Foodborne Bacterial Pathogens in Retail Prepacked Ready-to-Eat Mixed Ingredient Salads

A worldwide review of illness outbreaks involving mixed salads/dressings and factors influencing product safety and shelf life

The trends toward healthy living, vegetarianism, and busy schedules have increased salad popularity. Salads are usually consumed raw without any thermal treatment, and therefore, without proper care they can become major vehicles for foodborne illness outbreaks. This review examines the microbial quality of 'dressed' salads which contain two or more vegetables/fruits and salad dressings. The possible sources of ingredient contamination, recorded illnesses/outbreaks, and overall microbial quality observed worldwide, besides the antimicrobial treatments available are discussed in detail. Noroviruses were most frequently implicated in outbreaks. Salad dressings usually play a positive role in influencing microbial quality. However, this depends on several factors like the type of contaminating microorganism, storage temperature, dressing pH and ingredients, plus the type of salad vegetable. Very limited literature exists on antimicrobial treatments that can be used successfully with salad dressings and 'dressed' salads. The challenge with antimicrobial treatments is to find ones sufficiently broad in spectrum, compatible with produce flavour which can be applied at competitive cost. It is evident that renewed emphasis on prevention of produce contamination at the producer, processor, wholesale and retail levels plus enhanced hygiene vigilance at foodservice will have a major impact on reducing the risk of foodborne illnesses from salads.

Food safety risk perceptions and mitigation techniques in the dumpster diving community in Sweden

An online questionnaire was used to investigate behaviors and habits relating to food retrieval by Swedish dumpster divers. Respondents were also asked to share information on their background, reasons for engaging in dumpster diving, and perceptions of potential bacterial hazards associated with the consumption of discarded foods. The questionnaire was mainly distributed to Swedish social media groups focusing on dumpster diving, and a total of 92 responses were received. The most common reason given for engaging in dumpster diving was to reduce food waste and associated negative environmental impacts (60%, n = 55). Many of the respondents (65%) had completed university education, and around three-quarters (73%) had some form of employment or were enrolled in studies. Half the respondents (52%, n = 48) perceived a risk of falling ill after consuming food obtained through dumpster diving, but very few (2%) reported they had been made ill by food they had picked up from dumpsters. Around one-fifth (22%) of the respondents did not know of any bacteria that could cause foodborne infections or food poisoning. Salmonella was mentioned by 35% of the respondents, while Listeria was mentioned by 18%. The respondents reported employing various techniques to reduce the risk of encountering harmful microbiological agents, for example, rinsing their food retrievals or discarding food that appeared spoiled. These are novel findings on dumpster divers' perceptions of perceived health risks and the strategies they use to mitigate such risks with food obtained through dumpster diving.

Wholegenome sequencing as the gold standard approach for control of Listeria monocytogenes in the food chain

Listeria monocytogenes has been implicated in numerous outbreaks and related deaths of listeriosis. In food production, L. monocytogenes occurs in raw food material and above all, through postprocessing contamination. The use of next-generation sequencing technologies such as whole-genome sequencing (WGS) facilitates foodborne outbreak investigations, pathogen source tracking and tracing geographic distributions of different clonal complexes, routine microbiological/epidemiological surveillance of listeriosis, and quantitative microbial risk assessment. WGS can also be used to predict various genetic traits related to virulence, stress, or antimicrobial resistance, which can be of great benefit for improving food safety management as well as public health.

Assessment of the Microbiological Quality of Ready-to-Eat Salads-Are There Any Reasons for Concern about Public Health?

