Antimicrobial activity of some plant extracts and essential oils against foodborne pathogens in vitro and on the fate of inoculated pathogens in chocolate

The efficacy of commercially available plant extracts and essential oils used extensively as flavour ingredients in confectionery products were used as antimicrobials in laboratory media against the following microorganisms: Escherichia coli O157:H7, Salmonella Enteritidis, Salmonella Typhimurium, Staphylococcus aureus, Listeria monocytogenes, and Bacillus cereus. Using the disc diffusion method, inhibition zones in diameter >20 mm were observed by adding 10 μl of each antimicrobial substance on the following microorganisms: lemon flavour applied on E. coli O157:H7, lemongrass essences against S. aureus, plum using a B. cereus strain and strawberry flavour using a L. monocytogenes strain. E. coli O157:H7 strains were the most susceptible microorganisms inhibited by 18 extracts, followed by S. Typhimurium and S. aureus which were inhibited by 17 extracts. Lemon flavour, lemongrass essences, pineapple and strawberry flavour inhibited the foodborne pathogens at the lowest concentration (5 ml/100 ml). Plant extracts and essential oils with potent antimicrobial activities were tested in chocolate held at different temperatures (7 and 20 °C) in dry or humidified environment, which resulted in different a_w values of the product (i.e. 0.340, 0.450, and 0.822), in order to determine their efficacy on the fate of the inoculated pathogens. The most inhibitory action was observed by lemon flavour applied on chocolate inoculated with E. coli cocktail culture after storage at 20 °C for 9 days. Plant extracts tested on chocolate show an enhanced inhibitory effect during storage at 20 °C indicating that their application may provide protection in case of storage at the above temperature or even higher.     

Effects of natural antimicrobials on inhibition of Listeria monocytogenes and on chemical, physical and sensory attributes of naturally-cured frankfurters

Due to regulations for natural and organic processed meats, sodium nitrite and many antimicrobials cannot be used. Therefore, natural and organic processed meats are more susceptible to pathogenic bacterial growth, and natural alternatives to chemical preservatives are needed. Inhibition of Listeria monocytogenes, and quality characteristics of frankfurters manufactured with 3% cranberry powder, or with 1% or 2% cranberry powder each with either cherry powder (0.6%), lime powder (60 mg/kg), or a blend of cherry, lime and vinegar (1.4%) were investigated. Cranberry powder at 3% significantly reduced L. monocytogenes growth by 5.3 log CFU/g compared to the uncured co006Etrol (P < 0.05). However, cranberry addition over 1% also resulted in significant product pH decline and negatively impacted the color, texture and sensory attributes of the frankfurters.     

The effects of X-ray radiation on Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica and Shigella flexneri inoculated on whole Roma tomatoes

In the last two decades several foodborne disease outbreaks associated with produce were reported. Tomatoes, in particular, have been associated with several multi-state Salmonella outbreaks. Inactivation of inoculated Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica and Shigella flexneri on whole Roma tomato surfaces by X-ray at 0.1, 0.5, 0.75, 1.0, and 1.5 kGy was studied. The main purpose of this study was to achieve a 5 log reduction in consistent with the recommendations of the National Advisory Committee on Microbiological Criteria for Foods. Moreover, the effect of X-ray on inherent microflora (mesophilic counts, psychrotrophic counts and yeast and mold counts) of untreated and treated Roma tomatoes, during storage at ambient temperature (22 °C) for 20 days was also determined. Mixtures of three or two strains of each tested organism was spot inoculated (100 μl) onto the surface of Roma tomatoes (approximately 7–9 log per tomato), separately, and air-dried, followed by treatment with X-ray doses at 22 °C and 55–60% relative humidity. Surviving bacterial populations on tomato surfaces were evaluated using a nonselective medium (tryptic soy agar) with a selective medium overlay for each bacteria; E. coli O157:H7 (CT-SMAC agar), L. monocytogenes (MOA), and S. enterica and S. flexneri (XLD). Treatment with X-ray significantly reduced the population of the tested pathogens on whole Roma tomato surfaces, compared with the control. Approximately 4.2, 2.3, 3.7 and 3.6 log CFU reduction of E. coli O157:H7, L. monocytogenes, S. enterica and S. flexneri per tomato were achieved by treatment with 0.75 kGy X-ray, respectively. More than a 5 log CFU reduction per tomato was achieved at 1.0 or 1.5 kGy X-ray for all tested pathogens. Furthermore, treatment with X-ray significantly reduced the inherent microflora on Roma tomatoes. Inherent levels were significantly (p < 0.05) lower than the control sample throughout storage for 20 days.     

