The probability of growth of Listeria monocytogenes in cooked salmon and tryptic soy broth as affected by salt,smoke compound,and storage temperature

The objectives of this study were to examine and model the probability of growth of Listeria monocytogenes in cooked salmon containing salt and smoke (phenol) compound and stored at various temperatures. A growth probability model was developed, and the model was compared to a model developed from tryptic soy broth (TSB) to assess the possibility of using TSB as a substitute for salmon. A 6-strain mixture of L. monocytogenes was inoculated into minced cooked salmon and TSB containing 0–10% NaCl and 0–34 ppm phenol to levels of 10^2–3 cfu/g, and the samples were vacuum-packed and stored at 0-–25 °C for up to 42 days. A total 32 treatments, each with 16 samples, selected by central composite designs were tested. A logistic regression was used to model the probability of growth of L. monocytogenes as a function of concentrations of salt and phenol, and storage temperature. Resulted models showed that the probabilities of growth of L. monocytogenes in both salmon and TSB decreased when the salt and/or phenol concentrations increased, and at lower storage temperatures. In general, the growth probabilities of L. monocytogenes were affected more profoundly by salt and storage temperature than by phenol. The growth probabilities of L. monocytogenes estimated by the TSB model were higher than those by the salmon model at the same salt/phenol concentrations and storage temperatures. The growth probabilities predicted by the salmon and TSB models were comparable at higher storage temperatures, indicating the potential use of TSB as a model system to substitute salmon in studying the growth behavior of L. monocytogenes may only be suitable when the temperatures of interest are in higher storage temperatures (e.g., >12 °C). The model for salmon demonstrated the effects of salt, phenol, and storage temperature and their interactions on the growth probabilities of L. monocytogenes, and may be used to determine the growth probability of L. monocytogenes in smoked seafood.     

Listeria monocytogenes survival of UV-C radiation is enhanced by presence of sodium chloride, organic food material and by bacterial biofilm formation

The bactericidal effect on food processing surfaces of ceiling-mounted UV-C light (wavelength 254 nm) was determined in a fish smoke house after the routine cleaning and disinfection procedure. The total aerobic counts were reduced during UV-C light exposure (48 h) and the number of Listeria monocytogenes positive samples went from 30 (of 68) before exposure to 8 (of 68). We therefore in a laboratory model determined the L. monocytogenes reduction kinetics by UV-C light with the purpose of evaluating the influence of food production environmental variables, such as presence of NaCl, organic material and the time L. monocytogenes was allowed to adhere to steel before exposure. L. monocytogenes grown and attached in tryptone soy broth (TSB) with glucose were rapidly killed (after 2 min) by UV-C light. However, bacteria grown and adhered in TSB with glucose and 5% NaCl were more resistant and numbers declined with 4-5 log units during exposure of 8-10 min. Bacteria grown in juice prepared from cold-smoked salmon were protected and numbers were reduced with 2-3 log when UV-C light was used immediately after attachment whereas numbers did not change at all if bacteria had been allowed to form a biofilm for 7 days before exposure. It is not known if this enhanced survival is due to physiological changes in the attached bacterial cells, a physical protection of the cells in the food matrix or a combination. In conclusion, we demonstrate that UV-C light is a useful extra bacteriocidal step and that it, as all disinfecting procedures, is hampered by the presence of organic material.     

Inactivation of Escherichia coli O157:H7 on stainless steel upon exposure to Paenibacillus polymyxa biofilms

We investigated the potential use of biofilm formed by a competitive-exclusion (CE) microorganism to inactivate Escherichia coli O157:H7 on a stainless steel surface. Five microorganisms showing inhibitory activities against E. coli O157:H7 were isolated from vegetable seeds and sprouts. The microorganism with the greatest antimicrobial activity was identified as Paenibacillus polymyxa (strain T5). In tryptic soy broth (TSB), strain T5 reached a higher population at 25 °C than at 12 or 37 °C without losing inhibitory activity against E. coli O157:H7. When P. polymyxa (6 log CFU/mL) was co-cultured with E. coli O157:H7 (2, 3, 4, or 5 log CFU/mL) in TSB at 25 °C, the number of E. coli O157:H7 decreased significantly within 24 h. P. polymyxa formed a biofilm on stainless steel coupons (SSCs) in TSB at 25 °C within 24 h, and cells in biofilms, compared to attached cells without biofilm formation, showed significantly increased resistance to a dry environment (43% relative humidity [RH]). With the exception of an inoculum of 4 log CFU/coupon at 100% RH, upon exposure to biofilm formed by P. polymyxa on SSCs, populations of E. coli O157:H7 (2, 4, or 6 log CFU/coupon) were significantly reduced within 48 h. Most notably, when E. coli O157:H7 at 2 log CFU/coupon was applied to SSCs on which P. polymyxa biofilm had formed, it was inactivated within 1 h, regardless of RH. These results will be useful when developing strategies using biofilms produced by competitive exclusion microorganisms to inactivate foodborne pathogens in food processing environments.     

