Effect of osmotic, alkaline, acid or thermal stresses on the growth and inhibition of Listeria monocytogenes

Effectiveness of organic acids for inactivating pathogenic bacteria inoculated in laboratory media and foods: an updated minireview

Food processing industries commonly employ organic acids (OAAs) to determine bacterial contamination in acidified and fermented foods. OAAs are believed to possess potent antimicrobial properties by permeating cell membranes, altering proton and anion concentrations in the cytoplasm due to their lipophilic undissociated forms. The bacteriostatic or bactericidal effects of OAAs are influenced by various factors including microbial physiology, environmental pH, and acid dissociation ratios. Despite their utility, the precise mechanisms underlying OAA-mediated inhibition of pathogenic bacteria remain incompletely understood. Therefore, the objectives of this review are to compile a selected area of researches that focus on the current propensity of different OAAs for inactivating food-borne pathogens, and then to present a theoretical insight on the use of OAAs to prevent and control pathogenic bacteria present in acidic/acidified foods and their mode of mechanisms.

A stochastic approach for modelling the in-vitro effect of osmotic stress on growth dynamics and persistent cell formation in Listeria monocytogenes

Persistent bacteria (or persisters) can be defined as a microbial subpopulation that, exposed to bactericidal treatment, is killed more slowly than the rest of the population they are part of. They stochastically originate in response to environmental stressors or spontaneously without external signals. When transferred into a fresh medium, persisters can resume active replication although they spend more time adapting to the new conditions remaining in the lag phase longer. They were studied for decades for their ability to survive antibiotic treatments while studies on their formation in food and potential impact on their safety are lacking. The most common food preservation techniques may act as stressors that trigger the formation of persistent bacteria able to survive bactericidal treatments and grow later in foods during storage. This study aimed to investigate a possible relationship between exposure to different salt concentrations (osmotic stress) and the amount of persisters triggered in a strain of Listeria monocytogenes. Furthermore, we described this phenomenon from a mathematical perspective through predictive microbiology models commonly used in the food field. The lag time distribution of a L. monocytogenes ATCC 7644 strain grown in broth with additional 2 %, 4 % and 6 % NaCl was evaluated using the software ScanLag. It uses office scanners to automatically record the colony growth on agar plates and evaluate the frequency distribution of their appearance times (lag phase) by automated image analysis. The same broth cultures were diluted to equalize salt concentration and transferred into a fresh broth to evaluate how the previous salt exposure impacted their growth kinetics. The observed growth curves were reproduced using predictive models in which the mean duration of the lag phase of the whole population took into account the occurrence of persisters with a longer lag phase. The models were solved first using a deterministic approach and then a stochastic one introducing a stochastic term that mimics the variability of lag phase duration due to the persisters occurrence. Results showed that the growth of L. monocytogenes in broth with additional NaCl might trigger the formation of persistent cells whose number increased consistently with salt concentrations. The proposed predictive approach reproduced the observed real curves in strong agreement, especially through the stochastic resolution of the models. Persistence is currently a neglected bacterial defence strategy in the food sector but the persisters' formation during production cannot be excluded; therefore, further insights on the topic are certainly desirable.

Comparative Analysis of Growth, Survival, and Virulence Characteristics of Listeria monocytogenes Isolated from Imported Meat

Listeria monocytogenes is an important foodborne pathogen with worldwide prevalence. Understanding the variability in the potential pathogenicity among strains of different subtypes is crucial for risk assessment. In this study, the growth, survival, and virulence characteristics of 16 L. monocytogenes strains isolated from imported meat in China (2018-2020) were investigated. The maximum specific growth rate (μmax) and lag phase (λ) were evaluated using the time-to-detection (TTD) method and the Baranyi model at different temperatures (25, 30, and 37 °C). Survival characteristics were determined by D-values and population reduction after exposure to heat (60, 62.5, and 65 °C) and acid (HCl, pH = 2.5, 3.5, and 4.5). The potential virulence was evaluated via adhesion and invasion to Caco-2 cells, motility, and lethality to Galleria mellonella. The potential pathogenicity was compared among strains of different lineages and subtypes. The results indicate that the lineage I strains exhibited a higher growth rate than the lineage II strains at three growth temperatures, particularly serotype 4b within lineage I. At all temperatures tested, serotypes 1/2a and 1/2b consistently demonstrated higher heat resistance than the other subtypes. No significant differences in the log reduction were observed between the lineage I and lineage II strains at pH 2.5, 3.5, and 4.5. However, the serotype 1/2c strains exhibited significantly low acid resistance at pH 2.5. In terms of virulence, the lineage I strains outperformed the lineage II strains. The invasion rate to Caco-2 cells and lethality to G. mellonella exhibited by the serotype 4b strains were higher than those observed in the other serotypes. This study provides meaningful insights into the growth, survival, and virulence of L. monocytogenes, offering valuable information for understanding the correlation between the pathogenicity and subtypes of L. monocytogenes.

The influence of stress factors on selected phenotypic and genotypic features of Listeria monocytogenes - a pilot study

BACKGROUND

Listeria monocytogenes are Gram-positive rods, widespread in the environment due to their wide tolerance to changing conditions. The apilot study aimed to assess the impact of six various stresses (heat, cold, osmotic, acid, alkali, frozen) on phenotypic features: MIC of antibiotics (penicillin, ampicillin, meropenem, erythromycin, co-trimoxazole; gradient stripes), motility, ability to form a biofilm (crystal violet method) and growth rate (OD and quantitative method), expression level of sigB (stress induced regulator of genes), agrA, agrB (associated with biofilm formation) and lmo2230, lmo0596 (acid and alkali stress) (qPCR) for three strains of L. monocytogenes.