Ready-to-eat food products can be readily consumed without further preparation and are convenient for busy on-the-go consumers. The objective of the study was to assess the microbiological quality of ready-to-eat salads. Thirty RTE salads were tested for the presence of bacteria, yeasts, and molds using the TEMPO and agar plate method. The study demonstrated that most of the tested products were characterized by varying microbiological quality. The total number of mesophilic microbiotas was about 6 log CFU g^-1. The high number of microorganisms was due to yeast and molds or Enterobacteriaceae. Half of the salads were contaminated with E. coli and three salads were contaminated with S. aureus. LAB were also found, which can be explained mainly by a dairy ingredient. In some salads, Salmonella spp. and L. monocytogenes were detected (26.7% and 33.3% of the samples, respectively). Based on the conducted tests, it was found that the microbiological quality was not satisfactory. The results presented in this study indicate that there is a significant problem of the presence of pathogens. Manufacturers should strive to reduce the possibility of microbial contamination through the use of widely understood hygiene of the production process, using hurdle technology, including the modified atmosphere and refrigerated storage.

Effect of Non-Thermal Atmospheric Plasma on Food-Borne Bacterial Pathogens on Ready-to Eat Foods: Morphological and Physico-Chemical Changes Occurring on the Cellular Envelopes

Currently, there is a need for new technological interventions to guarantee the microbiological safety of ready-to-eat (RTE) foods. Non-thermal atmospheric plasma (NTAP) has emerged as a promising strategy for inactivating microorganisms on thermo-sensitive foods, and the elucidation of its mechanisms of action will aid the rational optimization and industrial implementation of this technology for potential applications in the food industry. In this study, the effectiveness of NTAP for inactivating strains of Salmonella Enteritidis, Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes contaminating the surface of different sliced RTE foods (“chorizo”, salami, bacon, smoked salmon, tofu and apple) was investigated. In addition, to further assess the bacterial inactivation mechanisms of NTAP, the morphological and physico-chemical damages in bacterial cells were analyzed. NTAP was effective for the surface decontamination of all products tested and, especially, of cut apple, where the microbial populations were reduced between 1.3 and 1.8 log units for the two Salmonella strains and E. coli O157: H7, respectively, after 15 min of exposure. In the rest of foods, no significant differences in the lethality obtained for the E. coli O157:H7 strain were observed, with inactivation rates of between 0.6 and 0.9 log cycles after a 15-min treatment. On the other hand, the strains from the rest of pathogenic microorganisms studied were extremely resistant on tofu, where barely 0.2–0.5 log units of inactivation were achieved after 15 min of plasma exposure. S. Enteritidis cells treated for 10 min exhibited noticeable morphological and structural changes, as observed by transmission electron microscopy, which were accompanied by a loss in membrane integrity, with an increased leakage of intracellular components and uptake of propidium iodide and marked changes in regions of their FTIR spectra indicating major alterations of the cell wall components. Overall, this indicates that loss of viability was likely caused for this microorganism by a significant damage in the cellular envelopes. However, the plasma-treated cells of L. monocytogenes did not show such obvious changes in morphology, and exhibited less marked effects on the integrity of their cytoplasmic membrane, what suggests that the death of this pathogenic microorganism upon NTAP exposure is more likely to occur as a consequence of damages in other cellular targets.

Cross-border outbreak of Yersinia enterocolitica O3 associated with imported fresh spinach, Sweden and Denmark, March 2019

In April 2019, a cross-border outbreak of Yersinia entercolitica O3 was identified in Sweden and Denmark and confirmed using whole genome sequencing. Close cross-border collaboration with representatives from human and food authorities helped direct resources and investigations. Combined epidemiological and trace-back investigations pointed to imported fresh spinach as the outbreak vehicle and highlight that other vehicles of Y. enterocolitica outbreaks than pork should be considered.