Chemical composition, cytotoxicity effect and antimicrobial activity of Ceratonia siliqua essential oil with preservative effects against Listeria inoculated in minced beef meat

The present study describes the phytochemical profile and the protective effects of Ceratonia siliqua pods essential oil (CsEO), a food and medicinal plant widely distributed in Tunisia. Twenty five different components were identified in the CsEO. Among them, the major detected components were: Nonadecane, Heneicosane , Naphthalene, 1,2-Benzenedicarboxylic acid dibutylester, Heptadecane, Hexadecanoic acid, Octadecanoic acid, 1,2-Benzenedicarboxylic acid, Phenyl ethyl tiglate, Eicosene, Farnesol 3, Camphor, Nerolidol and n-Eicosane. The antimicrobial activity of CsEO was evaluated against a panel of 13 bacteria and 8 fungal strains using agar diffusion and broth microdilution methods. Results have shown that CsEO exhibited moderate to strong antimicrobial activity against the tested species. In addition, the inhibitory effect of this CsEO was evaluated in vivo against a foodborne pathogens Listeria monocytogenes, experimentally inoculated in minced beef meat (2 × 10² CFU/g of meat) amended with different concentrations of the CsEO and stored at 7 °C for 10 days. The antibacterial activity of CsEO in minced beef meat was clearly evident and its presence led to a strong inhibitory effect against the pathogens at 7 °C. On the other hand, the cytotoxic effects of the essential oil against two tumoral human cell lines HeLa and MCF-7 were examined by MTT assay. The CsEO showed an inhibition of both cell lines with significantly stronger activity against HeLa cells. The IC₅₀ values were 210 and 800 μg/ml for HeLa and MCF-7 cells, respectively. Overall, results presented here suggest that the EO of C. siliqua possesses antimicrobial and cytotoxic properties, and is therefore a potential source of active ingredients for food and pharmaceutical industry.     

Antibacterial effects of natural tenderizing enzymes on different strains of Escherichia coli O157:H7 and Listeria monocytogenes on beef

This study determined the efficacy of actinidin and papain on reducing Listeria monocytogenes and three mixed strains of Escherichia coli O157:H7 populations on beef. The average reduction of E. coli O157:H7 was greater than that of L. monocytogenes and higher concentrations of either protease yielded greater reduction in bacterial populations. For instance, actinidin at 700 mg/ml significantly (p ≤ 0.05) reduced the population of L. monocytogenes by 1.49 log cfu/ml meat rinse after 3 h at 25 & 35 °C, and by 1.45 log cfu/ml rinse after 24 h at 5 °C, while the same actinidin concentration significantly reduced the populations of three mixed strains of E. coli O157:H7 by 1.81 log cfu/ml rinse after 3 h at 25 & 35 °C, and 1.94 log cfu/ml rinse after 24 h at 5 °C. These findings suggest that, in addition to improving the sensory attributes of beef, proteolytic enzymes can enhance meat safety when stored at suitable temperatures.     

