Presence of Listeria monocytogenes in Chilean food matrices

ObjectivesTo compare Listeria monocytogenes strains obtained from food matrices with those from environmental samples in the same food processing plant.Methods and resultsBetween 2008 and 2012 the presence of L. monocytogenes was evaluated in 2647 food samples. A total of 448 work surfaces and 92 equipment's were also evaluated from 6 plants which produce ready-to-eat (RTE) foods in Santiago, Chile. An additional selected sample of hand and nails samples was also obtained from 13 food handlers working in a sausage elaboration plant.As a whole L. monocytogenes was present in 265 (10%) food samples and 22 (4%) environmental samples. The foods with highest recovery were red meats 14/60 (23%), poultry 223/1196 (19%), the remaining samples accounted a total of 27/1391 (2%). The environmental samples positive for L. monocytogenes were obtained from two food plants both the cheese 8/8 (100%) and from a fresh peaches exporter 3/3 (100%). Finally L. monocytogenes was isolated from 5/13 (38%) food handlers studied.ConclusionsThe study confirms the presence of L. monocytogenes in different matrices, especially in meat and RTE products. Analyses conducted on work surfaces revealed that contamination comes mostly from both raw materials and surfaces in indirect contact with foods.Significance and impact of studyThe study reinforces the need for companies to apply regulations related to food quality and safety systems (HACCP, Hazard Analysis & Critical Control Points) to prevent L. monocytogenes contamination from food processing plants.     

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.     

Smoked salmon industry practices and their association with Listeria monocytogenes

This study was conducted to analyse the current practices used by the Scottish smoked salmon industry that will affect the likelihood of Listeria monocytogenes contamination in products. Sixteen visits to smoked salmon premises were conducted between June and November 2011, interviews were carried out based on a questionnaire. The results indicate that most processors carry out appropriate food safety practices, but some improvements are needed in order to minimize the risk of Listeria contamination. It was found that the larger processors achieved better temperature control than the smaller processors. Approximately half of the visited premises needed to improve their refrigerated storage. The risk of ceiling condensation dripping onto product was a common problem, but the smaller premises were the most affected. Small food business operators require additional information on how cleaning and sanitation throughout the process can reduce contamination of the final product. Furthermore, guidance describing the best way of determining shelf life was requested by small processors. Fifty six percent of the smoked salmon processors (mostly large and medium size) tested the product for L. monocytogenes and prevalence ranged widely (0–12%) between processors. Those processors having the highest Listeria prevalence were also those most concerned about what microbiological testing should be carried out and how to evaluate the quality of their products. Most processors rarely exceeded (i.e. once every several years) the statutory limit set by the European Union (>100 cfu/g or presence in 25 g). The small producers did not undertake product testing for Listeria because of high test costs and lack of technical expertise. Hence, it was concluded that sharing expertise between producers, especially to smaller processors would be beneficial in terms of consumer protection.     

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.     

Prevalence and characterization of Listeria monocytogenes isolated from retail food in Henan,China

Listeria monocytogenes, an important foodborne pathogen, is the causal agent of listeriosis. In this study, a total of 954 food samples originating from raw meat, cooked meat products, seafood, and vegetables purchased from supermarkets and open-air markets in Henan province, China, were analyzed for the presence of L. monocytogenes. All L. monocytogenes isolates were subjected to serotyping, pulsed-field gel electrophoresis (PFGE), and antimicrobial resistance. The overall percentage of L. monocytogenes prevalence was 6.2% (n = 59) with the highest rate of 7.4% for cooked meat products followed by raw meat (6.7%). The isolates belonged to five serotypes (1/2a, 1/2b, 1/2c, 4b, and 4c), with serotype 1/2a being predominant (55.9%). PFGE revealed a low genetic diversity among the isolates, irrespective of their sources, suggesting that dominant clones are widespread in different food products in Henan. Resistance to cefotaxime (30.5%) and ciprofloxacin (13.5%) was most often, whereas resistance to tetracycline, trimethoprim/sulfamethoxazole, and erythromycin was observed less frequently. The presence of L. monocytogenes in food products and antimicrobial resistance among the isolates represents a potential public health risk. Our results indicate that effective hygienic measures and bacteriological controls are necessary in China to reduce the contamination of retail food samples by L. monocytogenes.     

