Determination of Listeria monocytogenes Growth during Mushroom Production and Distribution

The Gamma concept approach as a tool to predict fresh produce supporting or not the growth of L. monocytogenes

Challenge tests are commonly employed to evaluate the growth behavior of L. monocytogenes in food matrices; they are known for being expensive and time-consuming. An alternative could be the use of predictive models to forecast microbial behavior under different conditions. In this study, the growth behavior of L. monocytogenes in different fresh produce was evaluated using a predictive model based on the Gamma concept considering pH, water activity (aw), and temperature as input factors. An extensive literature search resulted in a total of 105 research articles selected to collect growth/no growth behavior data of L. monocytogenes. Up to 808 L. monocytogenes behavior values and physicochemical characteristics were extracted for different fruits and vegetables. The predictive performance of the model as a tool for identifying the produce commodities supporting the growth of L. monocytogenes was proved by comparing with the experimental data collected from the literature. The model provided satisfactory predictions on the behavior of L. monocytogenes in vegetables (>80% agreement with experimental observations). For leafy greens, a 90% agreement was achieved. In contrast, the performance of the Gamma model was less satisfactory for fruits, as it tends to overestimate the potential of acid commodities to inhibit the growth of L. monocytogenes.

Growth of Listeria monocytogenes in fresh vegetables and vegetable salad products: An update on influencing intrinsic and extrinsic factors

The ability of foodborne pathogens to grow in food products increases the associated food safety risks. Listeria monocytogenes (Lm) is a highly adaptable pathogen that can survive and grow under a wide range of environmental circumstances, including otherwise inhibitory conditions, such as restrictive cold temperatures. It can also survive long periods under adverse environmental conditions. This review examines the experimental evidence available for the survival and growth of Lm on fresh vegetables and ready-to-eat vegetable salads. Published data indicate that, depending on certain intrinsic (e.g., nutrient composition) and extrinsic factors (e.g., storage temperature, packaging atmosphere), Lm can survive on and in a wide variety of vegetables and fresh-cut minimally processed vegetable salads. Studies have shown that temperature, modified atmosphere packaging, relative humidity, pH, water activity, background microbiota of vegetables, microbial strain peculiarities, and nutrient type and availability can significantly impact the fate of Lm in vegetables and vegetable salads. The influence of these factors can either promote its growth or decline. For example, some studies have shown that background microbiota inhibit the growth of Lm in vegetables and minimally processed vegetable salads, but others have reported a promoting, neutral, or insignificant effect on the growth of Lm. A review of relevant literature also indicated that the impact of most influencing factors is related to or interacts with other intrinsic or extrinsic factors. This literature synthesis contributes to the body of knowledge on possible strategies for improving food safety measures to minimize the risk of Lm-associated foodborne outbreaks involving vegetables and vegetable salads.

Variability in growth and biofilm formation of Listeria monocytogenes in Agaricus bisporus mushroom products

Foods and food production environments can be contaminated with Listeria monocytogenes and may support growth of this foodborne pathogen. This study aims to characterize the growth and biofilm formation of sixteen L. monocytogenes strains, isolated from mushroom production and processing environments, in filter-sterilized mushroom medium. Strain performance was compared to twelve L. monocytogenes strains isolated from other sources including food and human isolates. All twenty-eight L. monocytogenes strains showed rather similar growth performance at 20 °C in mushroom medium, and also significant biofilm formation was observed for all strains. HPLC analysis revealed the presence of mannitol, trehalose, glucose, fructose and glycerol, that were all metabolized by L. monocytogenes, except mannitol, in line with the inability of L. monocytogenes to metabolize this carbohydrate. Additionally, the growing behavior of L. monocytogenes was tested on whole, sliced and smashed mushroom products to quantify performance in the presence of product-associated microbiota. A significant increase of L. monocytogenes was observed with higher increase of counts when the mushroom products were more damaged, even with the presence of high background microbiota counts. This study demonstrated that L. monocytogenes grows well in mushroom products, even when the background microbiota is high, highlighting the importance to control (re)contamination of mushrooms.