Resistance of pathogenic bacteria on the surface of stainless steel depending on attachment form and efficacy of chemical sanitizers

Various bacteria including food spoilage bacteria and pathogens can form biofilms on different food processing surfaces, leading to potential food contamination or spoilage. Therefore, the survival of foodborne pathogens (Escherichia coli O157:H7, Listeria monocytogenes, Salmonella typhimurium, Staphylococcus aureus, Cronobacter sakazakii) in different forms (adhered cells, biofilm producing in TSB, biofilm producing at RH 100%) on the surface of stainless steel and stored at various relative humidities (RH 23%, 43%, 68%, 85%, and 100%) at room temperature for 5 days was investigated in this study. Additionally, the efficacy of chemical sanitizers (chlorine-based and alcohol-based commercial sanitizers) on inhibiting various types of biofilms of E. coli O157:H7 and S. aureus on the surface of stainless steel was investigated. The number of pathogens on the surface of stainless steel in TSB stored at 25 °C for 7 days or RH 100% at 25 °C for 7 days was significantly increased and resulted in the increase of 3 log₁₀ CFU/coupon after 1 day, and these levels were maintained for 7 days. When stainless steel coupons were stored at 25 °C for 5 days, the number of pathogens on the surface of stainless steel was significantly reduced after storage at RH 23%, 43%, 68%, and 85%, but not at 100%. When the bacteria formed biofilms on the surface of stainless steel in TSB after 6 days, the results were similar to those of the attached form. However, levels of S. aureus and C. sakazakii biofilms were more slowly reduced after storage at RH 23%, 43%, 68%, and 85% for 5 days than were those of the other pathogens. Formation of biofilms stored at RH 100% for 5 days displayed the highest levels of resistance to inactivation. Treatment with the alcohol sanitizer was very effective at inactivating attached pathogens or biofilms on the surface of stainless steel. Reduction levels of alcohol sanitizer treatment ranged from 1.91 to 4.77 log and from 4.35 to 5.35 log CFU/coupon in E. coli O157:H7 and S. aureus, respectively. From these results, the survival of pathogens contaminating the surfaces of food processing substrates such as stainless steel varied depending on RH and attachment form. Also, alcohol-based sanitizers can be used as a potential method to remove microbial contamination on the surfaces of utensils, cooking equipment, and other related substrates regardless of the microbial attached form.     

Combined effect of enterocin AS-48 and high hydrostatic pressure to control food-borne pathogens inoculated in low acid fermented sausages

The single and combined effects of enterocin AS-48 and high hydrostatic pressure (HHP) on Listeria monocytogenes, Salmonellaenterica, and Staphylococcus aureus was investigated in fuet (a low acid fermented sausage) during ripening and storage at 7 °C or at room temperature. AS-48 (148 AU g⁻¹) caused a drastic 5.5 log cfu g⁻¹ decrease in L. monocytogenes (P < 0.001) and a significant (P < 0.01) inhibition (1.79 logs) for Salmonella at the end of ripening (10 d). After pressurization (400 MPa) and storage Listeria counts remained below 5 cfu g⁻¹ in all fuets containing AS-48 (pressurized or not). HHP alone had no anti-Listeria effect. HHP treatment significantly reduced Salmonella counts, with lowest levels in pressurized fuets with AS-48. S. aureus showed similar growth for all treatments and storage conditions. These results indicate that AS-48 can be applied alone to control L. monocytogenes and combined with HHP treatment to control Salmonella in fuets.     