RESULTS

Applied stress conditions contributed to changes in phenotypic features and expression levels of sigB, agrA, agrB, lmo2230 and lmo0596. Stress exposure increased MIC value for penicillin (ATCC 19111 - alkaline stress), ampicillin (472CC - osmotic, acid, alkaline stress), meropenem (strains: 55 C - acid, alkaline, o smotic, frozen stress; 472CC - acid, alkaline stress), erythromycin (strains: 55 C - acid stress; 472CC - acid, alkaline, osmotic stress; ATCC 19111 - osmotic, acid, alkaline, frozen stress), co-trimoxazole (strains: 55 C - acid stress; ATCC 19111 - osmotic, acid, alkaline stress). These changes, however, did not affect antibiotic susceptibility. The strain 472CC (a moderate biofilm former) increased biofilm production after exposure to all stress factors except heat and acid. The ATCC 19111 (a weak producer) formed moderate biofilm under all studied conditions except cold and frozen stress, respectively. The strain 55 C became a strong biofilm producer after exposure to cold and produced a weak biofilm in response to frozen stress. Three tested strains had lower growth rate (compared to the no stress variant) after exposure to heat stress. It has been found that the sigB transcript level increased under alkaline (472CC) stress and the agrB expression increased under cold, osmotic (55 C, 472CC), alkali and frozen (472CC) stress. In contrast, sigB transcript level decreased in response to acid and frozen stress (55 C), lmo2230 transcript level after exposure to acid and alkali stress (ATCC 19111), and lmo0596 transcript level after exposure to acid stress (ATCC 19111).

CONCLUSIONS

Environmental stress changes the ability to form a biofilm and the MIC values of antibiotics and affect the level of expression of selected genes, which may increase the survival and virulence of L. monocytogenes. Further research on a large L. monocytogenes population is needed to assess the molecular mechanism responsible for the correlation of antibiotic resistance, biofilm formation and resistance to stress factors.

Assessment of the influence of selected stress factors on the growth and survival of Listeria monocytogenes

BACKGROUND

Listeria monocytogenes are Gram-positive rods, which are the etiological factor of listeriosis. L. monocytogenes quickly adapts to changing environmental conditions. Since the main source of rods is food, its elimination from the production line is a priority. The study aimed to evaluate the influence of selected stress factors on the growth and survival of L. monocytogenes strains isolated from food products and clinical material.

RESULTS

We distinguished fifty genetically different strains of L. monocytogenes (PFGE method). Sixty-two percent of the tested strains represented 1/2a-3a serogroup. Sixty percent of the rods possessed ten examined virulence genes (fbpA, plcA, hlyA, plcB, inlB, actA, iap, inlA, mpl, prfA). Listeria Pathogenicity Island 1 (LIPI-1) was demonstrated among 38 (76.0%) strains. Majority (92.0%) of strains (46) were sensitive to all examined antibiotics. The most effective concentration of bacteriophage (inhibiting the growth of 22 strains; 44.0%) was 5 × 108 PFU. In turn, the concentration of 8% of NaCl was enough to inhibit the growth of 31 strains (62.0%). The clinical strain tolerated the broadest pH range (3 to 10). Five strains survived the 60-min exposure to 70˚C, whereas all were alive at each time stage of the cold stress experiment. During the stress of cyclic freezing-defrosting, an increase in the number of bacteria was shown after the first cycle, and a decrease was only observed after cycle 3. The least sensitive to low nutrients content were strains isolated from frozen food. The high BHI concentration promoted the growth of all groups.

CONCLUSIONS

Data on survival in stress conditions can form the basis for one of the hypotheses explaining the formation of persistent strains. Such studies are also helpful for planning appropriate hygiene strategies within the food industry.

Transcriptomic and Phenotypic Analyses of the Sigma B-Dependent Characteristics and the Synergism between Sigma B and Sigma L in Listeria monocytogenes EGD-e

Numerous gene expression and stress adaptation responses in L. monocytogenes are regulated through alternative sigma factors σ^B and σ^L. Stress response phenotypes and transcriptomes were compared between L. monocytogenes EGD-e and its ΔsigB and ΔsigBL mutants. Targeted growth phenotypic analysis revealed that the ΔsigB and ΔsigBL mutants are impaired during growth under cold and organic-acid stress conditions. Phenotypic microarrays revealed increased sensitivity in both mutants to various antimicrobial compounds. Genes de-regulated in these two mutants were identified by genome-wide transcriptome analysis during exponential growth in BHI. The ΔsigB and ΔsigBL strains repressed 198 and 254 genes, respectively, compared to the parent EGD-e strain at 3 °C, whereas 86 and 139 genes, respectively, were repressed in these mutants during growth at 37 °C. Genes repressed in these mutants are involved in various cellular functions including transcription regulation, energy metabolism and nutrient transport functions, and viral-associated processes. Exposure to cold stress induced a significant increase in σ^B and σ^L co-dependent genes of L. monocytogenes EGD-e since most (62%) of the down-regulated genes uncovered at 3 °C were detected in the ΔsigBL double-deletion mutant but not in ΔsigB or ΔsigL single-deletion mutants. Overall, the current study provides an expanded insight into σ^B and σ^L phenotypic roles and functional interactions in L. monocytogenes. Besides previously known σ^B- and σ^L-dependent genes, the transcriptomes defined in ΔsigB and ΔsigBL mutants reveal several new genes that are positively regulated by σ^B alone, as well as those co-regulated through σ^B- and σ^L-dependent mechanisms during L. monocytogenes growth under optimal and cold-stress temperature conditions.

