Determination of Listeria monocytogenes growth potential on new fresh salmon preparations

The aim of this work was to evaluate the increase of Listeria monocytogenes on new salmon preparations (salt–sugar–pepper–dill salmon) in comparison with that obtained on cold-smoked salmon. Salmon preparations were inoculated with L. monocytogenes and were analyzed during storage at 4 °C then 8 °C. At 8 °C, the bacteria growth was of 4.53 log CFU g⁻¹ in cold-smoked salmon and of 2.06 log CFU g⁻¹ in salt–sugar–pepper–dill salmon without background microflora. The growth of L. monocytogenes was different in new salmon preparation because the mixture salt–sugar–pepper had an anti-Listeria activity and its presence could inhibitory to the growth. It is difficult to generalize findings observed with cold-smoked salmon to a new salmon preparation.     

Influence of Listeria innocua on the growth of Listeria monocytogenes

The growth of Listeria monocytogenes and Listeria innocua strains was monitored during this study: (i) in TSB–YE media and (ii) in a food matrix (pasteurized milk) according to the ISO 11290-1 methodology. Different inocula concentrations and mixtures were tested. The response was shown to be strain dependent. In TSB–YE the inhibition of a L. monocytogenes strain was observed in just one of the three mixtures (L. monocytogenes_1340 with L. innocua_11288) showing a reduction of 1.37 log cfu/ml after 42.5 h and 1.85 log after 66.5 h of incubation. In pasteurized milk the inhibition of L. monocytogenes by L. innocua was always observed when L. innocua was present in higher concentrations than L. monocytogenes. The reverse was also observed but only in one mixture (cocktail of six L. monocytogenes with L. innocua_2030c) when the initial concentration of L. monocytogenes was 100 times higher than L. innocua suggesting the phenomenon of quorum sensing. Furthermore, inhibitory activity was not caused by bacteriocins, and no correlation between the growth rate and inhibition was demonstrated.     

Effect of ozone and ultraviolet light on Listeria monocytogenes populations in fresh and spent chill brines

The efficacy of ozone and ultraviolet light (UV) treatment as hurdles against Listeria monocytogenes suspended in fresh (9% NaCl, 91.86% transmittance) and spent brines (20.5% NaCl, 0.01% transmittance) was evaluated. Brines were inoculated with a cocktail of L. monocytogenes-strains N1-227, N3-031, and R2-499. Ozonation was performed by sparging gaseous ozone into brine. This was followed by UV irradiation (253.7 nm) of the brine in sterile quartz cuvettes. Enumeration was performed by spread plating on modified Oxford medium and Trypticase Soy agar supplemented with yeast extract. In fresh brines containing L. monocytogenes, 10 min of ozonation lead to a 7.44 ± 0.13 log CFU/ml mean reduction and 10 min of UV radiation caused a 1.95 ± 0.41 log CFU/ml mean reduction. Sequential exposure of 10 min of ozonation and UV resulted in >9 log CFU/ml reduction in L. monocytogenes populations in fresh brine. Sixty minutes of ozonation of spent brines resulted in a 4.85 ± 0.61 log CFU/ml mean reduction of L. monocytogenes populations. Ten minutes of UV exposure in spent brines resulted in 0.49 ± 0.14 log CFU/ml mean reduction in L. monocytogenes. A sequential treatment of 60 min ozonation and 10 min UV resulted in an excess of 5 log CFU/ml reduction in L. monocytogenes cells in spent brine. Ozonation did not cause a significant increase in the transmittance of the spent brine to aid UV penetration but resulted in color change. Sequential treatments of Ozonation and UV maybe effective in reducing L. monocytogenes in chill brines.     