A filtration-based real-time PCR method for the quantitative detection of viable Salmonella enterica and Listeria monocytogenes in food samples

We developed a novel filtration-based method that can eliminate dead or severally damaged Salmonella enterica and Listeria monocytogenes in food samples. This new method can recover all viable bacteria in less than 30 min, and can be coupled with a subsequent bacterial DNA extraction and real-time PCR. No statically significant differences (p < 0.01) were found between real-time PCR results obtained separately from S. enterica and L. monocytogenes when different ratios of living and dead cells were used. The analytical sensitivity in both cases was 1 genome equivalent (GE), and the quantification was linear (R² > 0.9969) over a 5-log dynamic range with PCR efficiencies >0.9754. When compared with the standard microbiological methods for the detection of these foodborne pathogens, the relative accuracy was excellent ranging from 95.72% to 104.48%. Finally, we applied the pre-treatment method to the direct detection of viable forms of these foodborne pathogens in food samples using yogurt as a model, the results being similar to those obtained using pure cultures.     

A Weibull model to describe antimicrobial kinetics of oregano and lemongrass essential oils against Salmonella Enteritidis in ground beef during refrigerated storage

The antimicrobial effect of oregano (Origanum vulgare L.) and lemongrass (Cymbopogon citratus (DC.) Stapf.) essential oils (EOs) against Salmonella enterica serotype Enteritidis in in vitro experiments, and inoculated in ground bovine meat during refrigerated storage (4 ± 2 °C) for 6 days was evaluated. The Weibull model was tested to fit survival/inactivation bacterial curves (estimating of p and δ parameters). The minimum inhibitory concentration (MIC) value for both EOs on S. Enteritidis was 3.90 μl/ml. The EO concentrations applied in the ground beef were 3.90, 7.80 and 15.60 μl/g, based on MIC levels and possible activity reduction by food constituents. Both evaluated EOs in all tested levels, showed antimicrobial effects, with microbial populations reducing (p ≤ 0.05) along time storage. Evaluating fit-quality parameters (RSS and RSE) Weibull models are able to describe the inactivation curves of EOs against S. Enteritidis. The application of EOs in processed meats can be used to control pathogens during refrigerated shelf-life.     

Antibacterial activity of eicosapentaenoic acid (EPA) against foodborne and food spoilage microorganisms

Eicosapentaenoic acid (EPA), a long-chain polyunsaturated fatty acid of ω-3 type was evaluated for its antimicrobial action against the range of foodborne and food spoilage pathogens, using agar disc diffusion assay in Luria broth (LB) media. The EPA exhibited antimicrobial activity against Bacillus subtilis ATCC 6633, Listeria monocytogenes ATCC 19166, Staphylococcus aureus ATCC 6538, S. aureus KCTC 1916 and Pseudomonas aeruginosa KCTC 2004. The minimum inhibitory concentrations (MICs) of the EPA against the tested bacterial strains were found in the range of 500-1350 μg/ml using broth dilution method. EPA reduced the viability of S. aureus at 250, 125 and 62.5 μg/ml after 15 min exposure and a steep decline in colony forming units (CFUs) was observed at 125 μg/ml after 30 min exposure, while similar reduction in CFU rate was exhibited by EPA when treated with 62.5 μg/ml after 180 min. EPA also reduced the CFU numbers of P. aeruginosa at all the concentrations used in this study after 15 min exposure. On the other hand, scanning electron microscopy (SEM) study of bacterial cells clearly exhibited the antibacterial effect of EPA as evidenced by the damages found in the outer membrane of the cells when treated with EPA. The results demonstrated that EPA exerted significant bactericidal and bacteriostatic effects against both P. aeruginosa and S. aureus.     

Inactivation of Saccharomyces cerevisiae in pineapple, grape and cranberry juices under pulsed and continuous thermo-sonication treatments

Pineapple, grape and cranberry juice were thermo-sonicated (24 kHz, 400 W, 120 μm) at 40 °C, 50 °C and 60 °C during 10 min at continuous and pulsed mode. Inactivation of Saccharomyces cerevisiae was tested from 0 to 10 min; color and pH were measured. Survivor’s curves were fitted with Weibull distribution, four parameter model and modified Gompertz equation. The acoustic energy (AE) was also calculated. S. cerevisiae was inactivated in the treatments at 60 °C, with the continuous mode being more effective. Grape juice showed total inactivation (7-log) after 10 min. Results showed that pH and color changed significantly (p < 0.05); ultrasound may promote chemical reactions and extract some components. The modified Gompertz equation showed the best fit. Energy analysis showed that pineapple juice (4287.02 mW/ml) required a higher amount of energy; grape juice showed the lowest value (3112.13 mW/ml). Ultrasound represents a viable option for juice pasteurization.     

