Pre-growth conditions and strain diversity affect nisin treatment efficacy against Listeria monocytogenes on cold-smoked salmon

Unravelling the impact of fat content on the microbial dynamics and spatial distribution of foodborne bacteria in tri-phasic viscoelastic 3D models

The aim of the current study is to develop and characterise novel complex multi-phase in vitro 3D models, for advanced microbiological studies. More specifically, we enriched our previously developed bi-phasic polysaccharide (Xanthan Gum)/protein (Whey Protein) 3D model with a fat phase (Sunflower Oil) at various concentrations, i.e., 10%, 20%, 40% and 60% (v/v), for better mimicry of the structural and biochemical composition of real food products. Rheological, textural, and physicochemical analysis as well as advanced microscopy imaging (including spatial mapping of the fat droplet distribution) of the new tri-phasic 3D models revealed their similarity to industrial food products (especially cheese products). Furthermore, microbial growth experiments of foodborne bacteria, i.e., Listeria monocytogenes, Escherichia coli, Pseudomonas aeruginosa and Lactococcus lactis on the surface of the 3D models revealed very interesting results, regarding the growth dynamics and distribution of cells at colony level. More specifically, the size of the colonies formed on the surface of the 3D models, increased substantially for increasing fat concentrations, especially in mid- and late-exponential growth phases. Furthermore, colonies formed in proximity to fat were substantially larger as compared to the ones that were located far from the fat phase of the models. In terms of growth location, the majority of colonies were located on the protein/polysaccharide phase of the 3D models. All those differences at microscopic level, that can directly affect the bacterial response to decontamination treatments, were not captured by the macroscopic kinetics (growth dynamics), which were unaffected from changes in fat concentration. Our findings demonstrate the importance of developing structurally and biochemically complex 3D in vitro models (for closer proximity to industrial products), as well as the necessity of conducting multi-level microbial analyses, to better understand and predict the bacterial behaviour in relation to their biochemical and structural environment. Such studies in advanced 3D environments can assist a better/more accurate design of industrial antimicrobial processes, ultimately, improving food safety.

Reduction and Growth Inhibition of Listeria monocytogenes by Use of Anti-Listerial Nisin, P100 Phages and Buffered Dry Vinegar Fermentates in Standard and Sodium-Reduced Cold-Smoked Salmon

Cold-smoked salmon are ready-to-eat products that may support the growth of pathogenic Listeria monocytogenes during their long shelf-life. Consumption of such contaminated products can cause fatal listeriosis infections. Another challenge and potential risk associated with CS salmon is their high levels of sodium salt. Excess dietary intake is associated with serious health complications. In the present study, anti-listerial bacteriocin (nisin), P100 bacteriophages (Phageguard L, PGL) and fermentates (Verdad N6, P-NDV) were evaluated as commercial bio-preservation strategies for increased control of L. monocytogenes in standard (with NaCl) and sodium-reduced (NaCl partially replaced with KCl) CS salmon. Treatments of CS salmon with nisin (1 ppm) and PGL (5 × 107 pfu/cm2) separately yielded significant initial reductions in L. monocytogenes (up to 0.7 log) compared to untreated samples. Enhanced additive reductions were achieved through the combined treatments of nisin and PGL. Fermentates in the CS salmon inhibited the growth of Listeria but did not lead to its eradication. The lowest levels of L. monocytogenes during storage were observed in nisin- and PGL-treated CS salmon containing preservative fermentates and stored at 4 °C, while enhanced growth was observed during storage at an abusive temperature of 8 °C. Evaluation of industry-processed standard and sodium-replaced CS salmon confirmed significant effects with up to 1.7 log reductions in L. monocytogenes levels after 34 days of storage of PGL- and nisin-treated CS salmon-containing fermentates. No differences in total aerobic plate counts were observed between treated (PGL and nisin) or non-treated standard and sodium-reduced CS salmon at the end of storage. The microbiota was dominated by Photobacterium, but with a shift showing dominance of Lactococcus spp. and Vagococcus spp. in fermentate-containing samples. Similar and robust reductions in L. monocytogenes can be achieved in both standard and sodium-replaced CS salmon using the bio-preservation strategies of nisin, PGL and fermentates under various and relevant processing and storage conditions.