Modeling the Fate of Listeria monocytogenes and Salmonella enterica on Fresh Whole and Chopped Wood Ear and Enoki Mushrooms

Two recent foodborne illness outbreaks linked to specialty mushrooms have occurred in the United States, both representing novel pathogen-commodity pairings. Listeria monocytogenes and Salmonella enterica were linked to enoki and wood ear mushrooms, respectively. The aim of this study was therefore to examine the survival of both L. monocytogenes and S. enterica on raw whole and chopped enoki and wood ear mushrooms during storage at different temperatures. Fresh mushrooms were either left whole or chopped and subsequently inoculated with a cocktail of either S. enterica or rifampicin-resistant L. monocytogenes, resulting in an initial inoculation level of 3 log CFU/g. Mushroom samples were stored at 5, 10, or 25°C for up to 7 d. During storage, the population levels of S. enterica or L. monocytogenes on the mushrooms were enumerated. The primary Baranyi model was used to estimate the growth rates of both pathogens and the secondary Ratkowsky square root model was used to model the relationship between growth rates and temperature. Both L. monocytogenes and S. enterica survived on both mushroom types and preparations at all temperatures. No proliferation of either pathogen was observed on mushrooms stored at 5°C. At 10°C, moderate growth was observed for both pathogens on enoki mushrooms and for L. monocytogenes on wood ear mushrooms; no growth was observed for S. enterica on wood ear mushrooms. At 25°C, both pathogens proliferated on both mushroom types with growth rates ranging from 0.43 to 3.27 log CFU/g/d, resulting in 1 log CFU/g increases in only 0.31 d (7.44 h) to 2.32 d. Secondary models were generated for L. monocytogenes on whole wood ear mushrooms and S. enterica on whole enoki mushrooms with goodness-of-fit parameters of r² = 0.9855/RMSE = 0.0479 and r² = 0.9882/RMSE = 0.1417, respectively. Results from this study can aid in understanding the dynamics of L. monocytogenes and S. enterica on two types of specialty mushrooms.

Microflora and umami alterations of different packaging material preserved mushroom (Flammulina filiformis) during cold storage

In order to clarify the effect of nanocomposite-based packaging (NP) on umami and microflora characteristics of F. filiformis during cold storage, the contents of umami amino acids and 5'-nucleotides, equivalent umami concentration (EUC), and microflora succession were investigated. Results showed that NP could delay the degradation of umami components and inhibit bacterial growth in F. filiformis. At the initial stage, the dominant bacteria were Lactobacillus, Thermus and Acinetobacter. After 15 days of storage, the bacteria count in NP reached 7.63 lg cfu/g, which was significantly (P < 0.05) lower than that in control, and the major bacterial communities of packaged F. filiformis were Ewingella, Serratia and Pseudomonas. Moreover, the correlation analysis showed that Lactobacillus, Brevibacillus and Okibacterium were negatively correlated with AMP and IMP 5-nucleotides. Present work suggested that NP could enhance the umami flavor formation and improve the microbial community structure of F. filiformis, resulting in a better commercial quality. The results provided theoretical basis for large-scale applications of NP.

Microbiological safety of ready-to-eat fresh-cut fruits and vegetables sold on the Canadian retail market