The decontaminative effects of acidic electrolyzed water for Escherichia coli O157:H7, Salmonella typhimurium, and Listeria monocytogenes on green onions and tomatoes with differing organic demands

Acidic electrolyzed water (AC-EW) has strong bactericidal activity against foodborne pathogens on fresh vegetables. However, the efficacy of AC-EW is influenced by soil or other organic materials present. This study examined the bactericidal activity of AC-EW in the presence of organic matter, in the form of bovine serum against foodborne pathogens on the surfaces of green onions and tomatoes. Green onions and tomatoes were inoculated with a culture cocktail of Escherichia coli O157:H7, Salmonella typhimurium, and Listeria monocytogenes. Treatment of these organisms with AC-EW containing bovine serum concentrations of 5, 10, 15, and 20 ml/l was performed for 15 s, 30 s, 1 min, 3 min and 5 min. The total residual chlorine concentrations of AC-EW decreased proportional to the addition of serum. The bactericidal activity of AC-EW also decreased with increasing bovine serum concentration, whereas unamended AC-EW treatment reduced levels of cells to below the detection limit (0.7 logCFU/g) within 3 min.     

Combined effect of ultrasound and organic acids to reduce Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh lettuce

This study was performed to compare the effectiveness of individual treatments (ultrasound and organic acids) and their combination on reducing foodborne pathogens on organic fresh lettuce. Lettuce leaves were inoculated with a cocktail of three strains each of Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes and treated with ultrasound (40 kHz) alone, organic acids (0.3, 0.5, 0.7, 1.0, and 2.0% — malic acid, lactic acid, and citric acid) alone and combined with ultrasound and organic acids for 5 min. For all 3 pathogens, the combined treatment of ultrasound and organic acids resulted in additional 0.8 to 1.0 log reduction compared to individual treatments, without causing significant quality change (color and texture) on lettuce during 7 day storage. The maximum reductions of E. coli O157:H7, S. Typhimurium, and L. monocytogenes were 2.75, 3.18, and 2.87 log CFU/g observed after combined treatment with ultrasound and 2% organic acid for 5 min, respectively. Our results suggest that the combined treatment of ultrasound with organic acids was effective at increasing pathogen reduction compared to individual treatments without significantly affecting quality, and demonstrates its potential as a novel method to increase the microbial safety on organic fresh lettuce.     

Heat resistance of Listeria species to liquid whole egg ultrapasteurization treatment

The lethality of ultrapasteurization treatments (70 °C/1.5 min.) applied at constant temperature (isothermal condition) and at a constantly raising temperature of 2 °C/min (non-isothermal condition) in liquid whole egg (LWE) against two strains of Listeria monocytogenes (STCC 5672 and 4032) and one of Listeria innocua has been investigated. Isothermal survival curves up to 71 °C were obtained, which followed first-order inactivation kinetics. The obtained D_t values indicated that L. innocua was significantly (p < 0.05) more heat resistant than L. monocytogenes strains. Non-significant (p > 0.05) differences were observed among z values (12.4 ± 0.4 °C, 13.1 ± 0.4 °C and 12.2 ± 0.7 °C for L. innocua and L. monocytogenes 5672 and 4032, respectively). Based on obtained D_t and z values, isothermal ultrapasteurization treatment (70 °C/1.5 min.) would provide 3.5-, 5.0-, and 6.5-Log₁₀ cycles of L. innocua and L. monocytogenes 5672 and 4032, respectively. Non-isothermal heating lag phase increased the thermotolerance of Listeria species in LWE. The simulated industrial pasteurization treatment for LWE (heating-up phase from 25 to 70 °C followed by 1.5 min. at 70 °C) would attain 5-Log₁₀ reductions of L. monocytogenes 5672 and 4032, and 3.7-Log₁₀ reductions of L. innocua. Therefore, the safety level of industrial ultrapasteurization concerning L. monocytogenes could be lower than that estimated with data obtained under isothermal conditions.     

