon effluent and in the soil, genes involved in mobility and chemotaxis and in the transport of carbohydrates were the most frequently represented in the set of genes with higher transcript levels, and genes with phage-related functions were the most represented in the set of genes with lower transcript levels. A modification of the cell envelop was only found in the lagoon environment. Finally, the differential analysis included a large proportion of regulators, regulons, and ncRNAs.

Persistence and Potential Viable but Non-culturable State of Pathogenic Bacteria during Storage of Digestates from Agricultural Biogas Plants

Despite the development of on-farm anaerobic digestion as a process for making profitable use of animal by-products, factors leading to the inactivation of pathogenic bacteria during storage of digestates remain poorly described. Here, a microcosm approach was used to evaluate the persistence of three pathogenic bacteria (Salmonella enterica Derby, Campylobacter coli and Listeria monocytogenes) in digestates from farms, stored for later land spreading. Nine samples, including raw digestates, liquid fractions of digestate and composted digestates, were inoculated with each pathogen and maintained for 40 days at 24°C. Concentrations of pathogens were monitored using culture and qPCR methods. The persistence of L. monocytogenes, detected up to 20 days after inoculation, was higher than that of Salmonella Derby, detected for 7-20 days, and of C. coli (not detected after 7 days). In some digestates, the concentration of the pathogens by qPCR assay was several orders of magnitude higher than the concentration of culturable cells, suggesting a potential loss of culturability and induction of Viable but Non-Culturable (VBNC) state. The potential VBNC state which was generally not observed in the same digestate for the three pathogens, occurred more frequently for C. coli and L. monocytogenes than for Salmonella Derby. Composting a digestate reduced the persistence of seeded L. monocytogenes but promoted the maintenance of Salmonella Derby. The effect of NH[Formula: see text]/NH3 on the culturability of C. coli and Salmonella Derby was also shown. The loss of culturability may be the underlying mechanism for the regrowth of pathogens. We have also demonstrated the importance of using molecular tools to monitor pathogens in environmental samples since culture methods may underestimate cell concentration. Our results underline the importance of considering VBNC cells when evaluating the sanitary effect of an anaerobic digestion process and the persistence of pathogens during the storage of digestates and subsequent land spreading.