Control of Listeria monocytogenes in Caramel Apples by Use of Sticks Pretreated with Potassium Sorbate

A multistate listeriosis outbreak associated with caramel apples from 2014 to 2015 prompted research on the survival of Listeria monocytogenes in fresh apples and caramel apples. Research indicated that stem end-inoculated caramel apples with stick insertion allowed for the survival and growth of L. monocytogenes at both refrigeration and ambient temperatures. This study aimed to assess the effectiveness of chemical preservatives as pretreatments for the wooden stick component to reduce L. monocytogenes loads in stem end-inoculated caramel apples during storage. Wooden sticks were pretreated with 1, 3, or 5% ascorbic acid (vitamin C), Nisaplin (2.5% nisin), potassium sorbate, and sodium benzoate and then inoculated with L. monocytogenes at 7 log CFU per stick. After storage at 25°C, the pathogen was reduced most effectively by the ascorbic acid pretreatments. At all three ascorbic acid concentrations tested, L. monocytogenes levels were reduced below the level of enumeration (2.5 log CFU per apple) at 24 h and were no longer detectable by enrichment after 72 h. Ascorbic acid (5, 10, and 20%) and potassium sorbate (10, 20, 30, and 40%) were further tested as wooden stick pretreatments for pathogen reduction on stem end-inoculated caramel apples stored at 5 and 25°C. The 40% potassium sorbate solution at 25°C was the most effective pretreatment condition in caramel apples and demonstrated a 3.1-log CFU per apple overall decrease in L. monocytogenes population levels after 216 h. Pretreatment of the wooden stick component of a caramel apple with potassium sorbate may be a viable preventive measure to reduce postprocess L. monocytogenes population levels and hence reduce consumer risk associated with caramel apple consumption.

Toxicological effects of trichlorfon on hematological and biochemical parameters in Cyprinus carpio L. following thermal stress

Trichlorfon is a moderately toxic organophosphate pesticide that is widely used in aquaculture. This study investigated the effects of trichlorfon on hematological parameters, biochemical factors, and stress reaction in Cyprinus carpio L. The fish were exposed to acute concentrations of trichlorfon (0, 0.5, 1.0, 2.0, and 4.0 mg L^-1) at 25 °C and 15 °C for 1 and 2 weeks, after which several parameters were evaluated to assess the effects of the pesticide. Significant decreases were observed in red blood cell (RBC) Count, hemoglobin (Hb) level, hematocrit (Ht), and plasma protein levels after each exposure period. In contrast, notable increases in mean corpuscular volume (MCV), mean cell hemoglobin (MCH), calcium, and glucose levels were observed in the trichlorfon-treated groups. Additionally, there were significant increases in the plasma levels of glutamate-oxaloacetate transaminase (GOT), glutamate-pyruvate transaminase (GPT), and alkaline phosphatase (ALP) following the exposure to trichlorfon. Furthermore, the results showed a relationship between toxic stress and increment in HSP70 and cytochrome P450 1A (CYP1A) expression over time. Ht, MCV, MCH, and the value of other biochemical parameters were quite lower at 15 °C than their corresponding values were at 25 °C, which indicated the decreased physical activity at 15 °C. The results of the present work indicate that acute exposure to trichlorfon and thermal stimulus could damage erythropoietic tissue. Additionally, hepatocytes function and physiological mechanisms could be impaired. Ht, glucose, GOT, GPT, HSP70, and CYP1A levels might be useful biomarkers of trichlorfon toxicity in contaminated aquatic ecosystems.

Adaptive Response of Listeria monocytogenes to Heat, Salinity and Low pH, after Habituation on Cherry Tomatoes and Lettuce Leaves

Pathogens found on fresh produce may encounter low temperatures, high acidity and limited nutrient availability. The aim of this study was to evaluate the effect of habituation of Listeria monocytogenes on cherry tomatoes or lettuce leaves on its subsequent response to inhibitory levels of acid, osmotic and heat stress. Habituation was performed by inoculating lettuce coupons, whole cherry tomatoes or tryptic soy broth (TSB) with a three-strains composite of L. monocytogenes, which were further incubated at 5°C for 24 hours or 5 days. Additionally, cells grown overnight in TSB supplemented with 0.6% yeast extract (TSBYE) at 30°C were used as control cells. Following habituation, L. monocytogenes cells were harvested and exposed to: (i) pH 3.5 adjusted with lactic acid, acetic acid or hydrochloric acid (HCl), and pH 1.5 (HCl) for 6 h; (ii) 20% NaCl and (iii) 60°C for 150 s. Results showed that tomato-habituated L. monocytogenes cells were more tolerant (P < 0.05) to acid or osmotic stress than those habituated on lettuce, and habituation on both foods resulted in more stress resistant cells than prior growth in TSB. On the contrary, the highest resistance to heat stress (P < 0.05) was exhibited by the lettuce-habituated L. monocytogenes cells followed by TSB-grown cells at 5°C for 24 h, whereas tomato-habituated cells were highly sensitized. Prolonged starvation on fresh produce (5 days vs. 24 h) increased resistance to osmotic and acid stress, but reduced thermotolerance, regardless of the pre-exposure environment (i.e., tomatoes, lettuce or TSB). These results indicate that L. monocytogenes cells habituated on fresh produce at low temperatures might acquire resistance to subsequent antimicrobial treatments raising important food safety implications.

