The Canadian position on Listeria monocytogenes in ready-to-eat foods

Monitoring the Bioprotective Potential of Lactiplantibacillus pentosus Culture on Pathogen Survival and the Shelf-Life of Fresh Ready-to-Eat Salads Stored under Modified Atmosphere Packaging

Globally, fresh vegetables or minimally processed salads have been implicated in several foodborne disease outbreaks. This work studied the effect of Lactiplantibacillus pentosus FMCC-B281 cells (F) and its supernatant (S) on spoilage and on the fate of Listeria monocytogenes and Escherichia coli O157:H7 on fresh-cut ready-to-eat (RTE) salads during storage. Also, Fourier transform infrared (FTIR) and multispectral imaging (MSI) analysis were used as rapid and non-destructive techniques to estimate the microbiological status of the samples. Fresh romaine lettuce, rocket cabbage, and white cabbage were used in the present study and were inoculated with L. pentosus and the two pathogens. The strains were grown at 37 °C for 24 h in MRS and BHI broths, respectively, and then were centrifuged to collect the supernatant and the pellet (cells). Cells (F, ~5 log CFU/g), the supernatant (S), and a control (C, broth) were used to spray the leaves of each fresh vegetable that had been previously contaminated (sprayed) with the pathogen (3 log CFU/g). Subsequently, the salads were packed under modified atmosphere packaging (10%CO2/10%O2/80%N2) and stored at 4 and 10 °C until spoilage. During storage, microbiological counts and pH were monitored in parallel with FTIR and MSI analyses. The results showed that during storage, the population of the pathogens increased for lettuce and rocket independent of the treatment. For cabbage, pathogen populations remained stable throughout storage. Regarding the spoilage microbiota, the Pseudomonas population was lower in the F samples, while no differences in the populations of Enterobacteriaceae and yeasts/molds were observed for the C, F, and S samples stored at 4 °C. According to sensory evaluation, the shelf-life was shorter for the control samples in contrast to the S and F samples, where their shelf-life was elongated by 1-2 days. Initial pH values were ca. 6.0 for the three leafy vegetables. An increase in the pH of ca. 0.5 values was recorded until the end of storage at both temperatures for all cases of leafy vegetables. FTIR and MSI analyses did not satisfactorily lead to the estimation of the microbiological quality of salads. In conclusion, the applied bioprotective strain (L. pentosus) can elongate the shelf-life of the RTE salads without an effect on pathogen growth.

Analysis of the Bioprotective Potential of Different Lactic Acid Bacteria Against Listeria monocytogenes in Cold-Smoked Sea Bass, a New Product Packaged Under Vacuum and Stored at 6 ± 2°C

The aim of the work was to monitor the presence of Listeria monocytogenes in cold-smoked fish products (trout, salmon, and sea bass) marketed in Italy. Cold-smoked sea bass is a new product that has not yet been commercialized and was collected from the production facility. Monitoring data have shown that cold-smoked products can be contaminated by L. monocytogenes, the presence of which has been highlighted mainly by enrichment culture (presence in 25 g). The isolated Listeria were serotyped and belonged mainly to low-virulence serotypes (1/2c), followed by serotypes 1/2a, 1/2b, and 4b. Furthermore, considering the ability of L. monocytogenes to grow in these products due to their chemical-physical characteristics (pH > 6.0, Aw > 0.97) and long shelf life at 4°C, an additional aim was to verify the activity of different bioprotective starters, including Lactilactobacillus sakei (LAK-23, Sacco srl, Via Alessandro Manzoni 29/A, 22071 Cadorago, CO, Italy), Carnobacterium spp., Lacticaseibacillus casei (SAL 106), and Lacticaseibacillus paracasei (SAL 211), in cold-smoked sea bass. All starters were bacteriocin producers. For this experiment, smoked sea bass samples were intentionally inoculated with a mixture of three different strains of L. monocytogenes and of each starter culture. After inoculation, the smoked sea bass were vacuum-packed and stored at 6 ± 2°C for 60 days, simulating the typical abuse storage temperature of markets and home refrigerators. At 0, 15, 30, 45, and 60 days, the sea bass samples were analyzed to evaluate the effectiveness of the starters against L. monocytogenes. Listeria monocytogenes growth was prevented only by the addition of the LAK-23 starter. Indeed, at the end of the shelf life, the amount of L. monocytogenes observed was similar to that in the inoculum. Consequently, the use of this starter can allow the inclusion of cold-smoked sea bass or smoked fish products in category 1.3 of Regolamento CE 2073/2005, which are products that do not support the growth of this microorganism. Finally, the activity of the LAK-23 starter did not produce an off flavor or off odor in the smoked sea bass.

