Antimicrobial films and coatings for inactivation of Listeria innocua on ready-to-eat deli turkey meat

Edible antimicrobial coating solutions incorporating chitosan, lauric arginate ester (LAE) and nisin were developed to reduce foodborne pathogen contamination on ready-to-eat (RTE) meats. RTE deli meat samples were directly coated with the solutions, or treated with solution-coated polylactic acid (PLA) films. The antimicrobial efficacy of the coatings and films against Listeria innocua inoculated onto the surface of RTE meat samples was investigated. Antimicrobial coatings with 1.94 mg/cm² of chitosan and 0.388 mg/cm² of LAE reduced L. innocua by ca. 4.5 log CFU/cm². Nisin (486 IU/cm²) showed less effectiveness than LAE (0.388 mg/cm²) and addition of nisin to the antimicrobial coatings or films containing LAE (0.388 mg/cm²) did not enhance the total antimicrobial effectiveness. Combining antimicrobial coatings or films with flash pasteurization (FP), which uses short burst of steam under pressure, further reduced L. innocua, achieving over a 5 log reduction. There was no significant difference in the effectiveness of antimicrobial films versus the coatings (p > 0.05). These data show the potential use of antimicrobial packaging alone, or in combination with FP, in preventing foodborne illness due to post-processing contamination of RTE meat products.     

Heat inactivation of Listeria innocua in broth and food products under non-isothermal conditions

The objective of this work was to study the effect of three linear temperature profiles (heating rates of 1.5, 1.8 and 2.6 °C/min, from 20 to 65 °C) on Listeria innocua inactivation in liquid medium. The inactivation was also analyzed in artificially contaminated parsley (heating rate of 1.8 °C/min) and throughout a frying process, using a pre-cooked frozen food as case study. Inactivation showed a sigmoidal behaviour and all data was fitted with a Gompertz-inspired model. Results demonstrated that, in liquid media, Listeria inactivation is influenced by the temperature profile used. As heating rate increases, the shoulder decreases and the tail effect disappears. If Listeria was in parsley, its heat resistance increased (for identical experimental conditions in broth). Besides model adequacy was proven in all studied situations, the heating rate affected parameters’ precision.     

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.     

Thermal inactivation and growth of Listeria monocytogenes during production and storage of caramel apples

During the fall of 2014, commercially produced pre-packaged caramel apples were linked to 35 cases of listeriosis in 12 states. In response, this study aimed to assess 1) the reduction of different outbreak and non-outbreak strains of Listeria monocytogenes during caramel dipping of apples, and 2) subsequent growth of the apple outbreak strains within caramel apples during storage at 22 and 4 °C. In aim 1, three unwaxed Jonathan apples were dip-inoculated with three different 4-strain L. monocytogenes cocktails (apple outbreak, unrelated outbreak or unrelated environmental) at ∼8 log CFU/apple, dried for 1 h, dipped for 5 s in caramel at 82, 88, 93 or 99 °C, cooled for 1 h at room temperature and assessed for survivors. In aim 2, Jonathan apples were spot-inoculated with the apple outbreak cocktail (∼3 log CFU/apple) at the stem juncture, dried for 1 h, pushed onto wooden sticks, and dipped in caramel at 82 °C. During storage at 4 and 22 °C for 28 and 14 days, respectively, four different apple sections (top, middle, bottom and core) were cut from three apples, homogenized and plated for Listeria. After dipping apples in caramel at 82 and 99 °C, the apple outbreak, unrelated outbreak and environmental Listeria strains decreased 2.0 ± 0.6 and 2.7 ± 0.1, 1.8 ± 0.3 and 2.6 + 0.1, and 1.7 ± 0.1 and 2.9 ± 0.2 logs, respectively, with the environmental cocktail significantly less heat resistant (P < 0.05) at 99 °C compared to the other two cocktails. After 14 days of storage at 22 °C, Listeria populations were significantly higher (P < 0.05) in the core (7.4 ± 0.6 log CFU/g) compared to the other three sections (4.9–5.4 log CFU/g). The same trend was seen for the core (7.7 ± 0.6 log CFU/g) and the other three sections (5.0–5.4 log CFU/g) after 28 days of storage at 4 °C. Since dipping in hot caramel cannot ensure pathogen elimination, producers of caramel apples should implement good agricultural practices, post-harvest preventive controls and refrigeration of the final product to minimize the risks from Listeria.     