Fate of Pathogenic Bacteria Associated with Production of Pickled Sausage by Using a Cold Fill Process

Preservation by pickling has been used for many years to extend the shelf life of various types of food products. By storing meat products in a brine solution containing an organic acid, salt, spices, as well as other preservatives, the pH of the product is reduced, thus increasing the safety and shelf life of the product. Pickling may involve the use of heated brines to further add to the safety of the food product. When precooked, ready-to-eat (RTE) sausages are pickled with a heated brine solution, the process is referred to as hot filling. However, hot filling has been shown to affect the clarity of the brine, making the product cloudy and unappealing to consumers. Because of the potential quality defects caused by higher temperatures associated with hot fill pickling, cold fill pickling, which uses room temperature brine, is preferred by some pickled sausage manufacturers. Because little information exists on the safety of cold fill, pickled sausages, a challenge study was designed using a brine solution (5% acetic acid and 5% salt at 25°C) to pickle precooked, RTE sausages inoculated with a pathogen cocktail consisting of Salmonella Typhimurium, Salmonella Senftenberg, Salmonella Montevideo, Listeria monocytogenes , and Staphylococcus aureus . All pathogens were reduced ~6.80 log CFU/g in 72 h when enumerated on nonselective media. On selective media, Salmonella and L. monocytogenes decreased 6.33 and 6.35 log CFU/g in 12 h, respectively whereas S. aureus was reduced 6.80 log CFU/g in 24 h. Sausages experienced significant (P ≤ 0.05) decreases in pH over the 28 days of storage, whereas no significant differences were observed in water activity (P =0.1291) or salt concentration of the sausages (P =0.1445) or brine (P =0.3180). The results of this experiment demonstrate that cold fill pickling can effectively reduce and inhibit bacterial pathogens.

Use of Antimicrobial Food Additives as Potential Dipping Solutions to Control Pseudomonas spp. Contamination in the Frankfurters and Ham

This study evaluated the effect of sodium diacetate and sodium lactate solutions for reducing the cell count of Pseudomonas spp. in frankfurters and hams. A mixture of Pseudomonas aeruginosa (NCCP10338, NCCP10250, and NCCP11229), and Pseudomonas fluorescens (KACC10323 and KACC10326) was inoculated on cooked frankfurters and ham. The inoculated samples were immersed into control (sterile distilled water), sodium diacetate (5 and 10%), sodium lactate (5 and 10%), 5% sodium diacetate + 5% sodium lactate, and 10% sodium diacetate + 10% sodium lactate for 0-10 min. Inoculated frankfurters and ham were also immersed into acidified (pH 3.0) solutions such as acidified sodium diacetate (5 and 10%), and acidified sodium lactate (5 and 10%) in addition to control (acidified distilled water) for 0-10 min. Total aerobic plate counts for Pseudomonas spp. were enumerated on Cetrimide agar. Significant reductions (ca. 2 Log CFU/g) in Pseudomonas spp. cells on frankfurters and ham were observed only for a combination treatment of 10% sodium lactate + 10% sodium diacetate. When the solutions were acidified to pH 3.0, the total reductions of Pseudomonas spp. were 1.5-4.0 Log CFU/g. The order of reduction amounts of Pseudomonas spp. cell counts was 10% sodium lactate > 5% sodium lactate ≥ 10% sodium diacetate > 5% sodium diacetate > control for frankfurters, and 10% sodium lactate > 5% sodium lactate > 10% sodium diacetate > 5% sodium diacetate > control for ham. The results suggest that using acidified food additive antimicrobials, as dipping solutions, should be useful in reducing Pseudomonas spp. on frankfurters and ham.

Acquired acid adaptation of Listeria monocytogenes during its planktonic growth enhances subsequent survival of its sessile population to disinfection with natural organic compounds

The abilities of Listeria monocytogenes to adapt to stress and also to attach to surfaces are believed to contribute to its persistence in food processing environments. In this study, the possible influence of acid adaptation of L. monocytogenes cells during their planktonic growth on their subsequent resistance against 3 different natural disinfection treatments (0.05% lactic acid, 1% essential oil or 100% hydrosol of S. thymbra) upon their attachment to stainless steel (SS) was evaluated by simultaneously using the bead vortexing technique and a conductance method. Prior to disinfection, both nonadapted and acid-adapted stationary phase bacteria were left to attach to SS coupons statically incubated for 10days into Brain Heart Infusion (BHI) broth under 3 different temperatures (5, 16 or 30°C). Results revealed that both essential oil and hydrosol presented sufficient bactericidal activity against all formed sessile populations, always resulting in counts following disinfection below the plate counting detection limit. However, the conductance method, able to detect metabolically active sessile bacteria unable to be recovered by the bead vortexing, revealed the positive influence of previously acquired acid adaptation on disinfection resistance of attached cells against these plant extracts. A similar effect of acid adaptation was also evident for lactic acid disinfection.

Low, medium, and high heat tolerant strains of Listeria monocytogenes and increased heat stress resistance after exposure to sublethal heat

A group of 37 strains representing all 13 serotypes of Listeria monocytogenes with an initial cell density of 10(7) CFU/ml were analyzed for their heat tolerance at 60°C for 10 min. These L. monocytogenes strains were categorized into three heat tolerance groups: low (<2 log CFU/ml survival), medium (2 to 4 log CFU/ml survival), and high (4 to 6 log CFU/ml survival). Serotype 1/2a strains had relatively low heat tolerance; seven of the eight tested strains were classified as low heat tolerant. Of the two serotype 1/2b strains tested, one was very heat sensitive (not detectable) and the other was very heat resistant (5.4 log CFU/ml survival). Among the 16 serotype 4b strains, survival ranged from not detectable to 4 log CFU/ml. When one L. monocytogenes strain from each heat tolerance group was subjected to sublethal heat stress at 48°C for 30 or 60 min, the survival of heat-stressed cells at 60°C for 10 min increased by 5 log CFU/ml (D60°C-values nearly doubled) compared with the nonstressed control cells. Sublethal heat stress at 48°C for 60 or 90 min increased the lag phase of L. monocytogenes in tryptic soy broth supplemented with 0.6% yeast extract at room temperature by 3 to 5 h compared with nonstressed control cells. The heat stress adaptation in L. monocytogenes was reversed after 2 h at room temperature but was maintained for up to 24 h at 4°C. Our results indicate a high diversity in heat tolerance among strains of L. monocytogenes, and once acquired this heat stress adaptation persists after cooling, which should be taken into account while conducting risk analyses for this pathogen.