Phage Inactivation of Listeria monocytogenes on San Daniele Dry-Cured Ham and Elimination of Biofilms from Equipment and Working Environments

The anti-listerial activity of generally recognized as safe (GRAS) bacteriophage Listex P100 (phage P100) was demonstrated in broths and on the surface of slices of dry-cured ham against 5 strains or serotypes (i.e., Scott A, 1/2a, 1/2b, and 4b) of Listeria monocytogenes. In a broth model system, phage P100 at a concentration equal to or greater than 7 log PFU/mL completely inhibited 2 log CFU/cm² or 3 log CFU/cm² of L. monocytogenes growth at 30 °C. The temperature (4, 10, 20 °C) seemed to influence P100 activity; the best results were obtained at 4 °C. On dry-cured ham slices, a P100 concentration ranging from 5 to 8 log PFU/cm² was required to obtain a significant reduction in L. monocytogenes. At 4, 10, and 20 °C, an inoculum of 8 log PFU/cm² was required to completely eliminate 2 log L. monocytogenes/cm² and to reach the absence in 25 g product according to USA food law. Conversely, it was impossible to completely eradicate L. monocytogenes with an inoculum of approximately of 3.0 and 4.0 log CFU/cm² and with a P100 inoculum ranging from 1 to 7 log PFU/cm². P100 remained stable on dry-cured ham slices over a 14-day storage period, with only a marginal loss of 0.2 log PFU/cm² from an initial phage treatment of approximately 8 log PFU/cm². Moreover, phage P100 eliminated free L. monocytogenes cells and biofilms on the machinery surfaces used for dry-cured ham production. These findings demonstrate that the GRAS bacteriophage Listex P100 at level of 8 log PFU/cm² is listericidal and useful for reducing the L. monocytogenes concentration or eradicating the bacteria from dry-cured ham.

Potential uptake of Escherichia coli O157:H7 and Listeria monocytogenes from growth substrate into leaves of salad plants and basil grown in soil irrigated with contaminated water

Outbreaks of foodborne illness, resulting from the consumption of fresh produce contaminated with human pathogens, are increasing. Potential uptake and persistence of human pathogens within edible parts of consumed fresh vegetables become an important issue in food safety. This study was conducted to assess the potential uptake and internalization of Escherichia coli O157:H7 and Listeria monocytogenes from an autoclaved substrate into edible parts of basil and baby salad plants (lettuce, cultivated rocket, wild rocket and corn salad) from 20 to 60-80days after inoculation, when plants are ready to be harvested and commercialized. Plants were grown in mesocosms under different temperature conditions (24°C and 30°C) and the growing substrate was inoculated using contaminated irrigation water (7logCFU/mL). E. coli O157:H7 could be internalized in the leaves of the tested leafy vegetables through the roots and persist up to the harvesting time with negligible differences between 24°C and 30°C. Significant decreases in pathogen titers were observed over time in the growing substrate on which the plants grew, until the last sampling time. In contrast, L. monocytogenes internalized and persisted only in lettuce mesocosms at 24°C. Neither pathogen was observed in basil leaves. Similarly, in basil growing substrates, enteric bacteria were undetectable at the end of the experiments, suggesting that basil plants may produce and release antimicrobial compounds active against both bacteria in root exudates. These results suggest that enteric bacteria are able to persist within baby salad leaves up to market representing a risk for consumer's health.

Safety and quality of ready-to-eat dry fermented sausages subjected to E-beam radiation

The inactivation kinetics in the death of Listeriainnocua NTC 11288 (more radioresistant than five different strains of Listeriamonocytogenes) and Salmonella Enterica serovar Enteritidis and S. enterica serovar Typhimurium by E-beam irradiation has been studied in two types of vacuum-packed RTE dry fermented sausages ("salchichon" and "chorizo") in order to optimize the sanitation treatment of these products. A treatment of 1.29kGy was calculated to reach the food safety objective (FSO) according to the "zero tolerance" criterion for the three strains. No irradiation treatment was necessary to meet the 10(2)c.f.u./g microbiological criterion for L. monocytogenes. Dry fermented sausages treated with ⩽2kGy had negligible sensory (appearance, odour and taste) modifications. Therefore, this treatment produces safe dry fermented sausages with similar sensory properties to the non-irradiated product.