Genotypic characterization of Listeria monocytogenes isolated from milk and ready-to-eat indigenous milk products

Listeria monocytogenes is a food borne pathogen associated with severe diseases in humans and animals. We present the genotypic analysis of 18 L. monocytogenes isolates recovered from milk and ready-to-eat (RTE) indigenous milk products by multiplex-PCR, allowing serovar predictions, and by random amplification of polymorphic DNA (RAPD) assays. Multiplex-PCR serotyping assay revealed 72.2% (13/18) strains belonging to serovar group 4b, 4d, 4e, 22.2% (4/18) to serovar group 1/2b, 3b while 5.5% (1/18) to serovar group 1/2a, 3a. RAPD analysis revealed five RAPD profiles from eight L. monocytogenes strains from milk, and seven RAPD profiles from ten strains from milk products. Though RAPD analysis allowed discrimination among isolates of the same serotype and among isolates from the same sampling areas or those isolated from different areas, our results suggests that most of the L. monocytogenes isolates were indeed sporadically harboured by single food items. Thus, RAPD together with multiplex-PCR serotyping allowed rapid discrimination of L. monocytogenes strains and therefore could serve as an economical tool for typing L. monocytogenes strains. In addition, the predominance of L. monocytogenes serotype 4b in our study is of public health concern, as this serotype has been most frequently associated with human listeriosis.     

Listeria innocua growth in fresh cut mixed leafy salads packaged in modified atmosphere

To investigate Listeria innocua fate in fresh cut mixed leafy salads, packaged both in ordinary (OA) and modified atmosphere (MA), the type strain DSM 20649 was inoculated and its behaviour was monitored by conventional and molecular approaches, during storage at 4 °C.Results indicated that: fresh cut salads packaged under MA (at initial conditions of 6% CO₂ and 3% O₂) supported the growth of Listeria spp.; conventional plating technique could not detect L. innocua even into inoculated samples, whereas; PCR–DGGE analysis showed that the L. innocua became one of the dominant species in samples packaged in MA, starting from the 3rd day of storage.The study confirms that modified atmosphere has to be applied together with other preservative techniques in order to assure the inhibition of pathogenic micro-organisms in fresh cut vegetables.     

Prevalence of Listeria spp. and antibiotic susceptibility of Listeria monocytogenes isolated from raw chicken and ready-to-eat chicken products in Jordan

Listeria monocytogenes is the causal agent of listeriosis, a disease that can be serious and is often fatal in susceptible individuals. The objective of the study was to determine the prevalence of Listeria spp. in raw chicken and ready-to-eat (RTE) chicken products in Amman, Jordan and the antimicrobial resistance of L. monocytogenes isolates. A total of 280 raw chicken and RTE chicken products (chicken-shawirma, chicken-burger, chicken-sausage and mortadella) were collected from Amman abattoir and local retail markets in Amman city. Listeria spp. were isolated by the conventional International Organization for Standardization (ISO) method and L. monocytogenes identified by biochemical and Polymerase Chain Reaction (PCR). Results of conventional method showed that out of total 280 samples, 141 (50%) were found to be contaminated with Listeria spp. [L. monocytogenes (18.2%), Listeria ivanovi (26.1%), Listeria grayi (3.5%), Listeria seeligeri (1.8), Listeria welshimeri (0.7%)]. The PCR confirmed all L. monocytogenes isolates (51 isolates: 15 from raw dressed broiler chicken, 23 from chicken-burger, 9 from chicken-sausage, and 4 from chicken-shawirma). Five of the tested L. monocytogenes isolates were resistance to two antibiotics (tilimicosin and tetracycline) among the ten tested antibiotics as determined by microbroth dilution method. The results presented in this study indicate the potential risk of contamination of RTE chicken products with L. monocytogenes.     