Influence of phenolic compounds from wines on the growth of Listeria monocytogenes

The anti-microbial properties against Listeria monocytogenes of pure flavonoids rutin, catechin and quercetin; non-flavonoids gallic, vanillic, protocatechuic and caffeic acids and total polyphenols of three Argentinean wines, Cabernet Sauvignon, Malbec and Merlot varieties were investigated. The non-flavonoid caffeic acid and the flavonoids rutin and quercetin were the compounds with higher inhibitory activities on L. monocytogenes growth. The knowledge of the anti-listerial effect of different wines varieties could be the basis to demonstrate if the wine consumption with a meal may collaborate in the health protection against some foodborne organisms such as L. monocytogenes.     

The role of the consumer in the reduction of Listeria monocytogenes in lettuces by washing at home

Lettuce is highly appreciated for its nutritional properties; however microbial contamination through the food chain and its raw consumption may jeopardize these known benefits to the diet. The objective of this study was to determine the role of the consumer at the stage of washing at home, in relation to the probability of illness due to the presence of Listeria monocytogenes in lettuce. Survival curves of L. monocytogenes after washing (dipping with and without addition of bleach, and washing under a running tap) were studied. A mathematical model for each washing method was calculated by fitting experimental data. The obtained models were used to estimate the probability of illness after washing at home. Results show that although consumers can only deal with low loads of L. monocytogenes, their role is essential to reduce the normal contamination level of lettuces and ensure their safety.     

Occurrence of Listeria spp. in Brazilian fresh sausage and control of Listeria monocytogenes using bacteriophage P100

Since the 1980s, an increase in outbreaks of human listeriosis linked to contaminated food has been a concern of health authorities. Intensively manipulated foods, such as Brazilian fresh sausage, are frequently responsible for food-borne diseases. In this work the occurrence of Listeria monocytogenes and the efficacy of bacteriophage P100 (LISTEX?) to control the microorganism was evaluated in Brazilian fresh sausage. Eighty samples were analyzed, 40 each of swine and chicken Brazilian fresh sausage. Listeria spp. were isolated from 12 samples (15%), of which three (3.75%) were positive for L. monocytogenes. L. monocytogenes strains isolated belonged to serotype 1/2a. L. monocytogenes 1/2a was inoculated in Brazilian fresh sausage (2.1 × 10⁴ cfu/g) with the bacteriophage added thereafter (3.0 × 10⁷ pfu/g). Samples were analysed immediately (day zero) and then stored at 4 °C for 10 days. The bacteriophage P100 reduced L. monocytogenes counts by 2.5 log units at both 0 and 10 days compared to controls without bacteriophage. In spite of this, the populations of L. moncytogenes increased over the 10 day storage. Our data demonstrate that in one of the samples the use of the bacteriophage dropped the bacteria count below the level of direct detection. This study demonstrates a new alternative for pathogen control in the food industry, especially in the processes used to produce Brazilian fresh sausage.     

Efficacy of propionic acid against Listeria monocytogenes attached to poultry skin during refrigerated storage

This work evaluates the effect of propionic acid dip on the growth of Listeria monocytogenes on poultry legs stored at 4 °C for 8 days.Fresh inoculated chicken legs were dipped into either 1% or 2% propionic acid solution (v/v) or distilled water (control). Changes in mesophiles, enterobacteriaceae counts and sensorial characteristics (odour, colour and overall appearance) were also evaluated.The shelf life of the samples washed with propionic acid was extended by at least 2 days over the control samples washed with distilled water. Legs washed with 2% propionic acid showed a significant (p < 0.05) inhibitory effect on L. monocytogenes compared to control legs, with a decrease of about 2.72 log units after 3 days of storage. Sensory quality was not adversely affected by propionic acid.This study demostrates that while propionic acid did reduce growth of L. monocytogenes on meat, it did not completely inactivate the pathogen.     