Elimination of Listeria monocytogenes from vacuum-packed dry-cured ham by E-beam radiation

The inactivation kinetics for Listeria monocytogenes Scott A (CIP 103575, serotype 4b) and Listeria innocua (NTC 11288) after E-beam radiation were studied in vacuum-packed ready-to-eat dry-cured ham to optimize the sanitation treatment of this product. A treatment of 1.12 kGy was calculated to reach the food safety objective according to the U.S. Department of Agriculture criterion. No irradiation treatment is necessary to meet the European Union microbiological criterion for this bacterium. No changes (at doses < or =4 kGy) in the 2-thiobarbituric acid reactive substances values and texture were observed. Dry-cured hams treated with 1 and 2 kGy had negligible sensory modifications (appearance, odor, and flavor). However, the application of 3 and 4 kGy resulted in an increase in the intensity of off-odors and off-flavors. Despite these effects, all irradiated vacuum-packed dry-cured hams treated at < or =4 kGy were deemed acceptable for trading.

Survival and acid resistance of Listeria innocua in Feta cheese and yogurt, in the presence or absence of fungi

The objective of the study was to assess the survival of Listeria innocua, alone or coinoculated with fungal isolates, during storage of Feta cheese (pH 4.43 to 4.56) and yogurt (pH 4.01 to 4.27) at 3 to 15 degrees C. The acid resistance of the bacterium during subsequent exposure to pH 2.5 for 3 h was also evaluated in samples stored at 3 and 10 degrees C. In Feta cheese, L. innocua survived better than it did in yogurt at all temperatures. At 5, 10, and 15 degrees C, the pH of cheese increased due to fungal growth, and this enhanced the survival of L. innocua more than during storage at 3 degrees C. Moreover, during storage of Feta cheese, L. innocua was capable of surviving the subsequent exposure for 3 h in broth of pH 2.5, in contrast to cultures not inoculated in the product (control cultures; 24 h at 30 degrees C in broth). In yogurt, L. innocua reduced more than 5 log within 15 days of storage at 5, 10, and 15 degrees C, whereas extended survival was observed at 3 degrees C until day 22, with total reduction of approximately 4.5 log. In contrast to what was observed in Feta cheese, surviving populations of L. innocua in yogurt were eliminated after subsequent exposure for 3 h to pH 2.5. The findings indicate that growth of fungi on the surface of Feta cheese and yogurt may compromise the safety of these products by enhancing survival of the bacterium. Particularly, when fungi increase the pH of Feta cheese, L. innocua demonstrates better survival and prolonged storage may raise concerns for the development of acid-resistant Listeria populations.

Survival of Listeria monocytogenes Scott A on metal surfaces: implications for cross-contamination

Listeria monocytogenes is an important re-emerging pathogen which is commonly found in the environment. Many outbreaks have been associated with the contamination of food produce, often linked to cross-contamination from surfaces or equipment to prepared foodstuffs. In the present study a number of copper-base metal alloys have been used to assess the survival times of L. monocytogenes on different materials, in comparison with stainless steel. High concentrations (10(7)) of bacteria were placed on metal coupons cut from each alloy. After defined incubation times, coupons were placed in tubes containing phosphate buffered saline and vortexed to remove the cells. Aliquots were then plated onto tryptone blood agar plates and the number of colony forming units counted. The high concentration of bacteria was used to represent a "worst-case" scenario. The results indicate that survival is greatly reduced on a copper-base alloy compared to stainless steel. Viable cells could be detected on stainless steel after 24 h incubation at room temperature. On copper, brass, aluminium bronze and silicon bronze, no viable bacteria could be detected after 60 min incubation, indicating a 5 log reduction (the detection limit of the procedure was 100 bacteria). No cells could be detected from copper nickel and copper nickel zinc alloys, after 90 min incubation. The viability stain, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), confirmed these results, with actively respiring bacteria being clearly labelled on stainless steel after 24 h. The results suggest that careful choice of surface material could reduce the potential risk of cross-contamination in industrial, commercial and domestic environments.