Growth and membrane fluidity of food-borne pathogen Listeria monocytogenes in the presence of weak acid preservatives and hydrochloric acid

This study addresses a major issue in microbial food safety, the elucidation of correlations between acid stress and changes in membrane fluidity of the pathogen Listeria monocytogenes. In order to assess the possible role that membrane fluidity changes play in L. monocytogenes tolerance to antimicrobial acids (acetic, lactic, hydrochloric acid at low pH or benzoic acid at neutral pH), the growth of the bacterium and the gel-to-liquid crystalline transition temperature point (T m) of cellular lipids of each adapted culture was measured and compared with unexposed cells. The T m of extracted lipids was measured by differential scanning calorimetry. A trend of increasing T m values but not of equal extent was observed upon acid tolerance for all samples and this increase is not directly proportional to each acid antibacterial action. The smallest increase in T m value was observed in the presence of lactic acid, which presented the highest antibacterial action. In the presence of acids with high antibacterial action such as acetic, hydrochloric acid or low antibacterial action such as benzoic acid, increased T m values were measured. The T m changes of lipids were also correlated with our previous data about fatty acid changes to acid adaptation. The results imply that the fatty acid changes are not the sole adaptation mechanism for decreased membrane fluidity (increased T m). Therefore, this study indicates the importance of conducting an in-depth structural study on how acids commonly used in food systems affect the composition of individual cellular membrane lipid molecules.

Cellular and molecular investigations of the adhesion and mechanics of Listeria monocytogenes lineages' I and II environmental and epidemic strains

Atomic force microscopy (AFM) was used to probe the mechanical and adherence properties of eight Listeria monocytogenes' strains representative of the species' two phylogenetic lineages I and II. From a functional perspective, lineage' I strains were characterized by lower overall adhesion forces and higher specific and nonspecific forces compared to lineage' II strains. From a structural perspective, lineage' II strains were characterized by higher Young's moduli and longer and stiffer biopolymers compared to lineage' I strains. Both lineages' I and II strains were similar in their grafting densities. Finally, our results indicated that epidemic and environmental strains of L. monocytogenes and irrespective of their lineage group were characterized by similar Young's moduli of elasticities and adhesion forces at the cellular level. However, at the molecular level, epidemic strains were characterized by higher specific and nonspecific forces, shorter, denser, and more flexible biopolymers compared to environmental strains.

Effect of steam and lactic acid treatments on the survival of Salmonella Enteritidis and Campylobacter jejuni inoculated on chicken skin

Campylobacteriosis and salmonellosis are the most frequently reported zoonotic infectious diseases. The present work evaluated the effectiveness of steam treatment at 100 °C for 8s, a 5% lactic acid treatment for 1 min and their combination for inactivating Salmonella Enteritidis and Campylobacter jejuni inoculated on chicken skin. The impact of each treatment on the total aerobic mesophilic bacteria and the effect of rinsing after contact with lactic acid were also evaluated. Residual bacteria were counted immediately after treatment or after seven days of storage at 4 °C. Results demonstrated the immediate efficiency of the steam and the combined treatments with reductions of approximately 6 and 5 log cfu/cm2 respectively for S. Enteritidis and C. jejuni. They also showed significant reductions (equal to or >3.2 log cfu/cm2) in the total aerobic mesophilic plate count. Lactic acid had a persistent effect on pathogen growth during storage which was significantly higher when the skin was not rinsed, reaching reductions of 3.8 log cfu/cm2 for both S. Enteritidis and C. jejuni. Only the combined treatments significantly reduced the recovery of the total aerobic mesophilic bacteria during storage. The significant reductions in both pathogens and total aerobic mesophilic bacteria on treated chicken skins are possible ways to improve the safety and shelf life of the product although high levels of indigenous non-pathogenic bacteria may be beneficial due to their protective effect against potential re-contamination of chicken skin.

Exploration of the role of the non-coding RNA SbrE in L. monocytogenes stress response

SbrE is a ncRNA in Listeria monocytogenes, reported to be up-regulated by the alternative sigma factor σB. Initial quantitative RT-PCR (qRT-PCR) experiments on parent strains and isogenic ΔsigB strains demonstrated σB-dependent expression of SbrE across the four L. monocytogenes lineages and in L. innocua. Microarray and proteomics (MDLC/MS/MS with iTRAQ labeling) experiments with the L. monocytogenes parent strain and an isogenic ΔsbrE strain identified a single gene (lmo0636) and two proteins (Lmo0637 and Lmo2094) that showed lower expression levels in the ΔsbrE strain. qRT-PCR demonstrated an increase in SbrE transcript levels in stationary phase L. monocytogenes and in bacteria exposed to oxidative stress (mean log2 transcript levels 7.68 ± 0.57 and 1.70 ± 0.71 greater than in mid-log phase cells, respectively). However, no significant differences in growth or survival between the parent strain and ΔsbrE strain were confirmed under a variety of environmental stress conditions tested. Our data suggest that σB-dependent transcription of SbrE represents a conserved mechanism that contributes, across Listeria species, to fine-tuning of gene expression under specific environmental conditions that remain to be defined.