Differentiation of stx1A gene for detection of Escherichia coli serotype O157: H7 and Shigella dysenteriae type 1 in food samples using high resolution melting curve analysis

Escherichia coli serotype O157: H7 and Shigella dysenteriae type 1 as the Shiga toxin-producing bacteria cause some acute gastrointestinal and extraintestinal diseases such as hemorrhagic uremic syndrome and bloody diarrhea in human. Stx genes are the key virulence factors in these pathogens. The aim of this study was to develop HRMA assay to differentiate stx1A gene for detection of E. coli serotype O157: H7 and Sh. dysenteriae type 1 and determine the prevalence of these pathogens in food samples using this method. PCR-HRMA assay and gold standard methods have been carried out for identification of pathogens among 135 different food samples. We found HRMA method a sensitive and specific assay (100 and 100%, respectively) for differentiation of stx1A gene, consequently, detection of these pathogens in food samples. Also, the highest prevalence of E. coli serotype O157: H7 and Sh. dysenteriae type 1 harboring stx1A gene was observed in raw milk and vegetable salad samples, respectively. HRMA as a rapid, inexpensive, sensitive and specific method is suggested to be used for differentiation of stx1A gene to detect E. coli serotype O157: H7 and Sh. dysenteriae type 1 as the key pathogens for safety evaluation of food samples.

Hypervirulent Listeria monocytogenes clones' adaption to mammalian gut accounts for their association with dairy products

Listeria monocytogenes (Lm) is a major human and animal foodborne pathogen. Here we show that hypervirulent Lm clones, particularly CC1, are strongly associated with dairy products, whereas hypovirulent clones, CC9 and CC121, are associated with meat products. Clone adaptation to distinct ecological niches and/or different food products contamination routes may account for this uneven distribution. Indeed, hypervirulent clones colonize better the intestinal lumen and invade more intestinal tissues than hypovirulent ones, reflecting their adaption to host environment. Conversely, hypovirulent clones are adapted to food processing environments, with a higher prevalence of stress resistance and benzalkonium chloride tolerance genes and a higher survival and biofilm formation capacity in presence of sub-lethal benzalkonium chloride concentrations. Lm virulence heterogeneity therefore reflects the diversity of the ecological niches in which it evolves. These results also have important public health implications and may help in reducing food contamination and improving food consumption recommendations to at-risk populations.

A comprehensive review on the prevalence, pathogenesis and detection ofYersinia enterocolitica

Food safety is imperative for a healthy life, but pathogens are still posing a significant life threat. “Yersiniosis” is caused by a pathogen named Yersinia enterocolitica and is characterized by diarrheal, ileitis, and mesenteric lymphadenitis types of sicknesses. This neglected pathogen starts its pathogenic activity by colonizing inside the intestinal tract of the host upon the ingestion of contaminated food. Y. enterocolitica remains a challenge for researchers and food handlers due to its growth habits, low concentrations in samples, morphological similarities with other bacteria and lack of rapid, cost-effective, and accurate detection methods. In this review, we presented recent information about its prevalence, biology, pathogenesis, and existing cultural, immunological, and molecular detection approaches. Our ultimate goal is to provide updated knowledge regarding this pathogen for the development of quick, effective, automated, and sensitive detection methods for the systematic detection of Y. enterocolitica.

Food safety is imperative for a healthy life, but pathogens are still posing a significant life threat.

Determination of Temperature Dependent Growth Parameters in Psychrotrophic Pathogen Bacteria and Tentative Use of Mean Kinetic Temperature for the Microbiological Control of Food