Multiple regression model for thermal inactivation of Listeria monocytogenes in liquid food products

A multiple regression model was constructed for thermal inactivation of Listeria monocytogenes in liquid food products, based on 802 sets of data with 51 different strains and 6 cocktails of strains published from 1984 to 2010. Significant variables, other than inactivation temperature, were pH, sodium chloride content, sugar content, the temperature of growth or storage before inactivation, in addition to a heat shock before inactivation. The constructed model for thermal inactivation of L. monocytogenes has a reduced variability as these variables are known to influence the thermal resistance (and these are known or controllable in practice). Mean simulation results of inactivation of L. monocytogenes during pasteurisation (20 s, 76 °C) of raw milk (calculated mean level after growth 14 cfu/l) were comparable with results of a single regression model constructed from inactivation data found in experiments in milk only (175 data sets, 18 strains/cocktails). Both models predicted a probability of survival of less than 1 in a billion litres. The study shows that multiple regression modelling can be used to obtain a model from all data available, with a limited and realistic uncertainty level, while retaining the variability of heat resistance due to the 51 strains and 6 cocktails of strains (unknown and not controllable in practice).     

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.     

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.     

Listeria monocytogenes in ready-to-eat vacuum and modified atmosphere packaged meat and fish products of Estonian origin at retail level

The prevalence, counts and genetic diversity of Listeria monocytogenes in ready-to-eat (RTE) vacuum and modified atmosphere packaged meat and fish products was studied in Estonia. Within two consecutive years 370 RTE food samples were collected at retail level from which 11% were found to be positive for L. monocytogenes. Contamination was higher among RTE fish products (17%) than in RTE meat products (6%). Generally, the counts of L. monocytogenes in positive products remained under ten colony forming units (CFU) per gram of product. Only 1.6% of the RTE meat and fish products contained L. monocytogenes in range of 10–100 CFU/g and 0.3% more than 100 CFU/g at the end of shelf-life. The food category containing highest L. monocytogenes prevalence was RTE lightly salted fish products with the prevalence of 32%. Only one (0.3%) RTE food sample exceeded the 100 CFU/g food safety criterion set out in the EU Regulation 2073/2005. Pulsed-field gel electrophoresis (PFGE) characterization of the isolates showed an overall similarity higher than 70%, and nine clusters based on 100% similarity were revealed. PFGE genotyping revealed that the few predominant pulsotypes were associated with particular food plants.     

Survey of Listeria monocytogenes in ready-to-eat products: Prevalence by brands and retail establishments for exposure assessment of listeriosis in Northern Spain

The prevalence of Listeria monocytogenes in ready-to-eat products of markets in Northern Spain was studied, being analyzed 783 samples of deli meat products, smoked fish and pâté. RTE smoked fish was the most frequently contaminated food category (25% positive), with high occurrence in some brands (60% of lots positive). Significant differences in prevalence were found in in-store-packaged deli meat products (8.5%) with respect to manufacturer vacuum-packaged presentation (2.7%). Serological and molecular characterization of L. monocytogenes isolates confirmed the persistence of several clusters within manufacturing environments, as the same pulsotype was repeatedly isolated from different lots of the same brand and from different sliced products of the same store. These results reflect the need to improve hygiene and disinfection programs by addressing more accurate cleaning practices and continuous education of food workers.     