The effect of American cranberry (Vaccinium macrocarpon) constituents on the growth inhibition,membrane integrity,and injury of Escherichia coli O157:H7 and Listeria monocytogenes in comparison to Lactobacillus rhamnosus

The antimicrobial properties of the American cranberry were studied against Escherichia coli O157:H7, Listeria monocytogenes, and Lactobacillus rhamnosus to determine the effects on growth inhibition, membrane permeability, and injury. Cranberry powder was separated using a C-18 Sep-Pak cartridge into sugars plus organic acids (F1), monomeric phenolics (F2), and anthocyanins plus proanthocyanidins (F3). Fraction 3 was further separated into anthocyanins (F4) and proanthocyanidins (F5) using an LH-20 Sephadex column. Each fraction was diluted in the brain heart infusion (BHI) broth to determine the minimum inhibitory/bactericidal concentrations (MIC/MBC). L. monocytogenes was the most susceptible to cranberry fraction treatment with the lowest MIC/MBC for each treatment, followed by E. coli O157:H7 and L. rhamnosus. Membrane permeability and potential was studied using LIVE/DEAD viability assay and using Bis (1, 3-dibutylbarbituric acid) trimethine oxonol (DiBAC₄), respectively. L. rhamnosus demonstrated the highest permeability followed by E. coli O157:H7, and L. monocytogenes. L. rhamnosus demonstrated the highest recovery followed by E. coli O157:H7, and L. monocytogenes. Each cranberry fraction demonstrated membrane hyperpolarization at their native pH, while F2, F3, and F5 demonstrated membrane depolarization at neutral pH. With this knowledge cranberry compounds may be used to prevent maladies and potentially substitute for synthetic preservatives and antibiotics.     

Surface enhanced Raman scattering (SERS) with biopolymer encapsulated silver nanosubstrates for rapid detection of foodborne pathogens

A biopolymer encapsulated with silver nanoparticles was prepared using silver nitrate, polyvinyl alcohol (PVA) solution, and trisodium citrate. It was deposited on a mica sheet to use as SERS substrate. Fresh cultures of Salmonella Typhimurium, Escherichia coli, Staphylococcus aureus and Listeria innocua were washed from chicken rinse and suspended in 10 ml of sterile deionized water. Approximately 5 μl of the bacterial suspensions was placed on the substrate individually and exposed to 785 nm HeNe laser excitation. SERS spectral data were recorded over the Raman shift between 400 and 1800 cm^− 1 from 15 different spots on the substrate for each sample; and three replicates were done on each bacteria type. Principal component analysis (PCA) model was developed to classify foodborne bacteria types. PC1 identified 96% of the variation among the given bacteria specimen, and PC2 identified 3%, resulted in a total of 99% classification accuracy. Soft Independent Modeling of Class Analogies (SIMCA) of validation set gave an overall correct classification of 97%. Comparison of the SERS spectra of different types of gram-negative and gram-positive bacteria indicated that all of them have similar cell walls and cell membrane structures. Conversely, major differences were noted around the nucleic acid and amino acid structure information between 1200 cm^− 1 and 1700 cm^− 1 and at the finger print region between 400 cm^− 1 and 700 cm^− 1. Silver biopolymer nanoparticle substrate could be a promising SERS tool for pathogen detection. Also this study indicates that SERS technology could be used for reliable and rapid detection and classification of food borne pathogens.     