Effect of the competitive growth of Lactobacillus sakei MN on the growth kinetics of Listeria monocytogenes Scott A in model meat gravy

Listeria monocytogenes is a foodborne pathogen capable of growth under refrigeration temperatures. The use of bacteriocin-producing lactic acid bacteria to inhibit Gram-positive pathogen growth may be an important tool to enhance the safety of refrigerated foods. The influence of three different populations of the bacteriocin-producing strain Lactobacillus sakei MN on the growth kinetic parameters of three different populations of L. monocytogenes Scott A co-cultured in model meat gravy at 4, 10, 16, and 22 °C was studied. The Baranyi growth model was used to estimate the kinetic parameters of L. monocytogenes and L. sakei for each strain cultured alone or in co-culture. The highest L. monocytogenes populations were achieved by pure cultures, decreasing in co-culture with the different inocula of L. sakei, at all temperatures. A modified logistic model was applied which includes a factor β that adjusts the effect of L. sakei on L. monocytogenes depending on the environmental conditions. The co-cultures of low (∼1log) L. monocytogenes inocula showed a decrease in β values when temperature increased, indicating that inter-species competition changes with temperature; the 2log- and 4log-inocula of L. monocytogenes co-cultures also showed this behavior but only with the higher initial population of L. sakei.     

Combined effect of carvacrol and citral on the growth of Listeria monocytogenes and Listeria innocua and on the occurrence of damaged cells

The aim of this study was to evaluate the growth kinetics of Listeria innocua Serovar 6a (CECT 910) and Listeria monocytogenes Serovar 4b (CECT 4032) exposed to combinations of carvacrol and citral (0.0 μL/mL (control), 0.050 μL/mL of carvacrol and 0.075 μL/mL of citral, 0.050 μL/mL of carvacrol and 0.125 μL/mL of citral, 0.085 μL/mL of carvacrol and 0.075 μL/mL of citral, and 0.085 μL/mL of carvacrol and 0.125 μL/mL of citral), with two initial inoculum concentrations, and also the occurrence of sublethal damage in these cell populations. The terpene combinations exhibited antibacterial activity against L. innocua and L. monocytogenes and the effects were dependent on the concentration of terpenes present in the culture medium (p ≤ 0.05). When terpene-treated L. innocua and L. monocytogenes were incubated in TSB, significant differences in lag phase and growth rate were observed between low and high inoculum concentrations (p ≤ 0.05), indicating that the inoculum level should be taken into account in modeling studies. When bacterial cells were exposed to terpenes the proportion of sublethally injured cells increased with the increase in the terpene dose (p ≤ 0.05). In conclusion, all of these results show that carvacrol and citral can be used in combination at 25% of the MIC in order to control Listeria growth.     

Survival and growth of Listeria monocytogenes in lubricants used in the food industry

The survival and growth of three Listeria monocytogenes strains in 10 lubricants (synthetic and mineral-oil based) used in the food industry, and rapeseed oil, was investigated at room temperature (20 °C) and refrigerated (5 °C). Additionally, the transfer of L. monocytogenes from lubricants to stainless steel surfaces and vice versa was investigated. Though the amount of L. monocytogenes in most lubricants, both pure and soiled, decreased significantly (p < 0.05) during the 14 d test period, lubricants may act as sources of contamination on the basis of the results obtained on the survival of L. monocytogenes. In general, temperature had significant effect (p < 0.05) on listericidal effect of lubricants contrary to soiling (p > 0.05), however the effect of both factors was dependent on lubricant (p < 0.05). The results clearly showed that L. monocytogenes survived in synthetic conveyer belt lubricant diluted in water. In addition, L. monocytogenes was transferred significantly (p < 0.05) from stainless steel surfaces into conveyer-belt lubricants and into mineral-oil based hydraulic oil.     

Reduction of Listeria monocytogenes contamination on produce – A quantitative analysis of common liquid fresh produce wash compounds