Isolation and characterization of Listeria monocytogenes in Chinese food obtained from the central area of China

The objective of this study was to investigate the prevalence and characteristics of Listeria monocytogenes isolated from Chinese food, including frozen dumplings, flavored raw meat, roasted meat, braised meat, and a cold vegetable dish with sauce. A total of 900 food samples were collected from supermarkets, open-air markets, and delicatessens in three large cities in the central area of China to examine the presence of L. monocytogenes; 21 (2.3%) of the samples were positive for this pathogen. Among the different samples, braised meat showed the highest L. monocytogenes detection rate (4.4%). Samples obtained from delicatessens showed a much higher L. monocytogenes contamination rate (8.3%) than those from open-air markets (6.7%) or supermarkets (0%). Multilocus sequence typing (MLST) analysis indicated that the 21 bacterial isolates belonged to 12 ST subgroups. ST5 was the largest and contained 7 isolates (33.3%); it was followed by ST474, ST121 and ST9 (each containing 2 isolates [10.5%]). Antibiotic susceptibility analysis showed that the 21 L. monocytogenes isolates were thoroughly resistant to cefoxitin but highly susceptible to doxycycline and ciprofloxacin. The presence of 10 virulence genes was evaluated by PCR, which showed that inlA, inlC, inlJ, prfA, hlyA, and plcB were present in all isolates and that inlB, actA, plcA and iap were present in 71.4–90.5% of the isolates. This study provides a useful reference for risk assessment and control of L. monocytogenes contamination in Chinese food and for the treatment of clinical listeriosis.     

Adaptive response of Listeria monocytogenes to heat and its impact on food safety

Listeria monocytogenes is an important food associated pathogen because of its relatively high heat resistance and ability to multiply in refrigeration temperatures. Its thermotolerance can be increased when its cells are subjected to heat shock. One- to eight-fold increase of D values of L. monocytogenes have been reported, depending on the heat shock duration, the temperature and the heating menstrum. This acquisition of heat tolerance is related to the induction of the synthesis of heat shock proteins (HSPs).The adaptive response of food pathogens has important consequences on the safety of thermally processed foods. It is believed that this is responsible for the frequent occurrence of deviations (tails and shoulders) during heat treatments that are observed in the exponential model of microbial inactivation. These deviations from log-linear kinetic especially encountered under mild heat treatments, mean that prediction of food safety can no longer rely upon D and z values. Adaptive response to heat must be considered when quantifying and modeling microbial inactivation during thermal processing in order to achieve microbiologically safe products without overly conservative heat processes. Therefore a more mechanistic approach is needed for more accurate predictions of thermal inactivation. Prerequisite to this model are thorough studies to understand how L. monocytogenes and other pathogens adapt their cellular physiology to overcome heat and other stresses.     

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).     

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.     

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.     

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.     

Tracing the food sources of isolated strains of Listeria monocytogenes through fatty acid profiles analysis

Listeria monocytogenes widely exists in different kinds of food. L. monocytogenes infection could induce to a highly fatality ratio up to 30–50%. The objection of this study is to provide a new way to trace the food sources of isolated strains of L. monocytogenes when a foodborne poisoning of L. monocytogenes breaking up. In this paper, the whole-cell fatty acid profiles for the 22 strains were detected by the microorganism identification system (MIS). Results showed that the major fatty acid components of L. monocytogenes were branched acids: anteiso C_15:0, anteiso C_17:0, iso C_15:0, iso C_17:0, iso C_16:0, iso C_14:0, saturated chain C_16:0 and C_14:0 and hydroxy acid C_8:0 3OH. Based on the fatty acid profiles, 22 L. monocytogenes strains were statistically clustered into three major subgroups, which were highly relative to their food sources. Strains in the same cluster were isolated from the same food source, while strains in different clusters were isolated from different food sources.     

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.     

Recent Experiences with Listeria monocytogenes in New Zealand and development of a food control risk-based strategy

A risk-based strategy for Listeria monocytogenes has been developed by New Zealand Food Safety Authority. Incidents of L. monocytogenes in ready-to-eat foods reported to the regulatory authority were analysed to identify common factors that contributed to the contamination. In addition, the current regulatory framework and available food industry guidance were reviewed to prepare a strategy that will provide a consistent and informed approach to managing the risk of L. monocytogenes. Also recognised is the future impact of an aging population and an increase in the availability and range of chilled ready-to-eat foods that should be integrated into the strategy if an objective of ‘no increase in the reported incidence of foodborne listeriosis after five years’ is to be achieved.     