Effectiveness of chitosan on the inactivation of enteric viral surrogates

Chitosan is known to have bactericidal and antifungal activity. Although human noroviruses are the leading cause of non-bacterial gastroenteritis, information on the efficacy of chitosan against foodborne viruses is very limited. The objective of this work was to determine the effectiveness of different molecular weight chitosans against the cultivable human norovirus and enteric virus surrogates, feline calicivirus, FCV-F9, murine norovirus, MNV-1, and bacteriophages, MS2 and phiX174. Five purified chitosans (53, 222, 307, 421, ~1150 kDa) were dissolved in water, 1% acetic acid, or aqueous HCl pH = 4.3, sterilized by membrane filtration, and mixed with equal volume of virus to obtain a final concentration of 0.7% chitosan and 5 log(10) PFU/ml virus. Virus-chitosan suspensions were incubated for 3 h at 37 °C. Untreated viruses in PBS, in PBS with acetic acid, and in PBS with HCl were tested as controls. Each experiment was run in duplicate and replicated at least twice. Water-soluble chitosan (53 kDa) reduced phiX174, MS2, FCV-F9 and MNV-1 titers by 0.59, 2.44, 3.36, and 0.34 log(10) PFU/ml respectively. Chitosans in acetic acid decreased phiX174 by 1.19-1.29, MS2 by 1.88-5.37, FCV-F9 by 2.27-2.94, and MNV-1 by 0.09-0.28 log(10) PFU/ml, respectively. Increasing the MW of chitosan corresponded with an increasing antiviral effect on MS2, but did not appear to play a role for the other three tested viral surrogates. Overall, chitosan treatments showed the greatest reduction for FCV-F9, and MS2 followed by phiX174, and with no significant effect on MNV-1.

Studies on the effect of temperature and pH on the inactivation of fish viral and bacterial pathogens

Disposal of fish by-products in the European Community must comply with Regulation (EC) No 1069/2009 which categorizes animal by-products according to risk, and specifies methods of disposal of by-products according to that risk. There is provision under the regulation for composting or ensiling to be used for by-products from aquatic animals. Biosecurity considerations require knowledge of the parameters of time and temperature, or time and pH, required to inactivate any fish pathogens that may be present. To provide those data, we undertook laboratory studies on the inactivation of a number of fish pathogenic viruses and bacteria at 60 °C, pH 4.0 and pH 12.0 as a preliminary to conducting subsequent trials with the most resistant viruses and bacteria in fish tissues. The most resistant bacterium to 60 °C, pH 4.0 as well as pH 12.0 was Lactococcus garvieae. Its concentration was reduced to the level of sensitivity of the test after 24-48 h exposure to 60 °C, but it survived for at least 7 days at pH 4.0 and 14 days at pH 12.0. The most resistant virus to 60 °C was infectious pancreatic necrosis virus, and to pH 12.0 was infectious salmon anaemia virus. The majority of the viruses tested survived exposure to pH 4.0 for up to 28 days. The results suggest that the process of acid ensiling alone is not an effective method for the inactivation of many viral and bacterial pathogens, and fish by-products would need further treatment by a method approved under the regulation following ensiling, whereas alkaline or heat treatment are likely to provide an increased degree of biosecurity for on-farm processing of mortalities.

Spatial competition with Lactococcus lactis in mixed-species continuous-flow biofilms inhibits Listeria monocytogenes growth

Surfaces in industrial settings provide a home for resident biofilms that are likely to interact with the attachment, growth and survival of pathogens such as Listeria monocytogenes. Experimental results have indicated that L. monocytogenes cells were inhibited by the presence of a model resident flora (Lactococcus lactis) in dual-species continuous flow-biofilms, and are spatially restricted to the lower biofilm layers. Using a new, simplified individual-based model (IBM) that simulates bacterial cell growth in a three-dimensional space, the spatial arrangements of the two species were reconstructed and their cell counts successfully predicted. This model showed that the difference in generation times between L. monocytogenes and L. lactis cells during the initial stages of dual-species biofilm formation was probably responsible for the species spatialization observed and the subsequent inhibition of growth of the pathogen.

Rapid, transient, and proportional activation of σ(B) in response to osmotic stress in Listeria monocytogenes

The osmotic activation of sigma B (σ(B)) in Listeria monocytogenes was studied by monitoring expression of four known σ(B)-dependent genes, opuCA, lmo2230, lmo2085, and sigB. Activation was found to be rapid, transient, and proportional to the magnitude of the osmotic stress applied, features that underpin the adaptability of this pathogen.

Survival of Listeria innocua in rainbow trout protein recovered by isoelectric solubilization and precipitation with acetic and citric acids

During mechanical fish processing, a substantial amount of protein is discarded as by-products. Isoelectric solubilization and precipitation (ISP) is a process that uses extreme pH shifts to solubilize and precipitate protein from by-products to recover previously discarded protein. Typically, strong acids are used for pH reduction, but these acids do not have a pasteurization effect (6 log reduction) on bacterial load; therefore, organic acids were used during ISP processing to test the impact on Listeria innocua concentrations. Headed and gutted rainbow trout (Oncorhynchus mykiss) were inoculated with L. innocua, homogenized, and brought to the target pH with granular citric acid (pH 2.0 and 2.5) or glacial acetic acid (pH 3.0 and 3.5). Proteins were solubilized for 10 min at 4°C, and insoluble components (e.g., skin and insoluble protein) were removed by centrifugation. The remaining solution was pH shifted to the protein isoelectric point (pH 5.5) with sodium hydroxide, and precipitated protein was separated from the water. Microbial cells for each component (proteins, insolubles, and water) were enumerated on modified Oxford agar (MOX) and tryptic soy agar with 6% yeast extract (TSAYE). The sums of the surviving cells from each component were compared with the initial inoculum levels. No significant differences were observed between results obtained from TSAYE and from MOX (P > 0.05). Significant reductions in microbial populations were detected, regardless of pH or acid type (P < 0.05). The greatest reduction was at pH 3.0 with glacial acetic acid, resulting in a mean reduction of 6.41 log CFU/g in the recovered protein and 5.88 log CFU/g in the combined components. These results demonstrate the antimicrobial potential of organic acids in ISP processing.