Contaminated produce has been identified as the cause of several listeriosis outbreaks in recent years. Listeria monocytogenes is widely distributed in the environment and complete prevention of produce contamination is therefore challenging. Mitigation options that reduce contamination on produce are valuable, especially for produce commodities that are commonly consumed fresh or minimally processed. We performed a systematic review and meta-analysis of the available peer-reviewed literature to evaluate the efficacy of liquid fresh produce wash compounds in reducing produce contamination with L. monocytogenes, and derive quantitative estimates of treatment efficacy for a variety of common liquid fresh produce wash compounds. Treatment efficacy differed considerably across produce commodities, with liquid fresh produce wash compounds generally showing considerably greater efficacy for some tested commodities than for other commodities. Most but not all of the evaluated liquid fresh produce wash compounds were significantly more effective in reducing L. monocytogenes contamination than water alone, with mean reductions in L. monocytogenes levels ranging from less than 1 log₁₀ cfu to more than 5 log₁₀ cfu. Liquid fresh produce wash compounds are therefore a possible tool for reducing contamination with L. monocytogenes, for certain produce commodities.     

Effects of Salmonella bacteriophage,nisin and potassium sorbate and their combination on safety and shelf life of fresh chilled pork

Salmonella is an important foodborne pathogen and a serious threat to human health worldwide. This study was to reduce Salmonella and the spoilage bacteria on fresh chilled pork using bacteriophage, nisin, and potassium sorbate (PS) along with their combinations. Microbial, chemical, and sensory qualities of the fresh chilled pork (artificially contaminated with Salmonella 3 log CFU/g) treated with bacteriophage (9 log PFU/g), nisin (5000 IU/g), PS (2 mg/g) and their combinations were evaluated. The result showed that all the samples treated with phage could significantly (P < 0.05) reduce Salmonella population on fresh chilled pork. The combination treatment of nisin, PS and phage (N-PS-P) could significantly lower total viable counts (TVC), TVB-N and TBARS of the chilled pork during the storage period. The TVC of sample treated by N-PS-P was reduced by 2.3 log CFU/g at 7th day. It was also found through the electronic nose detection that the N-PS-P treatment was able to significantly reduce odour and maintain good sensory of the chilled pork. Hence, the N-PS-P treatment extended the shelf life of fresh chilled pork up to 14 days. No adverse effect of the phage on the chilled pork was observed. In conclusion, this study suggests that the phage and its combination with nisin and PS have great potential to be used as a good preservative for fresh chilled pork.     

A review of minimal and defined media for growth of Listeria monocytogenes

Listeria monocytogenes is known to be an important foodborne pathogen and is the causative agent of one of the deadliest foodborne illnesses. The organism has a wide range of environmental conditions under which it will survive and grow, and often contaminates processing plants and retail environments in which ready-to-eat foods are manufactured, prepared and served. Although L. monocytogenes is not a fastidious organism and can grow in a variety of rich media, defined and minimal media are necessary to elucidate the minimal environmental nutrients that are required for the survival and growth of this organism. In addition, many of the virulence factors required for L. monocytogenes to invade and multiply in mammalian host cells are not produced in rich media and thus induction of these factors is best studied in a minimal medium. This review covers the historical development of minimal media for Listeria spp. and explores the various factors required for survival and growth of this organism. To the best of our knowledge, this is the first time that these media have been compared side by side. In order to better compare studies using different chemical substrates such as a different carbon source, all concentrations of components in each medium have been converted to molar concentrations.     

Casamino Acids and Oxyrase enhance growth of Listeria monocytogenes in multi-pathogen enrichments

Rapid methods have been developed as relatively faster alternatives to plate culture for the detection of pathogenic bacteria in foods. However, since most rapid methods are subject to logistical limitations (e.g., sample volume size, analysis time, matrix effects) and/or a detection scheme with insufficient sensitivity needed to detect very low levels of bacteria in foods, culture enrichment is often employed to increase the concentration of targeted pathogens prior to detection. Multiplexed rapid detection platforms, capable of simultaneous detection of different bacteria in a single sample, necessitate co-enrichment (or mixed culture enrichment) of as many different targeted microorganisms as possible in a timely manner. This investigation compares the growth of four major foodborne pathogens (Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica, and Yersinia enterocolitica) inoculated into pristine media or ground pork and enriched in various culture media. Initial results revealed that, after 24 h incubation, the growth of L. monocytogenes (the slowest-growing pathogen examined) was increased by approximately 1-log by the supplementation of Universal Preenrichment Broth with Casamino Acids and/or Oxyrase. Overnight (24 h) growth of L. monocytogenes in ground pork enrichment cultures was enhanced up to ca. 2-log by the addition of either Casamino Acids or Casamino Acids and Oxyrase for each of the tested growth media. Ultimately, an overnight culture of the inoculated pathogens in any of the selected media containing both Casamino Acids and Oxyrase was observed to yield target bacterial concentrations that were at sufficient levels (between 10e5 and 10e6 CFU/mL) for detection by most rapid methods.     