Temperature is the main factor to control the microbial growth in perishable foods. The psychrotrophic pathogen bacteria are microorganisms of concern for food products with extended shelf life in chilling conditions. The aims of this work were two. Firstly, to evaluate growth behavior of Aeromonas hydrophila DSM-30187, Listeria monocytogenes DSM-20600, and Yersinia enterocolitica DSM-27689 strains, at different temperatures (4, 7, 10, 15, 25, and 30°C) and starting cell concentrations (10 and 106 CFU/mL), in order to determine the activation energies (E a) of the relevant lag phases and growth rates. Secondly, to investigate if Mean Kinetic Temperature (MKT) might be applied in recording temperature devices to alert a thermal abuse in a management control system for food safety. As expected, lag phase and growth rate proved to be heavily affected by temperature whereas the inoculum size did not. The E a values involved in the duration of latent periods, calculated on the basis of the Arrhenius model, were comparable for A. hydrophila and L. monocytogenes strains (from 21.3 to 24.4 kcal/mol), while significantly differed for Y. enterocolitica (16.6 kcal/mol). The E a values of growth rates were similar for A. hydrophila and L. monocytogenes strains (from 20.9 to 21.1 kcal/mol), while were considerably lower for Y. enterocolitica (from 14.2 to 16.7 kcal/mol). The use of MKT is widespread and well-accepted in pharmaceutical field as convenient method for estimating drugs degradation in relation to storage temperature. The E a value of the lag phase found for L. monocytogenes (23.9 ± 1.2 kcal/mol) was included in the MKT formula. In this work, the air temperature of two chilling rooms was monitored during the normal operating activity in a catering company for a period of 8 months. The MKT profiles were then compared with those of mean temperatures in different conditions (short or prolonged events of thermal abuse) with the purpose to evaluate if it may be applicable to reduce false alarms without lowering the safety level of stored food.

The prevalence of Campylobacter spp. in vegetables, fruits, and fresh produce: a systematic review and meta-analysis

There are a number of reports indicating correlation between outbreaks of campylobacteriosis and the consumption of raw vegetables. This study is a meta-analysis on the prevalence of Campylobacter in fresh vegetables and fruits without any location limitation, which was performed through a documented review of the available resources. Relevant literature was reviewed by trained reviewers, who examined the results for the inclusion of articles in the meta-analysis. The prevalence of Campylobacter in raw vegetables, the sample source, the Campylobacter species, and the method of detection were extracted. The prevalence of Campylobacter in vegetables, fruits, and fresh produce were estimated to be 0.53%. Analysis of the various sample groups initially showed that the bean and sprouts group was the vegetable with the highest prevalence of Campylobacter (11.08%). The rate of contamination was higher when both the molecular and conventional methods were employed. The highest prevalence of Campylobacter was found in Asia (33.4%). Despite the low prevalence, consumption of raw vegetables is inherently risky because no treatment is used to inactivate the pathogens. Therefore, proper sanitation methods are recommended to treat the raw products.

Is your lunch salad safe to eat? Occurrence of bacterial pathogens and potential for pathogen growth in pre-packed ready-to-eat mixed-ingredient salads

As part of a trend toward healthy convenience foods, ready-to-eat (RTE) mixed-ingredient salads have become popular products among consumers. A mixed-ingredient salad contains combinations of raw (e.g. leafy vegetables and tomatoes) and processed (e.g. chicken, salmon, ham, pasta and couscous) ingredients. Contamination of leafy vegetables can occur during any step in the production chain and, since there is no step that kills pathogens, a completely safe final product can never be guaranteed. Meat ingredients, for example poultry meat and ham, are generally heat-treated before preparation, but may be contaminated after this treatment, e.g. when diced or sliced. When several ingredients are mixed together, cross-contamination may occur. Preparation of mixed-ingredient salads requires human handling, which presents an additional risk of bacterial contamination. With high-protein ingredients, e.g. cooked meat, the mixed-ingredient salad represents an excellent substrate for bacterial growth. This article reviews current knowledge regarding human bacterial pathogen prevalence in mixed-ingredient salads and the potential for pathogen growth in this product during storage.

A review on microbiological decontamination of fresh produce with nonthermal plasma

Food safety is a critical public health issue for consumers and the food industry because microbiological contamination of food causes considerable social and economic burdens on health care. Most foodborne illness comes from animal production, but as of the mid-1990s in the United States and more recently in the European Union, the contribution of fresh produce to foodborne outbreaks has rapidly increased. Recent studies have suggested that sterilization with nonthermal plasma could be a viable alternative to the traditional methods for the decontamination of heat-sensitive materials or food because this technique proves capable of eliminating micro-organisms on surfaces without altering the substrate. In the last 10 years, researchers have used nonthermal plasma in a variety of food inoculated with many bacterial species. All of these experiments were conducted exclusively in a laboratory and, to our knowledge, this technique has not been used in an industrial setting. Thus, the purpose of this review is to understand whether this technology could be used at the industrial level. The latest researches using nonthermal plasma on fresh produce were analysed. These evaluations have focused on the log reduction of micro-organisms and the treatment time.