Impact of food-relevant conditions and food matrix on the efficacy of prenylated isoflavonoids glabridin and 6,8-diprenylgenistein as potential natural preservatives against Listeria monocytogenes

Prenylated isoflavonoids can be extracted from plants of the Leguminosae/Fabaceae family and have shown remarkable antimicrobial activity against Gram-positive food-borne pathogens, such as Listeria monocytogenes. Promising candidates from this class of compounds are glabridin and 6,8-diprenylgenistein. This research aimed to investigate the potential of glabridin and 6,8-diprenylgenistein as food preservatives against L. monocytogenes. Their antimicrobial activity was tested in vitro at various conditions relevant for food application, such as different temperatures (from 10 °C to 37 °C), pH (5 and 7.2), and in the presence or absence of oxygen. The minimum inhibitory concentrations of glabridin and 6,8-diprenylgenistein in vitro were between 0.8 and 12.5 μg/mL in all tested conditions. Growth inhibitory activities were similar at 10 °C compared to higher temperatures, although bactericidal activities decreased when the temperature decreased. Notably, lower pH (pH 5) increased the growth inhibitory and bactericidal activity of the compounds, especially for 6,8-diprenylgenistein. Furthermore, similar antimicrobial efficacies were shown anaerobically compared to aerobically at the tested conditions. Glabridin showed a more stable inhibitory and bactericidal activity when the temperature decreased compared to 6,8-diprenylgenistein. Therefore, we further determined the antimicrobial efficacy of glabridin against L. monocytogenes growth on fresh-cut cantaloupe at 10 °C. In these conditions, concentrations of glabridin of 50, 100 and 250 μg/g significantly reduced the growth of L. monocytogenes compared to the control, resulting on average in >1 Log CFU/g difference after 4 days compared to the control. Our results further underscored the importance of considering the food matrix when assessing the activity of novel antimicrobials. Overall, this study highlights the potential of prenylated isoflavonoids as naturally derived food preservatives.

The efficacy of nisin against Listeria monocytogenes on cold-smoked salmon at natural contamination levels is concentration-dependent and varies by serotype

Cold-smoked salmon is a ready-to-eat food product capable of supporting Listeria monocytogenes growth at refrigeration temperatures. While the FDA-approved antimicrobial nisin can be used to mitigate L. monocytogenes contamination, stresses associated with cold-smoked salmon and the associated processing environments may reduce nisin efficacy. A previous study in our laboratory showed that, at high inoculation levels, pre-exposure of L. monocytogenes to sublethal concentrations of quaternary ammonium compounds had an overall detrimental effect on nisin efficacy. The objective of this study was to investigate the impact of nisin concentration and storage temperature on nisin efficacy against L. monocytogenes inoculated on salmon at natural contamination levels. Three L. monocytogenes strains were pre-grown in the presence of sublethal levels of benzalkonium chloride prior to inoculation at ~10² CFU/g on salmon slices that were pre-treated with either 0, 25, or 250 ppm nisin, followed by vacuum-packing and incubation at 4 or 7°C for up to 30 days. L. monocytogenes was enumerated on days 1, 15, and 30 using direct plating and/or most probable number methods. A hurdle model was constructed to describe the odds of complete elimination of L. monocytogenes on salmon and the level of L. monocytogenes when complete elimination was not achieved. Our data showed that (i) nisin efficacy (defined as L. monocytogenes reduction relative to the untreated control) was concentration-dependent with increased efficacy at 250 ppm nisin, and that (ii) 250 ppm nisin treatments led to a reduction in L. monocytogenes prevalence, independent of storage temperature and serotype; this effect of nisin could only be identified since low inoculation levels were used. While lower storage temperatures (i.e., 4°C) yielded lowered absolute L. monocytogenes counts on days 15 and 30 (as compared to 7°C), nisin efficacy did not differ between these two temperatures. Finally, the serotype 1/2b strain was found to be more susceptible to nisin compared with serotype 1/2a and 4b strains on samples incubated at 7°C or treated with 25 ppm nisin. This variation of nisin susceptibility across serotypes, which is affected by both the storage temperature and nisin concentration, needs to be considered while evaluating the efficacy of nisin.