Mixed species biofilms of Listeria monocytogenes and Lactobacillus plantarum show enhanced resistance to benzalkonium chloride and peracetic acid

We investigated the formation of single and mixed species biofilms of Listeria monocytogenes strains EGD-e and LR-991, with Lactobacillus plantarum WCFS1 as secondary species, and their resistance to the disinfectants benzalkonium chloride and peracetic acid. Modulation of growth, biofilm formation, and biofilm composition was achieved by addition of manganese sulfate and/or glucose to the BHI medium. Composition analyses of the mixed species biofilms using plate counts and fluorescence microscopy with dual fluorophores showed that mixed species biofilms were formed in BHI (total count, 8-9 log₁₀ cfu/well) and that they contained 1-2 log₁₀ cfu/well more L. monocytogenes than L. plantarum cells. Addition of manganese sulfate resulted in equal numbers of both species (total count, 8 log₁₀ cfu/well) in the mixed species biofilm, while manganese sulfate in combination with glucose, resulted in 1-2 log₁₀ more L. plantarum than L. monocytogenes cells (total count, 9 log₁₀ cfu/well). Corresponding single species biofilms of L. monocytogenes and L. plantarum contained up to 9 log₁₀ cfu/well. Subsequent disinfection treatments showed mixed species biofilms to be more resistant to treatments with the selected disinfectants. In BHI with additional manganese sulfate, both L. monocytogenes strains and L. plantarum grown in the mixed species biofilm showed less than 2 log₁₀ cfu/well inactivation after exposure for 15 min to 100 μg/ml benzalkonium chloride, while single species biofilms of both L. monocytogenes strains showed 4.5 log₁₀ cfu/well inactivation and single species biofilms of L. plantarum showed 3.3 log₁₀ cfu/well inactivation. Our results indicate that L. monocytogenes and L. plantarum mixed species biofilms can be more resistant to disinfection treatments than single species biofilms.     

Effects of binary solvent extraction system,extraction time and extraction temperature on phenolic antioxidants and antioxidant capacity from mengkudu (Morinda citrifolia)

An investigation into the effects of ethanol concentration (0–100%, v/v), extraction time (20–120 min) and extraction temperature (25–65 °C) on the extraction of phenolic antioxidants from mengkudu (Morinda citrifolia) was performed using a single-factor experiment. Total phenolic content (TPC) and total flavonoid content (TFC) assays were used for determination of phenolic compounds. Antioxidant capacity was evaluated by measuring the scavenging effect on 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and 2,2′-diphenyl-1-picrylhydrazyl (DPPH) radicals. Experimental results showed that extraction conditions had significant effect on extraction of phenolic compounds and antioxidant capacities. The optimised conditions were 40% ethanol for 80 min at 65 °C, with values of 919.95 mg GAE/100 g DW for TPC, 472.73 mg CE/100 g DW for TFC, 791.71 μmol TEAC/100 g DW for ABTS and 1928.5 μmol TEAC/100 g DW for DPPH. TPC was significantly correlated with DPPH under the effects of ethanol concentration (r = 0.932) and extraction time (r = −0.938).     

Bactericidal activity of lauric arginate in milk and Queso Fresco cheese against Listeria monocytogenes cold growth

Lauric arginate (LAE) at concentrations of 200 ppm and 800 ppm was evaluated for its effectiveness in reducing cold growth of Listeria monocytogenes in whole milk, skim milk, and Queso Fresco cheese (QFC) at 4°C for 15 to 28 d. Use of 200 ppm of LAE reduced 4 log cfu/mL of L. monocytogenes to a nondetectable level within 30 min at 4°C in tryptic soy broth. In contrast, when 4 log cfu/mL of L. monocytogenes was inoculated in whole milk or skim milk, the reduction of L. monocytogenes was approximately 1 log cfu/mL after 24 h with 200 ppm of LAE. When 800 ppm of LAE was added to whole or skim milk, the initial 4 log cfu/mL of L. monocytogenes was nondetectable following 24 h, and no growth of L. monocytogenes was observed for 15 d at 4°C. With surface treatment of 200 or 800 ppm of LAE on vacuum-packaged QFC, the reductions of L. monocytogenes within 24 h at 4°C were 1.2 and 3.0 log cfu/g, respectively. In addition, the overall growth of L. monocytogenes in QFC was decreased by 0.3 to 2.6 and by 2.3 to 5.0 log cfu/g with 200 and 800 ppm of LAE, respectively, compared with untreated controls over 28 d at 4°C. Sensory tests revealed that consumers could not determine a difference between QFC samples that were treated with 0 and 200 ppm of LAE, the FDA-approved level of LAE use in foods. In addition, no differences existed between treatments with respect to flavor, texture, and overall acceptability of the QFC. Lauric arginate shows promise for potential use in QFC because it exerts initial bactericidal activity against L. monocytogenes at 4°C without affecting sensory quality.     