Effects of salt, sodium pyrophosphate, and sodium lactate on the probability of growth of Escherichia coli O157:H7 in ground beef

Ground beef products are susceptible to contamination with Escherichia coli O157:H7. The objective of this study was to examine the effect of salt, sodium pyrophosphate (SPP), and sodium lactate on the probability of growth of E. coli O157:H7 in ground beef under a temperature abuse condition. Ground beef containing 0 to 2.25% salt, 0 to 0.5% SPP, and 0 to 3% lactate was inoculated with a four-strain mixture of E. coli O157:H7, vacuum packaged, and stored at 10°C for 15 days. A total of 25 combinations of the three additives, each with 20 samples, were tested. A logistic regression was used to model the probability of growth of E. coli O157:H7 (with a 1.0-log CFU/g increase during storage) as a function of salt, SPP, and lactate. The resultant probability model indicated that lactate at higher concentrations decreased the probability of growth of E. coli O157:H7 in ground beef, and the effect was more pronounced at higher salt concentrations. At salt concentrations below 1.3%, the increase of SPP concentration marginally increased the growth probabilities of E. coli O157:H7. The model illustrated the effect of salt, SPP, and lactate on the growth probabilities and growth or no-growth behavior of E. coli O157:H7 in ground beef and can be used to improve the microbial food safety of ground beef products.

Implications of salt and sodium reduction on microbial food safety

Excess sodium consumption has been cited as a primary cause of hypertension and cardiovascular diseases. Salt (sodium chloride) is considered the main source of sodium in the human diet, and it is estimated that processed foods and restaurant foods contribute 80% of the daily intake of sodium in most of the Western world. However, ample research demonstrates the efficacy of sodium chloride against pathogenic and spoilage microorganisms in a variety of food systems. Notable examples of the utility and necessity of sodium chloride include the inhibition of growth and toxin production by Clostridium botulinum in processed meats and cheeses. Other sodium salts contributing to the overall sodium consumption are also very important in the prevention of spoilage and/or growth of microorganisms in foods. For example, sodium lactate and sodium diacetate are widely used in conjunction with sodium chloride to prevent the growth of Listeria monocytogenes and lactic acid bacteria in ready-to-eat meats. These and other examples underscore the necessity of sodium salts, particularly sodium chloride, for the production of safe, wholesome foods. Key literature on the antimicrobial properties of sodium chloride in foods is reviewed here to address the impact of salt and sodium reduction or replacement on microbiological food safety and quality.

Alteration of the phospho- or neutral lipid content and fatty acid composition in Listeria monocytogenes due to acid adaptation mechanisms for hydrochloric, acetic and lactic acids at pH 5.5 or benzoic acid at neutral pH

This study provides a first approach to observe the effects on Listeria monocytogenes of cellular exposure to acid stress at low or neutral pH, notably how phospho- or neutral lipids are involved in this mechanism, besides the fatty acid profile alteration. A thorough investigation of the composition of polar and neutral lipids from L. monocytogenes grown at pH 5.5 in presence of hydrochloric, acetic and lactic acids, or at neutral pH 7.3 in presence of benzoic acid, is described relative to cells grown in acid-free medium. The results showed that only low pH values enhance the antimicrobial activity of an acid. We suggest that, irrespective of pH, the acid adaptation response will lead to a similar alteration in fatty acid composition [decreasing the ratio of branched chain/saturated straight fatty acids of total lipids], mainly originating from the neutral lipid class of adapted cultures. Acid adaptation in L. monocytogenes was correlated with a decrease in total lipid phosphorus and, with the exception of cells adapted to benzoic acid, this change in the amount of phosphorus reflected a higher content of the neutral lipid class. Upon acetic or benzoic acid stress the lipid phosphorus proportion was analysed in the main phospholipids present: cardiolipin, phosphatidylglycerol, phosphoaminolipid and phosphatidylinositol. Interestingly only benzoic acid had a dramatic effect on the relative quantities of these four phospholipids.

Relative transcription of Listeria monocytogenes virulence genes in liver pâtés with varying NaCl content

Quantitative real time polymerase chain reaction (qRT PCR) was used to compare the relative transcription of prfA, inlA, sigB and clpC for three Listeria monocytogenes strains after incubation in i) a standard liver pâté versus brain heart infusion (BHI) broth and ii) the standard liver pâté versus three liver pâtés with reduced NaCl content of which one also has been supplied with organic acids (Ca-acetate and Ca-lactate). The three strains (EGD-e: reference strain; O57: more NaCl sensitive; 6896: more NaCl tolerant) were selected out of twelve strains based on their growth in BHI broth adjusted to 6%, 8%, 10% (w/v) NaCl. The three strains were spiked into the liver pâtés (10(9) cfu/g) and the BHI (10(9) cfu/ml) and incubated for 48 h at 7 degrees C; all incubation conditions supported growth of the strains. Extraction of intact listerial RNA from the liver pâtés was complicated by the complexity of the liver pâté matrix. However, a method has been optimized and described, and the quality of RNA extracted from liver pâtés was equal to the quality of RNA extracted from BHI. The amplification efficiencies of the six genes used for the transcription analyses (the four target genes and two reference genes, gap and rpoB) were within the acceptable range from 90% to 110% for all three strains in both liver pâté and BHI. Comparison of the three strains after incubation in the standard liver pâté and BHI showed that the relative transcription of prfA for O57 and the relative transcription of inlA and sigB for both O57 and 6896 were significantly higher when the strains were grown in BHI compared to the standard liver pâté. Reducing the NaCl content of the standard liver pâté did not change relative transcription levels of prfA, inlA, sigB or clpC (except for prfA in O57 and sigB in 6896). However, the presence of Ca-acetate and Ca-lactate induced relative transcription of the stress response gene, clpC, for all three strains. This study demonstrates that relative microbial gene transcription can be measured in complex food matrices and points to the need for designing experimental set-ups in real food matrices to replace the laboratory model systems. With respect to L. monocytogenes, it seems that the NaCl content of liver pâté can be lowered within the investigated range without significant changes in relative virulence gene transcription while more caution should be taken when adding organic acids such as acetate and lactate.