Lactic acid bacteria in an alginate film inhibit Listeria monocytogenes growth on smoked salmon

Antimicrobial packaging with lactic acid bacteria incorporated into the film matrix is a novel approach for controlling the growth of food-borne pathogens in ready-to-eat food. The overall objective of this study was to assess the effect of two strains of lactic acid bacteria (LAB) and nisin trapped in an alginate matrix, on Listeria monocytogenes growth on vacuum packed cold-smoked salmon. A film was formulated containing two LAB strains and nisin (100 IU/mL). LAB viability and bacteriocin like substance production (BLS) were assessed using the plate antagonism technique. To check the film antagonistic activity, pieces of salmon (4.0 × 4.0 cm²), inoculated with L. monocytogenes at a final concentration of 10⁴ CFU/cm², were covered with film containing both LAB strains plus nisin and stored at 4 °C. L. monocytogenes colonies on OXA agar were counted after 0, 7, 14, 21 and 28 days to evaluate pathogen inhibition. All treatments led to effective diffusion of the BLS that inhibited L. monocytogenes for 20 days after film preparation, with inhibition zones of 5.7 cm² for film coupons of 8 mm in diameter. After 28 days, salmon pieces covered with the film without inhibitors showed an increase of 2.4 log cycles in L. monocytogenes growth. In contrast, films with either LAB strain or a combination of both strains and nisin had a bacteriostatic effect on the pathogen over a period of 28 days, which exceeds the industrial standard shelf life for smoked salmon. The results demonstrate that these films inhibit L. monocytogenes growth on salmon during refrigerated storage.     

Anti-listerial properties of garlic shoot juice at growth and morphology of Listeria monocytogenes

The anti-listerial effect of garlic shoot juice (GSJ) was investigated against the four strains of Listeria monocytogenes ATCC 19116, 19118, 19166 and 15313. Various concentrations of (1%, 2.5% and 5%) were used and applied for 0, 4, 7, 10 and 14 days at 4 °C and 0, 6, 12, 18 and 24 h at 37 °C. 5% GSJ showed the strongest anti-listerial effect of 3–4 log cfu/ml against all the bacterial strains tested as compared to control at 37 °C. At 4 °C, 2.5% GSJ showed a strong growth inhibitory effect of about 2–3 log cfu/ml against all the bacterial strains when compared to control after 14 days, and 5% GSJ rather decreased the number of bacterial cell when compared to initial polulation and showed general lysis of cytoplasm, lysis of cell wall and cellular swelling, and on scanning electron microscopy (SEM) observation, strain L. monocytogenes ATCC 19118 showed swelling, partially distorted shape, pore formation and empty cell formation inside the cell.     

Effect of hydrogen peroxide vapor treatment for inactivating Salmonella Typhimurium,Escherichia coli O157:H7 and Listeria monocytogenes on organic fresh lettuce