Development of a Modeling Tool To Assess and Reduce Regulatory and Recall Risks for Cold-Smoked Salmon Due to Listeria monocytogenes Contamination

ABSTRACT

Although public health risk assessments for Listeria monocytogenes (Lm) have been published for various foods, firm-level decision making on interventions targeting Lm involves considerations of both public health and enterprise risks. Smoked seafood is a ready-to-eat product with a high incidence of Lm contamination and has been associated with several recalls. We used cold-smoked salmon as a model product to develop a decision support tool (the regulatory and recall risk [3R] model) to estimate (i) baseline regulatory and recall (RR) risks (i.e., overall risks of a lot sampled and found positive for Lm, e.g., by food regulatory agencies) due to Lm contamination and (ii) the RR risk reduction that can be achieved through interventions with underlying mechanisms such as reducing the prevalence and/or level of Lm and retarding or preventing Lm growth. Given that a set number of samples (e.g., 10) are tested for a given lot, the RR risk equals the likelihood of detecting Lm in at least one sample. Under the baseline scenario, which assumes a 4% Lm prevalence and no interventions, the median predicted RR risk for a given production lot was 0.333 (95% credible interval: 0.288, 0.384) when 10 25-g samples were tested. Nisin treatments, which reduce both the prevalence and initial level of Lm, reduced RR risks in a concentration-dependent manner to 0.109 (0.074, 0.146) with 5 ppm, 0.049 (0.024, 0.083) with 10 ppm, and 0.017 (0.007, 0.033) with 20 ppm. In general, more effective reduction in RR risks can be achieved by reducing Lm prevalence than by retarding Lm growth; the RR risk was reduced to 0.182 (0.153, 0.213) by a 50% prevalence reduction but to only 0.313 (0.268, 0.367) by bacteriostatic growth inhibitors. Sensitivity analysis indicated that prevalence and initial level of Lm and storage temperature have the greatest impact on predicting RR risks, suggesting that reliable data for these parameters will improve model performance.

HIGHLIGHTS

Cold Shock Proteins Promote Nisin Tolerance in Listeria monocytogenes Through Modulation of Cell Envelope Modification Responses

Listeria monocytogenes continues to be a food safety challenge owing to its stress tolerance and virulence traits. Several listeriosis outbreaks have been linked to the consumption of contaminated ready-to-eat food products. Numerous interventions, including nisin application, are presently employed to mitigate against L. monocytogenes risk in food products. In response, L. monocytogenes deploys several defense mechanisms, reducing nisin efficacy, that are not yet fully understood. Cold shock proteins (Csps) are small, highly conserved nucleic acid-binding proteins involved in several gene regulatory processes to mediate various stress responses in bacteria. L. monocytogenes possesses three csp gene paralogs; cspA, cspB, and cspD. Using a panel of single, double, and triple csp gene deletion mutants, the role of Csps in L. monocytogenes nisin tolerance was examined, demonstrating their importance in nisin stress responses of this bacterium. Without csp genes, a L. monocytogenes ΔcspABD mutant displayed severely compromised growth under nisin stress. Characterizing single (ΔcspA, ΔcspB, and ΔcspD) and double (ΔcspBD, ΔcspAD, and ΔcspAB) csp gene deletion mutants revealed a hierarchy (cspD > cspB > cspA) of importance in csp gene contributions toward the L. monocytogenes nisin tolerance phenotype. Individual eliminations of either cspA or cspB improved the nisin stress tolerance phenotype, suggesting that their expression has a curbing effect on the expression of nisin resistance functions through CspD. Gene expression analysis revealed that Csp deficiency altered the expression of DltA, MprF, and penicillin-binding protein-encoding genes. Furthermore, the ΔcspABD mutation induced an overall more electronegative cell surface, enhancing sensitivity to nisin and other cationic antimicrobials as well as the quaternary ammonium compound disinfectant benzalkonium chloride. These observations demonstrate that the molecular functions of Csps regulate systems important for enabling the constitution and maintenance of an optimal composed cell envelope that protects against cell-envelope-targeting stressors, including nisin. Overall, our data show an important contribution of Csps for L. monocytogenes stress protection in food environments where antimicrobial peptides are used. Such knowledge can be harnessed in the development of better L. monocytogenes control strategies. Furthermore, the potential that Csps have in inducing cross-protection must be considered when combining hurdle techniques or using them in a series.