Fate of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella on fresh-cut celery

Illnesses from Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella have been associated with the consumption of numerous produce items. Little is known about the effect of consumer handling practices on the fate of these pathogens on celery. The objective of this study was to determine pathogen behavior at different temperatures under different storage conditions. Commercial fresh-cut celery was inoculated at ca. 3 log CFU/g onto either freshly cut or outer uncut surfaces and stored in either sealed polyethylene bags or closed containers. Samples were enumerated following storage for 0, 1, 3, 5, and 7 days when held at 4 °C or 12 °C, and after 0, 8, and 17 h, and 1, and 2 days when held at 22 °C. At 4 °C, all populations declined by 0.5–1.0 log CFU/g over 7 days. At 12 °C, E. coli O157:H7 and Salmonella populations did not change, while L. monocytogenes populations increased by ca. 0.5 log CFU/g over 7 days. At 22 °C, E. coli O157:H7, Salmonella, and L. monocytogenes populations increased by ca. 1, 2, or 0.3 log CFU/g, respectively, with the majority of growth occurring during the first 17 h. On occasion, populations on cut surfaces were significantly higher than those on uncut surfaces. Results indicate that populations are reduced under refrigeration, but survive and may grow at elevated temperatures.     

A comparison between E-beam irradiation and high pressure treatment for cold-smoked salmon sanitation: microbiological aspects

The effectiveness of electron beam irradiation and high pressure treatment for the sanitation of cold-smoked salmon from two points of view, microbial safety and shelf-life extension, was compared. From the response of L. monocytogenes INIA H66a to irradiation, a D value of 0.51 kGy was calculated. For samples stored at 5 °C, 1.5 kGy would be sufficient to attain a Food Safety Objective (FSO) of 2 log₁₀cfu/g L. monocytogenes for a 35-day shelf-life, whereas 3 kGy would be needed in the case of a temperature abuse (5 °C + 8 °C). Pressurization at 450 MPa for 5 min was considered to be an insufficient treatment, since the FSO of 2 log₁₀cfu/g L. monocytogenes was only attained for a shelf-life of 21 days at 5 °C. However, treatment at 450 MPa for 10 min achieved this FSO for samples held during 35 days at 5 °C, or during 21 days under temperature abuse (5 °C + 8 °C) conditions. Irradiation at 2 kGy kept the microbial population of smoked salmon below 6 log₁₀cfu/g after 35 days at 5 °C, with negligible or very light changes in its odor. Pressurization at 450 MPa for 5 min also kept the microbial population below 6 log₁₀cfu/g after 35 days at 5 °C and did not alter odor, but affected negatively the visual aspect of smoked salmon.     

Strain dependent expression of stress response and virulence genes of Listeria monocytogenes in meat juices as determined by microarray

A subgenomic array, encompassing 54 probes targeting genes responsible for virulence, adhesion and stress response in Listeria monocytogenes, was used in order to study their expression in food systems. RNA extracted from L. monocytogenes inoculated in BHI and in situ (i.e. in minced meat and fermented sausage juices) and incubated at 4 °C, was hybridized on the array and the results obtained were compared in order to understand the effect that the food juice has on the expression. Three different strains of L. monocytogenes were tested, in order to determine the effect of the strain provenience. As determined by cluster analysis, each strain behaved in a different way when inoculated in food juices. The goal was to respond to acidic and osmotic stresses encountered in the food, particularly in the fermented sausage juice. No differences in the expression profile between the three strains were observed, when they were inoculated in BHI. On the other hand, in the meat and sausage juices, the iap, gadC and gadE genes, together with different internalin encoding genes, were significantly differentially expressed in the three strains.