The effect of sodium reduction with and without potassium chloride on the survival of Listeria monocytogenes in Cheddar cheese

Sodium chloride (NaCl) in cheese contributes to flavor and texture directly and by its effect on microbial and enzymatic activity. The salt-to-moisture ratio (S/M) is used to gauge if conditions for producing good-quality cheese have been met. Reductions in salt that deviate from the ideal S/M range could result in changing culture acidification profiles during cheese making. Lactococcus lactis ssp. lactis or Lc. lactis ssp. cremoris are both used as cultures in Cheddar cheese manufacture, but Lc. lactis ssp. lactis has a higher salt and pH tolerance than Lc. lactis ssp. cremoris. Both salt and pH are used to control growth and survival of Listeria monocytogenes and salts such as KCl are commonly used to replace the effects of NaCl in food when NaCl is reduced. The objectives of this project were to determine the effects of sodium reduction, KCl use, and the subspecies of Lc. lactis used on L. monocytogenes survival in stirred-curd Cheddar cheese. Cheese was manufactured with either Lc. lactis ssp. lactis or Lc. lactis ssp. cremoris. At the salting step, curd was divided and salted with a concentration targeted to produce a final cheese with 600 mg of sodium/100 g (control), 25% reduced sodium (450 mg of sodium/100 g; both with and without KCl), and low sodium (53% sodium reduction or 280 mg of sodium/100 g; both with and without KCl). Potassium chloride was added on a molar equivalent to the NaCl it replaced to maintain an equivalent S/M. Cheese was inoculated with a 5-strain cocktail of L. monocytogenes at different times during aging to simulate postprocessing contamination, and counts were monitored over 27 or 50 d, depending on incubation temperature (12 or 5°C, respectively). In cheese inoculated with 4 log₁₀ cfu of L. monocytogenes/g 2 wk after manufacture, viable counts declined by more than 3 log₁₀ cfu/g in all treatments over 60 d. When inoculated with 5 log₁₀ cfu/g at 3 mo of cheese age, L. monocytogenes counts in Cheddar cheese were also reduced during storage, but by less than 1.5 log₁₀ cfu/g after 50 d. However, cheese with a 50% reduction in sodium without KCl had higher counts than full-sodium cheese at the end of 50 d of incubation at 4°C when inoculated at 3 mo. When inoculated at 8 mo postmanufacture, this trend was only observed in 50% reduced sodium with KCl, for cheese manufactured with both cultures. This enhanced survival for 50% reduced-sodium cheese was not seen when a higher incubation temperature (12°C) was used when cheese was inoculated at 3 mo of age and monitored for 27 d (no difference in treatments was observed at this incubation temperature). In the event of postprocessing contamination during later stages of ripening, L. monocytogenes was capable of survival in Cheddar cheese regardless of which culture was used, whether or not sodium had been reduced by as much as 50% from standard concentrations, or if KCl had been added to maintain the effective S/M of full-sodium Cheddar cheese.     

Control of Growth of L. monocytogenes in Fresh Salmon using Microgard and Nisin

Pathogens like Listeria monocytogenes in fresh, or even in cold smoked salmon, have become a major public health concern for the salmon processing industry and government agencies. The effect of bacteriocin solutions (Microgard^™ and Nisin) on reducing total microbial counts, inhibiting Listeria monocytogenes, and prolonging the shelf-life was evaluated. Listeria monocytogenes was inoculated onto chilled and on frozen and thawed salmon samples. The combination of Nisin and Microgard reduced the total aerobic bacteria populations of fresh chilled salmon by 2 log (P<0.05) and increased its shelf-life, at 6 °C, by 3-4 d, as compared with the control. The above bacteriocin combination also reduced the growth of inoculated Listeria monocytogenes in frozen-thawed salmon and increased its shelf-life from 5 to 10 d at 6 °C. The bacteriocin treatment did not affect surface pH values or color of the fish.     

Use of titanium dioxide (TiO2) photocatalysts as alternative means for Listeria monocytogenes biofilm disinfection in food processing

The aim of this work was to study the photocatalytic activity of titanium dioxide (TiO₂) against Listeria monocytogenes bacterial biofilm. Different TiO₂ nanostructured thin films were deposited on surfaces such as stainless steel and glass using the doctor-blade technique. All the surfaces were placed in test tubes containing Brain Heart (BH) broth and inoculated with L. monocytogenes. Test tubes were then incubated for 10 days at 16 °C in order to allow biofilm development. After biofilm formation, the surfaces were illuminated by ultraviolet A light (UVA; wavelength of 315-400 nm). The quantification of biofilms was performed using the bead vortexing method, followed by agar plating and/or by conductance measurements (via the metabolic activity of biofilm cells). The presence of the TiO₂ nanoparticles resulted in a fastest log-reduction of bacterial biofilm compared to the control test. The biofilm of L. monocytogenes for the glass nanoparticle 1 (glass surface modified by 16% w/v TiO₂) was found to have decreased by 3 log CFU/cm² after 90 min irradiation by UVA. The use of TiO₂ nanostructured photocatalysts as alternative means of disinfecting contaminated surfaces presents an intriguing case, which by further development may provide potent disinfecting solutions. Surface modification using nanostructured titania and UV irradiation is an innovative combination to enhance food safety and economizing time and money.     