Following implementation of Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods by Canadian food safety authorities in 2011, a four-year study (2012-2016) was carried out to gain baseline information on the occurrence of bacterial pathogens, notably the prevalence and levels of Listeria monocytogenes (L. monocytogenes) in various product types of ready-to-eat (RTE) fresh-cut fruits and fresh-cut vegetables sold at retail in Canada. A total of 10,070 pre-packaged samples, including 4691 fresh-cut fruit and 5379 fresh-cut vegetable samples were collected from retail stores across Canada and analyzed for bacterial pathogens and generic Escherichia coli (E. coli). Salmonella species (spp.), E. coli O157:H7, Shigella and Campylobacter were not detected in any of the tested samples. L. monocytogenes was identified in 0.51% (95% CI [0.34, 0.76]) of the fresh-cut fruit and in 0.24% (95% CI [0.14, 0.41]) of the fresh-cut vegetable samples. Of the 37 L. monocytogenes positive samples identified, levels of L. monocytogenes <5 CFU/g, 5-<100 CFU/g, and ≥100 CFU/g were found in 67.6% (25/37), 24.3% (9/37) and 8.1% (3/37) of the samples, respectively. The results of this study indicate that the vast majority of fresh-cut fruits and vegetables sold on the Canadian retail market are safe for consumption. However, contamination by L. monocytogenes can infrequently occur in fresh-cut fruits and vegetables, with certain types of fresh-cut fruits (i.e., melons, apples) and vegetables (i.e., mushrooms, cauliflower) being more likely to be contaminated than others. Safe handling practices are recommended for producers, retailers and consumers including storage at refrigerated temperatures.

Sources and survival of Listeria monocytogenes on fresh, leafy produce

Listeria monocytogenes is an intracellular human pathogen which enters the body through contaminated food stuffs and is known to contaminate fresh leafy produce such as spinach, lettuce and rocket. Routinely, fresh leafy produce is grown and processed on a large scale before reaching the consumer through various products such as sandwiches and prepared salads. From farm to fork, the fresh leafy produce supply chain (FLPSC) is complex and contains a diverse range of environments where L. monocytogenes is sporadically detected during routine sampling of produce and processing areas. This review describes sources of the bacteria in the FLPSC and outlines the physiological and molecular mechanisms behind its survival in the different environments associated with growing and processing fresh produce. Finally, current methods of source tracking the bacteria in the context of the food supply chain are discussed with emphasis on how these methods can provide additional, valuable information on the risk that L. monocytogenes isolates pose to the consumer.

Diversity of Listeria monocytogenes strains isolated from Agaricus bisporus mushroom production

AIMS

The aims of this study were to characterize the genetic diversity of Listeria monocytogenes isolates obtained from commercial mushroom production, to establish the persistence, recontamination and the risk of cross-contamination from the working environment to the final products, creating awareness about the presence of L. monocytogenes thus helping to prevent the possibility of cross-contamination.

METHODS AND RESULTS

From an extensive analysis of commercial mushroom production, analysed with BS EN ISO 11290-1:1996/Amd 1:2004 and BS EN ISO 11290-2:1998/Amd 1:2004, 279 L. monocytogenes isolates were obtained. All of the isolates were characterized by pulsed-field gel electrophoresis, species PCR and serogroup PCR. All the isolates were confirmed as L. monocytogenes; 30·1% were serogroup 1/2b-3b-7, 40·8% were serogroup 1/2a-3a and 29·1% were serogroup 4b-4d-4e. There were 77 pulsotypes from the 279 isolates, 40 of the pulsotypes had only one strain and 37 had two or more strains, indicating great diversity in the isolates.

CONCLUSIONS

The high genetic diversity is indicative of the fact that current hygiene practices are successful at removing L. monocytogenes but that recontamination of the production environment is frequent.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained are very valuable in creating awareness of L. monocytogenes in mushroom production and for the improvement of hygiene practices.

Inhibition of Listeria monocytogenes biofilms by bacteriocin-producing bacteria isolated from mushroom substrate

AIMS

This study was designed to investigate the ability of naturally occurring bacteria isolated from mushroom substrate to prevent biofilm formation by Listeria monocytogenes or to remove existing biofilms in mushroom production facilities.