Prevalence,genetic diversity and antimicrobial resistance of Listeria monocytogenes isolated from ready-to-eat meat products in Nanjing,China

The prevalence of Listeria monocytogenes were investigated in a total of 628 ready-to-eat (RTE) meat products collected from different supermarkets and open-air markets in Nanjing, China. All isolates were further examined for the serogroup, virulence marker gene, genotype and antibiotic resistance. Thirty-three out of 628 samples (5.3%) were positive confirmed by the bacteriological method and PCR including 7.2% (17/236) of sauce pickled products, 4.2% (11/260) of cured products and 5.6% (5/90) of smoked and roasted products. Fifteen isolates (45.5%) belonged to serogroup 1/2a, 3a; 16 (48.5%) belonged to serogroup 1/2b, 3b and 2 (6.1%) belonged to serogroup 1/2c, 3c. All of them were positive for the virulence marker genes-iap, inlA, inlC, inlJ and lmo2672. Thirty-three isolates were grouped into 11 sequence types (STs) by multilocus sequence typing (MLST). The results of the antimicrobial susceptibility revealed that the isolates were sensitive to most of the antimicrobials used in the study except trimethoprim–sulfamethoxazole (100%), chloramphenicol (33.3%), ciprofloxacin (30.3%) and tetracycline (12.1%). Our findings indicated high prevalence of L. monocytogenes especially in sauce pickled products and from open-air markets, high prevalence of serogroups 1/2a, 3a and 1/2b, 3b that involved in the majority of foodborne outbreaks could be a public health concern. In addition, resistance of the isolates to the antimicrobials was also a potential health hazard for consumers.     

Temperature distribution in Spanish domestic refrigerators and its effect on Listeria monocytogenes growth in sliced ready-to-eat ham

In order to evaluate the increase of L. monocytogenes concentration during home storage, a low level of the pathogen (<10 CFU/g) was inoculated in sliced ham. Two storage temperatures (5 °C and 9 °C) were selected from a previous study in 33 home refrigerators. Three days of storage or less were necessary to reach the regulated limited concentration (100 CFU/g), at both temperatures. When storage time was extended to 5 days, the pathogen achieved risky values (>10³ CFU/g).     

Effects of process severity on survival and growth of Escherichia coli and Listeria innocua on minimally processed vegetables

The effects of different slicing methods on subsequent growth and survival of Escherichia coli, Listeria innocua, and background microflora, during storage (8 °C) on modified atmosphere packaged vegetables (sliced carrots, sliced iceberg and butterhead lettuce) were evaluated. E. coli and L. innocua were used as models for E. coli O157:H7 and Listeria monocytogenes. Gas atmospheres within packages of minimally processed vegetables (MPV) were monitored to identify any effects of slicing treatments on oxygen and carbon dioxide levels. In general, the slicing method had no significant effect on initial inoculation levels. L. innocua grew better and E. coli survived better on vegetables sliced with blades that caused the most damage to cut surfaces. Slicing manually with a blunt knife or with machine blades gave consistently higher E. coli and L. innocua counts during storage than slicing manually with a razor blade. The effects of hand tearing were similar to slicing with a razor blade. The slicing method also affected the growth of the total background microflora; razor sliced vegetables tended to have lower counts than other treatments. Results also indicated that product respiration was affected by slicing method.     

Growth capacity of Listeria monocytogenes in ingredients of ready-to-eat salads

Raw minimally processed vegetables and fruits as well as boiled pasta and potato are the main ingredients of ready-to-eat (RTE) salads. These ingredients are diverse in their ability to support growth of Listeria monocytogenes. The objective of this study was to determine to what extent different ingredients can contribute to growth of L. monocytogenes in RTE salads. This was done by assessing the growth capacity of L. monocytogenes in ingredients of RTE salads by means of challenge tests and determination of the effect of product characteristics and presence of competitive flora. The majority of the tested products did not support or hardly supported growth of L. monocytogenes. The calculated maximum increase of 3.4 log CFU/g was exceeded in Galia melon, potato and pasta products. In RTE dinner salads, growth of L. monocytogenes was supported by the carbohydrate component. Galia melon, which is often used as ingredient of fruit salads, can contribute to growth of L. monocytogenes in these products. Growth inhibition, or differences in relative increase, could not always be explained by a_w, pH or presence of competitive flora. The effect of competitive flora, especially lactic acid bacteria, on growth inhibition was observed in non-pasteurized potato, white cabbage and mango.     