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.     

A predictive model for heat inactivation of Listeria monocytogenes biofilm on buna-N rubber

The purpose of this study was to develop a predictive model for the heat inactivation of Listeria monocytogenes in monoculture (strains Scott A and 3990) and with competing bacteria (Pseudomonas sp. and Pantoea agglomerans) formed on buna-N rubber with and without the presence of food-derived soil. Biofilms were produced on rubber disks in dilute Tryptic Soy broth (dTSB) with incubation for 48 h at 25 °C. Duplicate biofilm samples were heat treated for 1, 3, 5, and 15 min at 70, 72, 75, 77 and 80 °C and tested for survivors using enrichment media. The experiment was repeated six times. A predictive model was developed and plots were generated showing the percent probability of L. monocytogenes inactivation in biofilms after heat treatment. For example, to achieve a 95% probability level of complete inactivation required heat treatment of 76 °C for 6 min. The predicted model was validated using a five-strain cocktail of L. monocytogenes. The validated prediction model indicates that with proper maintenance of the time/temperature controls L. monocytogenes in biofilms on rubber surfaces will be inactivated. This model can be used as a tool in the selection of hot water sanitation processes for rubber surfaces.     

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.     

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.     

High-pressure processing and water-holding capacity of fresh and cold-smoked salmon (Salmo salar)

The influence of high pressure on the water-holding capacity (WHC) of fresh and cold-smoked salmon (CSS) was investigated up to a pressure level of 200 MPa at room temperature for 10- and 20-min periods. Changes in moisture content and WHC were determined by two methods, namely filter paper and spin-spin relaxation proton nuclear magnetic resonance. Both pressure (p<0.05) and process time (p<0.05) had significant effects on the moisture content of CSS, but not on fresh Atlantic salmon. Fresh salmon had less WHC than smoked salmon and a pressure of 150 MPa for 10 min caused a 2% increase in WHC of smoked salmon (p<0.001). The spin-spin proton relaxation (T₂) values were heavily affected by high pressure in both the samples, with substantial effects seen in CSS. At 150 MPa, both fresh and smoked fish behaved differently with respect to T₂ values compared with other pressure levels used.     

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.     

Loss of viability of Listeria monocytogenes in contaminated processed cheese during storage at 4, 12 and 22 °C

The behaviour of Listeria monocytogenes in a processed cheese product was evaluated over time by inoculating the product with three different L. monocytogenes strains (Scott A, CA and a strain isolated from processed cheese) at three different inoculation levels (ca. 6 × 10⁵, ca. 6 × 10³ and 10² CFU/g of cheese or less) and after storage of the contaminated products at 4, 12 or 22 °C. Growth of L. monocytogenes was not observed in any of the experimental trials (experiments involving different combinations of strain, inoculum level and storage temperature) throughout the storage period. L. monocytogenes populations decreased over time with a rate that was strain- and storage temperature-dependent. Nonetheless, for cheeses that had been inoculated with the higher inoculum and stored at 4 °C viable populations of L. monocytogenes could be detected for up to nine months post-inoculation. The L. monocytogenes survival curves obtained from the different trials were characterised by a post-inoculation phase during which the populations remained essentially unchanged (lag phase) followed by a phase of logarithmic decline. The duration of the lag phase and the rate of inactivation of L. monocytogenes in the different trials were estimated based on data from the linear descending portions of the survival curves. In addition, a non-linear Weibull-type equation was fitted to the data from each survival curve with satisfactory results. The results of the present study emphasize that, according to the definition laid down in the European Union Regulation 1441/2007, the processed cheese product tested in this work should be considered and classified as one that does not support the growth of L. monocytogenes under reasonable foreseeable conditions of distribution and storage. However, post-processing contamination of the product should be austerely avoided as the pathogen can survive in the product for extended periods of time, particularly under refrigerated storage (4 °C).     

Habituation to organic acid anions induces resistance to acid and bile in Listeria monocytogenes

We evaluated the intrinsic and inducible resistance of four human pathogenic strains of Listeria monocytogenes to acid and bile, factors associated with virulence. Cells were grown in media at pH 7.4, or in media at pH 6.0 containing 0 (HCl control) or 4.75 mM of different organic acids, harvested at stationary or mid log phase, and challenged for 1 h in acid or bile. Stationary phase cells were intrinsically more resistant to either challenge than log phase cells, and large differences between strains were evident among the latter. Compared to the HCl control, habituation to log phase with organic acids induced significant (p < 0.05) and meaningful (≥ 1 log) increases in acid resistance of three of four strains tested, and in bile resistance of two strains suggesting that exposure to organic acid anions may enhance virulence in L. monocytogenes.     