Control of Listeria monocytogenes in turkey deli loaves using organic acids as formulation ingredients

The growth of Listeria monocytogenes in further-processed meat products has become a major concern and an important food safety issue. The meat and poultry industries have incorporated interventions such as organic acids in marinades to inhibit the growth of L. monocytogenes. In this study, organic acids were utilized in the raw product and as a postcook dip to determine their inhibitory effect on the growth of L. monocytogenes in turkey deli loaves. The turkey deli loaves were processed, cooked, cooled, inoculated with streptomycin-resistant L. monocytogenes, and then dipped. Treatments were potassium lactate (PL) in the raw product with sodium lactate (SL), sodium diacetate (SD) dip, PL with SL/PL/SD dip, SL with SL/SD dip, and SL with SL/PL/SD dip. There was also a positive (inoculated) and negative (noninoculated) control, which was dipped in distilled water. Days 0, 7, 14, 21, 28, 42, and 56 were sampled for L. monocytogenes. There were no differences (P>0.05) among the organic acid treatments in the turkey deli loaves at any time points; therefore, all of the treatments increased the lag phase of L. monocytogenes, extending the shelf-life of the product. However, there was a difference between the treatments and the positive control at d 7, 14, 21, 28, 42, and 56. The growth of L. monocytogenes increased immediately in the positive control, whereas the negative control appeared to have no growth. These organic acids can provide meat processors with a useful method for extending the lag phase of L. monocytogenes in ready-to-eat meat and poultry products.

Validation of a lactic acid- and citric acid-based antimicrobial product for the reduction of Escherichia coli O157: H7 and Salmonella on beef tips and whole chicken carcasses

The objectives of this study were to determine the effects of a lactic acid- and citric acid-based antimicrobial product on the reduction of Salmonella on whole broiler carcasses during processing and the reduction of Salmonella and Escherichia coli O157:H7 on beef trim. Freshly harvested broiler carcasses were inoculated with an inoculum of Salmonella strains to yield a 10(5) CFU/ml pathogen load on the surface of the carcass. The beef tips were inoculated as well with an inoculum of either E. coli O157:H7 or Salmonella to yield 10(4) CFU/100 cm(2). After 30 min for attachment, the broiler carcasses were treated with Chicxide applied for 5 s via a spray or immersed in Chicxide for 5, 10, or 20 s. Broiler carcasses were rinsed in poultry rinse bags with 400 ml of Butterfield's phosphate buffer in which Salmonella was enumerated from the diluents and Butterfield's phosphate. Chicxide significantly reduced Salmonella by 1.3 log CFU/ml with spray treatment and 2.3 log CFU/ml for all dip treatments. Following 30 min of attachment, the beef tips were placed into a spray cabinet with either Beefxide or sterilized water (control) and sprayed at 1 ft/2.5 s chain speed at 40 lb/in(2). The external surface of each beef tip was swabbed (100 cm(2)) to determine pathogen loads. Beefxide significantly reduced E. coli O157:H7 by 1.4 log CFU/100 cm(2) and Salmonella by 1.1 log CFU/100 cm(2) (P < 0.05) compared with the control samples.

Effect of acid stress, antibiotic resistance, and heat shock on the resistance of Listeria monocytogenes to UV light when suspended in distilled water and fresh brine

Exposure to sublethal processing treatments can stimulate bacterial stress responses. The purpose of this research was to determine whether adaptation to common food processing stresses encountered during the preparation of ready-to-eat foods affects the dose of UV light required to significantly reduce Listeria monocytogenes populations in sterile distilled water and a 9% NaCl solution, using uridine as a chemical actinometer. L. monocytogenes strains N1-227 (from hot dog batter), N3-031 (from turkey franks), and R2-499 (from ready-to-eat meat) were acid stressed for 3 h at 35 degrees C in Trypticase soy broth with yeast extract acidified to pH 5.0, heat shocked for 1 h at 48 degrees C in brain heart infusion broth (BHIB), and selected for sulfanilamide resistance (512 microg/ml). These strains were then mixed in equal proportions and suspended in water and 9% NaCl solution, each containing 10(-4) M uridine. Samples were exposed to UV light (253.7 nm) for 0, 5, 10, 15, 20, 25, or 30 min. Inactivation was evaluated by surface plating onto modified Oxford agar and Trypticase soy agar with yeast extract and by enrichment in BHIB followed by incubation at 37 degrees C for 24 h. The absorbance of each sample was measured before and after irradiation to calculate the dose of UV light. There were no significant differences between population estimates based on medium or suspension solution. There were no population differences between acid-stressed and antibiotic-resistant or unstressed and heat-shocked L. monocytogenes strains. However, acid-stressed and antibiotic-resistant strains were significantly more resistant to UV light than were unstressed and heat-shocked strains (P < or = 0.05).