Growth potential of Listeria monocytogenes in traditional Austrian cooked-cured meat products

European Union legislation limits for Listeria monocytogenes in ready-to-eat foods are based on whether or not foods favour the multiplication of this bacterium. The latter is defined by criteria for water activity (a_w), pH and shelf-life. We studied a peculiar group of traditional Austrian meats implicated in foodborne listeriosis made from cured cooked comminuted meat with gelatin/aspic (Presswurst), blood (Blutwurst) or fat as a binder (Leberpaté, Streichwurst, Zwiebelstreichwurst). Average pH values were 5.74 ± 0.45; 6.62 ± 0.31; 6.18 ± 0.36; 6.19 ± 0.15; 6.28 ± 0.04 for Presswurst (n = 15), Blutwurst (n = 15), Leberpaté (n = 10), Streichwurst (n = 18) and Zwiebelstreichwurst (n = 3), respectively. Corresponding a_w values were: 0.968 ± 0.004; 0.965 ± 0.004; 0.961 ± 0.005; 0.963 ± 0.003 and 0.957 ± 0.005. There were no statistically significant differences of pH among spreadable meat products. Presswurst had significantly lower pH values, but a significantly higher level of lactic acid bacteria. With the exception of one low pH Presswurst sample, all foods under study would favour the growth of L. monocytogenes. In a 9 days challenge test, Blutwurst showed a strong potential for supporting L. monocytogenes growth (2.4–4.6 log). In contrast, Presswurst was not able to support growth in all temperatures tested. A pH vs. a_w chart was designed delineating the growth/no-growth border (defined as 0.43 log increase over 216 h) at 2, 4 and 8 °C. For a given sample (i.e. a pH/a_w data pair), it could be easily assessed if the product would likely be “safe” for 9 days at temperatures of <2 °C, 2–4 °C etc. by simply plotting the data point in the chart. Agreement of predicted bacterial growth and multiplication in food samples was studied in one Presswurst and three Blutwurst products. In our conservative approach, additional anti-listerial effects of lactic acid bacteria and food additives were not considered, but could be integrated, if desired. The usefulness of such a pH vs. a_w chart for small businesses, the competent authority and for didactic purposes is discussed.     

Influence of Listeria innocua on the attachment of Listeria monocytogenes to stainless steel and aluminum surfaces

Listeria monocytogenes is an important foodborne pathogen that may be transmitted from the food-processing environment to food; however, the ecology and interaction of this organism with microbial residents on surfaces within the food industry is not well understood. The current study was undertaken to investigate the influence of Listeria innocua on the growth and attachment of L. monocytogenes to stainless steel or aluminum surfaces at 23 °C. When grown in broth as a mixed culture, L. innocua reached a higher cell count at 24 h than did L. monocytogenes. Attachment was evaluated by placing an aliquot containing 10³ CFU/ml of L. innocua and 10³ CFU/ml of L. monocytogenes on the coupons and by quantifying attached cells after 24 and 72 h. Attachment of L. monocytogenes was decreased by the presence of L. innocua. When compared to L. monocytogenes alone, there was a significant reduction of attachment of L. monocytogenes at 24 and 72 h on stainless steel and 72 h on aluminum surface when L. innocua was added at the same time. L. innocua exhibited an effect on the attachment of L. monocytogenes, increasing our knowledge of the behavior of L. monocytogenes in the presence of another Listeria species.     

Thermal inactivation of Listeria monocytogenes from alheiras,traditional Portuguese sausage during cooking

D values were calculated at 55, 60 and 65 °C for five Listeria monocytogenes isolates using alheira (traditional Portuguese sausage) food matrix as heating menstrum. z values were also calculated and combined with internal alheiras temperature profiles during cooking in multiple ways, allowed estimations of percentages of L. monocytogenes survival.Survival percentages estimations showed that except for roasting, the remaining evaluated cooking methods might not be sufficient to inactivate this foodborne pathogen in alheiras at the minimum temperature profiles. However, it is important to note that all evaluated cooking methods were able to inactivate L. monocytogenes in alheiras at their maximum internal temperature profiles.