Effect of gamma and electron beam irradiation on the survival of pathogens inoculated into salted, seasoned, and fermented oyster

The objective of this study was to identify the efficacy of gamma and electron beam irradiation of the food-borne pathogens including 3-strain cocktail of Listeria monocytogenes (ATCC 19114, 19115, and 19111), Staphylococcus aureus (ATCC 6538, 25923, and 29213), and Vibrio parahaemolyticus (ATCC 17802, 33844, and 27969) in salted, seasoned, and fermented oyster (oyster Jeotkal, 8% salt), commercially available in the market. Irradiation (0, 0.5, 1, 2, and 5 kGy) significantly reduced the initial microbial level not only immediately after irradiation but also during storage at 10 °C for 4 weeks (P ≤ 0.05). No viable cell was detected at 5 kGy of irradiation at a detection limit of 10¹ CFU/g. Gamma irradiation was more effective than electron beam irradiation, and yielded D₁₀ values of 0.60, 0.71, and 0.29 kGy for L. monocytogenes, S. aureus, and V. parahaemolyticus, and those of electron beam irradiation were 0.69, 0.94, and 0.29 kGy, respectively. V. parahaemolyticus was most sensitive to irradiation and storage among all pathogens tested. Sensory quality was not affected by irradiation treatment. Results suggest that a low dose irradiation can improve the microbial quality and reduce the risk by the food-borne pathogens of oyster Jeotkal, which has limited alternative sterilization methods due to the temperature sensitivity of food products.     

SigB plays a major role in Listeria monocytogenes tolerance to bile stress

The ability of Listeria monocytogenes to tolerate high levels of bile stress is critical to its successful infection and colonization in the human gastrointestinal tract. L. monocytogenes encodes bile salt hydrolase by a bsh gene which plays a significant role in hydrolyzing high concentrations of bile salt when L. monocytogenes grows under hypoxemic condition. As the bsh promoter contains consensus SigB and PrfA binding sites, we investigated the role of SigB (σ^B) and PrfA in L. monocytogenes tolerance against bile stress by comparing the survival of isogenic deletion mutants of L. monocytogenes EGD_ΔsigB, EGD_ΔprfA and EGD_ΔprfAΔsigB with their parent strain EGD at high levels of bile salt. Our results show that the sigB deletion significantly reduced the MICs of bile salt for EGD_ΔsigB and EGD_ΔprfAΔsigB (2.6% and 2.2% vs 3.5% in wild type strain EGD), while the growth rates of these two sigB deletion mutants (EGD_ΔsigB and EGD_ΔprfAΔsigB) were affected the most in the presence of 3% bile salt. Pre-exposure to alkali (pH 9.0) and osmotic (0.3 M NaCl) stresses for a short period of time (30 min) resulted in improved growth of L. monocytogenes as well as its prfA-sigB isogenic mutants even under sublethal concentrations of bile salt, while pre-exposure to acid pH (pH 4.5) failed to provide cross-protection against subsequent bile stress. Furthermore, the sigB gene had more remarkable influence than that of prfA on bsh expression, as much lower levels of bsh transciption were observed in EGD_ΔsigB and EGD_ΔprfAΔsigB. Meanwhile, bsh expression in the deletion mutants did not respond to elevated levels of bile salt. These data indicate that σ^B might play a crucial role in Listeria survival under bile salt environment in the gastrointestinal tract before its successful colonization, invasion and intracellular propagation.     

The effect of high hydrostatic pressure,sodium nitrite and salt concentration on the growth of Listeria monocytogenes on RTE ham and turkey

Growth of Listeria monocytogenes was evaluated for up to 182 days after inoculation on ready-to-eat (RTE) sliced ham and turkey breast formulated with sodium nitrite (0 or 200 ppm), sodium chloride (1.8% or 2.4%), and treated (no treatment or 600 MPa) with high hydrostatic pressure (HHP). HHP at 600 MPa for 3 min resulted in a 3.85–4.35 log CFU/g reduction in L. monocytogenes. With formulations at similar proximate analyses, one of the evaluation days (day 21) without HHP showed significantly greater growth of L. monocytogenes in ham than in turkey breast, but there were no significant differences on other evaluation days or with HHP. There were no differences in growth of L. monocytogenes due to sodium chloride level. Sodium nitrite provided a small, but significant inhibition of L. monocytogenes without HHP, but addition of sodium nitrite did not significantly affect growth of L. monocytogenes with use of HHP.