Fate of Listeria monocytogenes , Pathogenic Yersinia enterocolitica , and Escherichia coli O157:H7 gfp+ in Ready-to-Eat Salad during Cold Storage: What Is the Risk to Consumers?

In this study, we investigated the fate of Listeria monocytogenes , pathogenic Yersinia enterocolitica , and Escherichia coli O157:H7 gfp⁺ inoculated in low numbers into ready-to-eat baby spinach and mixed-ingredient salad (baby spinach with chicken meat). Samples were stored at recommended maximum refrigerator temperature (8°C in Sweden) or at an abuse temperature (15°C) for up to 7 days. Mixed-ingredient salad supported considerable growth when stored at 15°C during shelf life (3 days), with populations of L. monocytogenes , pathogenic Y. enterocolitica , and E. coli O157:H7 gfp⁺ increasing from less than 2.0 log CFU/g on day 0 to 7.0, 4.0, and 5.6 log CFU/g, respectively. However, when mixed-ingredient salad was stored at 8°C during shelf life, only L. monocytogenes increased significantly, reaching 3.0 log CFU/g within 3 days. In plain baby spinach, only pathogenic Y. enterocolitica populations increased significantly during storage for 7 days, and this was exclusively at an abuse temperature (15°C). Thus, mixing ready-to-eat leafy vegetables with chicken meat strongly influenced levels of inoculated strains during storage. To explore the food safety implications of these findings, bacterial numbers were translated into risks of infection by modeling. The risk of listeriosis (measured as probability of infection) was 16 times higher when consuming a mixed-ingredient salad stored at 8°C at the end of shelf life, or 200,000 times higher when stored at 15°C, compared with when consuming it on the day of inoculation. This indicates that efforts should focus on preventing temperature abuse during storage to mitigate the risk of listeriosis. The storage conditions recommended for mixed-ingredient salads in Sweden (maximum 8°C for 3 days) did not prevent growth of L. monocytogenes in baby spinach mixed with chicken meat. Manufacturers preparing these salads should be aware of this, and recommended storage temperature should be revised downwards to reduce the risk of foodborne disease.

Detection of Listeria spp. in food handling areas of retail food stores in the state of Pernambuco, Brazil

Abstract The identification of Listeria spp. in food handling areas is of great concern to health surveillance agencies, and their control is often hampered by the ability of the bacteria to grow and maintain themselves even under adverse conditions. The present study aimed to isolate and identify Listeria spp. in the food handling areas of 10 retail food stores in the state of Pernambuco, Brazil. Eighty-six swab samples were collected from equipment, utensils and surfaces used for processing ready-to-eat meat products. The Dry and Wet Swabbing Methods (3M™ Quick Swabs) and 3M™ Petrifilm™ Plates were used to identify Listeria spp. Contamination by Listeria monocytogenes was confirmed by the Real-time Polymerase Chain Reaction (qPCR). The hygienic and sanitary conditions of the food handling areas of each store were also assessed. Listeria spp. was isolated in eight stores (80%). Of the 86 swab samples analyzed, 27 (31.2%) [confidence interval 21.81% to 42.30%] were positive for Listeria spp. and only one (3.7%) was confirmed as Listeria monocytogenes. The main contamination sites were the floor (50.0%), the plastic cutting board (42.9%) and the knife (40.0%). None of the hygienic and sanitary conditions assessed in the present study were associated with contamination by Listeria spp. (p = 0.700). It was concluded that Listeria spp. was widely distributed in the retail food stores studied, being a possible risk factor for public health.