Identification of Closely Related Listeria monocytogenes Isolates with No Apparent Evidence for a Common Source or Location: A Retrospective Whole Genome Sequencing Analysis

ABSTRACT

Public health and regulatory agencies worldwide sequence all Listeria monocytogenes isolates obtained as part of routine surveillance and outbreak investigations. Many of these entities submit the sequences to the National Center for Biotechnology Information Pathogen Detection (NCBI PD) database, which groups the L. monocytogenes isolates into single nucleotide polymorphism (SNP) clusters based on a pairwise SNP difference threshold of 50 SNPs. Our goal was to assess whether isolates with metadata that suggest different sources or locations could show evidence for close genetic relatedness indicating a recent common ancestor and a possible unknown common source. We compared the whole genome sequencing (WGS) data of 249 L. monocytogenes isolates sequenced here, which have detailed metadata, with WGS data of nonclinical isolates on NCBI PD. The 249 L. monocytogenes isolates originated from natural environments (n = 91) as well as from smoked fish (n = 62), dairy (n = 56), and deli meat (n = 40) operations in the United States. Using a combination of subtyping by core genome multilocus sequence typing and high-quality SNP, we observed five SNP clusters in which study isolates and SNP cluster isolates seemed to be closely related and either (i) shared the same geolocation but showed different source types (one SNP cluster); (ii) shared the same source type but showed different geolocations (two SNP clusters); or (iii) shared neither source type nor geolocation (two SNP clusters). For one of the two clusters under (iii), there was, however, no strong bootstrap support for a common ancestor shared between the study isolates and SNP cluster isolates, indicating the value of in-depth evolutionary analyses when WGS data are used for traceback and epidemiological investigations. Overall, our results demonstrate that some L. monocytogenes subtypes may be associated with specific locations or commodities; these associations can help in investigations involving multi-ingredient foods such as sandwiches. However, at least some L. monocytogenes subtypes can be widespread geographically and can be associated with different sources, which may present a challenge to traceback investigations involving these subtypes.

HIGHLIGHTS

Novel 2,3-Dialdehyde Cellulose-Based Films with Photodynamic Inactivation Potency by Incorporating the β-Cyclodextrin/Curcumin Inclusion Complex

Antibacterial packaging film mediated by photodynamic inactivation (PDI) is a new concept in food industry. The objective of this study was to fabricate a green 2,3-dialdehyde cellulose (DAC)-based antimicrobial film with PDI potency by incorporating the β-cyclodextrin/curcumin (β-CD/Cur) complex as a photosensitizer. The PDI-mediated films were characterized by evaluating the surface morphology, chemical structure, light transmittance, mechanical properties, photochemical and thermal stability, and water solubility. The results showed that the DAC-CD/Cur films were soluble in water and mechanically strong with a tensile strength of 63.87 MPa and an elongation break of 1.32%, which was attributed to the formation of hydrogen bonds between DAC and β-CD/Cur molecules. Meanwhile, the composite films possessed a good light transmittance but impeded the penetration of ultraviolet light and efficiently delayed the degradation of curcumin. More importantly, the PDI-mediated films exhibited a broad-spectrum ability to kill Listeria monocytogenes, Vibrio parahaemolyticus, and Shewanella putrefaciens in pure culture. Notably, they also potently inactivated these harmful bacteria on ready-to-eat salmon with a maximum of ∼4 Log CFU/g (99.99%) reduction after 60 min irradiation (13.68 J/cm²). Therefore, the PDI-mediated DAC-CD/Cur films are novel and promising antimicrobial food packaging films in food industry.

β-Phenylethylamine as a Natural Food Additive Shows Antimicrobial Activity against Listeria monocytogenes on Ready-to-Eat Foods

Listeria monocytogenes is an important foodborne pathogen and a major cause of death associated with bacterial foodborne infections. Control of L. monocytogenes on most ready-to-eat (RTE) foods remains a challenge. The potential use of β-phenylethylamine (PEA) as an organic antimicrobial against L. monocytogenes was evaluated in an effort to develop a new intervention for its control. Using a collection of 62 clinical and food-related isolates we determined the minimum inhibitory concentration (MIC) of PEA against L. monocytogenes in different broth and agar media. Bologna type sausage (lyoner) and smoked salmon were used as food model systems to validate the in vitro findings. PEA had a growth inhibitory and bactericidal effect against L. monocytogenes both in in vitro experiments as well as on lyoner and smoked salmon. The MIC’s ranged from 8 to 12.5 mg/mL. Furthermore, PEA also inhibited L. monocytogenes biofilm formation. Based on good manufacturing practices as a prerequisite, the application of PEA to RTE products might be an additional hurdle to limit L. monocytogenes growth thereby increasing food safety.