In this study, the efficacy of hydrogen peroxide vapor (HPV) for reducing Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes on lettuce was investigated as well as its effect on lettuce quality. Lettuce was inoculated with a cocktail containing three strains of each pathogen then treated with vaporized hydrogen peroxide for 0, 2, 4, 6, 8 and 10 min. The concentrations of hydrogen peroxide used were 0, 1, 3, 5 and 10%. With increasing treatment time and hydrogen peroxide concentration, HPV treatment showed significant (P < 0.05) reduction compared to the control (0%, treated with vaporized distilled water). In particular, vaporized 10% hydrogen peroxide treatment for 10 min was the most effective combination for reducing the three pathogens on lettuce. The reduction levels of S. Typhimurium, E. coli O157:H7 and L. monocytogenes on lettuce were 3.12, 3.15 and 2.95 log₁₀ CFU/g, respectively. Furthermore, there were no significant (P > 0.05) quality changes (color and texture) of lettuce among all tested samples, and hydrogen peroxide residues were not detected after 36 h storage time in any of the treated samples. These results suggest that HPV treatment could be an alternative method for reducing S. Typhimurium, E. coli O157:H7 and L. monocytogenes on fresh produce.     

Effect of temperature and salt on thermal inactivation of Listeria monocytogenes in salmon roe

Listeria monocytogenes is a potentially fatal foodborne pathogen that can be found in ready-to-eat seafood products, such as fresh salmon roe. Once contaminated, salmon roe must be decontaminated prior to human consumption. This study was conducted to determine the thermal inactivation kinetics of L. monocytogenes in raw salmon roe as affected by bacterial strain, temperature, and salt concentration. Three different strains of L. monocytogenes, including serotype 4b (F2365), 1/2b (F4260), and 1/2a (V7), were individually inoculated to salmon roe supplemented with salt (0–4.5%), and heated under different temperatures (57.5–65.0 °C) to evaluate the survival of the bacterium during heating and determine the D-values. Results showed that the thermal resistance (log D) of L. monocytogenes was significantly affected by bacterial strain, temperature, and salt and by their interactive effects, with strain F2365 being the most heat-resistant among all three strains tested. Salt added to salmon roe significantly increased the thermal resistance of the bacteria. For L. monocytogenes F2365, the z value of the bacterium in salmon roe was 5.99 °C, and its heat resistance increased with the level of salt in a linear manner. The results of kinetic analysis and the models obtained in this study may be used by the seafood industry to develop proper thermal processes to eliminate L. monocytogenes in raw salmon roe and to ensure microbial safety and prevent foodborne illness.     

Characterisation of the resistance and the growth variability of Listeria monocytogenes after high hydrostatic pressure treatments

The use of non-thermal methods for food preservation is due to consumer demands for microbiological safe products, without changes in the sensory and nutritional qualities of the product. High hydrostatic pressure (HHP) has emerged as an alternative to traditional thermal processing methods for foods.Listeria monocytogenes CECT 5672 was treated under HHPs (350, 400 and 450 MPa) for 3, 16 and 23 min. The effects of pH (5, 6 and 7) and sodium chloride concentration (0, 0.5 and 1.0) of the recovery medium were studied on L. monocytogenes. The kinetic parameters were estimated by the method described by Métris, George, and Baranyi (2006). From results obtained, histograms of the lag phase were generated and distributions were fitted. The duration of the lag phase of HHP damaged cells increased with the application of additional stresses. Histograms showed a shift to longer lag phases and an increase in variability with high stress levels in the recovery medium.Using a primary model together with Monte Carlo simulation, predictions of time to growth (100 cfu/g) of L. monocytogenes were established and they were compared with deterministic predictions. It was evidenced that deterministic predictions do not give a good indication of the probability of a certain level of growth.     

The determination of ready-to-eat foods into Listeria monocytogenes growth and no growth categories by challenge tests

To assess the potential for a food to allow the growth of Listeria monocytogenes throughout its shelf life and be compliant with European Commission regulations, a challenge test was designed by the EU Community Reference Laboratory. The procedure for the determination of the growth potential includes the following: product characteristics, shelf-life of the product, number of batches, choice of the strain(s), preparation of the inoculum, preparation and inoculation of the test units, storage conditions, measurement of physico-chemical characteristics of the food and microbiological analysis to measure any increase in the pathogen load. The results are then used to assign a ready-to-eat food into a growth or no growth category, and if the food is able to support the growth of L. monocytogenes, to quantify the behavior of this bacteria in a food between production and consumption.