METHODS AND RESULTS

It is generally recognized that L. monocytogenes forms biofilms that can facilitate its survival in food-processing environments. Eleven bacteriocin-producing isolates were identified and the bacteriocins characterized based on heat and enzyme inactivation studies. Further characterization was undertaken by MALDI-TOF mass spectrometry, PCR and sequencing. Production of nisin Z (by Lactococcus lactis isolates), subtilomycin (by Bacillus subtilis isolates) and lichenicidin (by Bacillus licheniformis and Bacillus sonorensis isolates) was detected. In co-culture with L. monocytogenes, the bacteriocin-producing strains could prevent biofilm formation and reduce pre-formed biofilms.

CONCLUSIONS

Mushroom substrate can be a source of bacteriocin-producing bacteria that can antagonize L. monocytogenes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results highlight the potential of bacteriocin-producing strains from mushroom substrate to reduce L. monocytogenes biofilm in food production environments, contributing to a reduction in the risk of food contamination from the environment.

Assessment of Primary Production of Horticultural Safety Management Systems of Mushroom Farms in South Africa

Growing global consumer concern over food safety in the fresh produce industry requires producers to implement necessary quality assurance systems. Varying effectiveness has been noted in how countries and food companies interpret and implement food safety standards. A diagnostic instrument (DI) for global fresh produce industries was developed to measure the compliancy of companies with implemented food safety standards. The DI is made up of indicators and descriptive grids for context factors and control and assurance activities to measure food safety output. The instrument can be used in primary production to assess food safety performance. This study applied the DI to measure food safety standard compliancy of mushroom farming in South Africa. Ten farms representing almost half of the industry farms and more than 80% of production were independently assessed for their horticultural safety management system (HSMS) compliance via in-depth interviews with each farm's quality assurance personnel. The data were processed using Microsoft Office Excel 2010 and are represented in frequency tables. The diagnosis revealed that the mushroom farming industry had an average food safety output. The farms were implementing an average-toadvanced HSMS and operating in a medium-risk context. Insufficient performance areas in HSMSs included inadequate hazard analysis and analysis of control points, low specificity of pesticide assessment, and inadequate control of suppliers and incoming materials. Recommendations to the industry and current shortcomings are suggested for realization of an improved industry-wide food safety assurance system.

Predominance and Distribution of a Persistent Listeria monocytogenes Clone in a Commercial Fresh Mushroom Processing Environment

A longitudinal study was conducted to determine the prevalence of Listeria spp. in a commercial fresh mushroom slicing and packaging environment. Samples were collected at three different sampling periods within a 13-month time interval. Of the 255 environmental samples collected, 18.8% tested positive for L. monocytogenes, 4.3% for L. innocua, and 2.0% for L. grayi. L. monocytogenes was most often found on wet floors within the washing and slicing and packaging areas. Each of the 171 L. monocytogenes isolates found in the environment could be placed into one of three different serotypes; 1/2c was predominant (93.6%), followed by 1/2b (3.5%) and 1/2a (2.9%). Of 58 isolates subtyped using multi-virulence-locus sequence typing, all 1/2c isolates were identified as virulence type (VT) 11 (VT11), all 1/2b isolates were VT105, and 1/2a isolates were either VT107 or VT56. VT11 was designated as the predominant and persistent clone in the environment because it was isolated repeatedly at numerous locations throughout the study. The overall predominance and persistence of VT11 indicates that it likely colonized the mushroom processing environment. Areas adjacent to the trench drain in the washing and slicing area and a floor crack in the packaging area may represent primary harborage sites (reservoirs) for VT11. Improvements made to sanitation procedures by company management after period 2 coincided with a significant (P ≤ 0.001) reduction in the prevalence of L. monocytogenes from 17.8% in period 1 and 30.7% in period 2 to 8.5% in period 3. This suggests that targeted cleaning and sanitizing procedures can be effective in minimizing the occurrence of L. monocytogenes contamination in processing facilities. Additional research is needed to understand why VT11 was predominant and persistent in the mushroom processing environment.