Environmental sampling for Listeria monocytogenes control in food processing facilities reveals three contamination scenarios

Listeria monocytogenes enters the food processing facility via environment, or contaminated raw materials. To increase the understanding of L. monocytogenes environmental contamination in the meat and dairy food sector, six European scientific institutions sampled twelve food processing environments (FPEs) in a harmonized methodological approach. The selection of six previously assumed uncontaminated (UC) FPEs and six contaminated (C) FPEs was based on the L. monocytogenes occurrence information originating from the time prior to the current study. An aim of the study was to highlight, that FPEs regarded for years as uncontaminated, may also become L. monocytogenes contaminated and repeated environmental sampling could help to identify the potential sources of contamination.From a total of 2242 FPE samples, L. monocytogenes was present in 32% and 8.8% of meat and dairy processing environments, respectively. In the actual study, each FPE was contaminated with L. monocytogenes on at least one sampling occasion. Three contamination scenarios could be observed: (i) sporadic contamination in the interface of raw material reception and hygienic areas, (ii) hotspot contamination in the hygienic processing areas (iii), and widely disseminated contamination in the entirely FPE. These data demonstrate that L. monocytogenes are common colonizers of FPEs in the European processing facilities sampled and that a consistent cross-contamination risk exists. To avoid food contamination, a risk assessment approach should assign risk levels to critical control areas (CCAs) and identify those where cross-contamination should be essentially excluded.     

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.     

Influence of food soiling matrix on cleaning and disinfection efficiency on surface attached Listeria monocytogenes

Cleaning and disinfection are essential steps in preventing contamination of foods with pathogenic and spoilage bacteria. The efficacy of cleaning and disinfection products differ depending on target bacteria and type of soiling. We evaluated the effectiveness of cleaning and disinfecting products against Listeria monocytogenes attached on food soiled inert surfaces in a laboratory model. The number of bacteria on surfaces before and after treatment was quantified using indirect conductometric measurements. L. monocytogenes attached to stainless steel surfaces in fish broth systems reached approx. 10⁴ CFU/cm². However, all cleaning and disinfection products were equally effective in this system since all bacteria were removed or killed. When the steel disks were immersed in a fish or meat emulsions, a level of approx. 10⁵–10⁶ L. monocytogenes per cm² was reached after 2–3 days at 20 °C. In this case, 2–3 log were removed or killed by alkaline cleaning products (MC103 or FC140). Direct use of a peracetic acid based disinfectant (Oxivit Active Plus) killed all bacteria when attached in salmon emulsions, whereas only 1–2 log were removed or killed in the meat emulsion system. Thus, the efficiency of cleaning and disinfection products against L. monocytogenes is strongly influenced by the food matrix.     

Listeria monocytogenes isolated from the smoked salmon industry: Growth potential under different environmental conditions

Forty isolates of Listeria monocytogenes were obtained from smoked salmon and production environment in a processing plant. Molecular characterisation evidenced the presence of biotypes corresponding to differences in technological conditions, particularly in salting.The growth behaviour of eight selected strains was evaluated in two media (BHI and Salmon Broth) at different temperatures and salt concentrations. In sub-optimal conditions, the physiological biodiversity of the strains was enhanced, especially in Salmon Broth, underlying the crucial importance of the medium in the evaluation of growth potential.     

Effect of potassium sorbate washing on the growth of Listeria monocytogenes on fresh poultry

This work evaluated the effect of potassium sorbate washing on the growth of Listeria monocytogenes on poultry legs stored at 4 °C for 7 days. Fresh inoculated chicken legs were dipped into either a 2.5% (w/v) or 5% potassium sorbate solution or distilled water (control). Changes in mesophiles, pychrotrophic counts and sensorial characteristics (odor, color, texture and overall appearance) were also evaluated.The shelf life of the samples washed with potassium sorbate was extended by at least 2 days over the control samples washed with distilled water. Legs washed with 5% potassium sorbate showed a significant (p < 0.05) inhibitory effect on L. monocytogenes compared to control legs, with a decrease of about 1.3 log units after 7 days of storage. Sensory quality was not adversely affected by potassium sorbate.