Design of challenge testing experiments to assess the variability of Listeria monocytogenes growth in foods

The assessment of the evolution of micro-organisms naturally contaminating food must take into account the variability of biological factors, food characteristics and storage conditions. A research project involving eight French laboratories was conducted to quantify the variability of growth parameters of Listeria monocytogenes obtained by challenge testing in five food products. The residual variability corresponded to a coefficient of variation (CV) of approximately 20% for the growth rate (μ_max) and 130% for the parameter K = μ_max × lag. The between-batch and between-manufacturer variability of μ_max was very dependent on the food tested and mean CV of approximately 20 and 35% were observed for these two sources of variability, respectively. The initial physiological state variability led to a CV of 100% for the parameter K. It appeared that repeating a limited number of three challenge tests with three different batches (or manufacturers) and with different initial physiological states seems often necessary and adequate to accurately assess the variability of the behavior of L. monocytogenes in a specific food produced by a given manufacturer (or for a more general food designation).     

Use of titanium dioxide (TiO2) photocatalysts as alternative means for Listeria monocytogenes biofilm disinfection in food processing

The aim of this work was to study the photocatalytic activity of titanium dioxide (TiO₂) against Listeria monocytogenes bacterial biofilm. Different TiO₂ nanostructured thin films were deposited on surfaces such as stainless steel and glass using the doctor-blade technique. All the surfaces were placed in test tubes containing Brain Heart (BH) broth and inoculated with L. monocytogenes. Test tubes were then incubated for 10 days at 16 °C in order to allow biofilm development. After biofilm formation, the surfaces were illuminated by ultraviolet A light (UVA; wavelength of 315-400 nm). The quantification of biofilms was performed using the bead vortexing method, followed by agar plating and/or by conductance measurements (via the metabolic activity of biofilm cells). The presence of the TiO₂ nanoparticles resulted in a fastest log-reduction of bacterial biofilm compared to the control test. The biofilm of L. monocytogenes for the glass nanoparticle 1 (glass surface modified by 16% w/v TiO₂) was found to have decreased by 3 log CFU/cm² after 90 min irradiation by UVA. The use of TiO₂ nanostructured photocatalysts as alternative means of disinfecting contaminated surfaces presents an intriguing case, which by further development may provide potent disinfecting solutions. Surface modification using nanostructured titania and UV irradiation is an innovative combination to enhance food safety and economizing time and money.     

Antibacterial activity of extracts from some edible plants commonly consumed in Asia

Extracts of edible plants (26 species) from China, Japan, Thailand and Yemen were screened for their antibacterial activity against Bacillus cereus, Staphylococcus aureus, Listeria monocytogenes, Escherichia coli and Salmonella infantis. Buffered methanol (80% methanol and 20% PBS) and acetone extracted inhibitory substances against tested bacteria from 16 plants, as revealed by the disc assay. The minimum inhibitory concentrations (MICs) of extracts determined by the agar dilution method ranged from 165 to 2640 mg l⁻¹. The most sensitive microorganism to extracts from Azadirachta indica, Cinnamomum cassia, Rumex nervosus, Ruta graveolens, Thymus serpyllum and Zingiber officinale was B. cereus, with MIC of 165 to 660 mg l⁻¹. E. coli and S. infantis were only inhibited by Cinnamomum cassia extracts at the highest MIC (2640 mg l⁻¹). L. monocytogenes (Tottori) was more resistant than the ATCC 7644 strain to extracts from Ruta chalepensis, Artemisia absinthium and Cissus spp. EDTA (0.85 mM) reduced the MICs of Cinnamomum cassia and Cissus rotundifolia by at least 50% when tested against E. coli, S. infantis, S. aureus and L. monocytogenes.     

Development of a Safety Monitoring and Assurance System for chilled food products

The principles of a novel chill chain management policy, coded Safety Monitoring and Assurance System (SMAS) for the optimisation of the distribution of chilled food products within the chill chain are developed. In this system, a new approach based on actual risk evaluation at important points of the chill chain is used in order to promote products to the next stage of distribution. This evaluation based on product's time–temperature history, variation in product's characteristics (e.g. a_w, pH, etc.), and the use of predictive models for the growth of food pathogens, allows to give priority to products in such a way that risk at consumption time is minimized. The effectiveness of SMAS was evaluated against the First In First Out (FIFO) approach, the current method for food distribution, in a case study on the risk of listeriosis of cooked ham using the Monte Carlo simulation technique. Furthermore, the two approaches were compared for their effect on the quality of the products in terms of remaining shelf life at the time of consumption. The results showed that following the SMAS approach the risk of listerisosis is significantly lower while the spoiled products at the time of consumption are significantly reduced compared to FIFO approach.     

Efficacy of citric acid against Listeria monocytogenes attached to poultry skin during refrigerated storage

The aim of this study was to evaluate the effect of citric acid washing on the growth of Listeria monocytogenes on poultry legs stored at 4 °C for 8 days. Fresh inoculated chicken legs were dipped into either a 0.052, 0.104 or 0.156  m citric acid solution for 5 min or distilled water (control). Surface pH values, sensorial characteristics and L . monocytogenes , mesophiles and psychrotrophs counts were evaluated. Legs washed with 0.156  m citric acid for 5 min showed a significant ( P  < 0.05) inhibitory effect on L . monocytogenes compared with control legs, being about 1.55 log units lower in the first ones than in control legs after 1 day of storage. Treatments with 0.156  m citric acid reduced bacterial growth and preserved reasonable sensorial quality after storage at 4 °C for 8 days.     

Rapid detection of Listeria monocytogenes in food using culture enrichment combined with real-time PCR

A rapid method for the detection of Listeria monocytogenes in foods combining culture enrichment and real-time PCR was compared to the ISO 11290-1 standard method. The culture enrichment component of the rapid method is based on the ISO standard and includes 24 h incubation in half-Fraser broth, 4 h incubation in Fraser broth followed by DNA extraction and real-time PCR detection of the ssrA gene of L. monocytogenes. An internal amplification control, which is co-amplified with the same primers as the L. monocytogenes DNA, was also included in the assay. The method has a limit of detection of 1–5 CFU/25 g food sample and can be performed in 2 working days compared to up to 7 days for the ISO standard. A variety of food samples from retail outlets and food processing plants (n = 175) and controls (n = 31) were tested using rapid and conventional methods. The rapid method was 99.44% specific, 96.15% sensitive and 99.03% accurate when compared to the standard method. This method has the potential to be used as an alternative to the standard method for food quality assurance providing rapid detection of L. monocytogenes in food.     

Differences in pressure tolerance of Listeria monocytogenes strains are not correlated with other stress tolerances and are not based on differences in CtsR

Thirty strains of Listeria monocytogenes were screened for their pressure tolerance phenotype at 400 MPa for 2 min at 21 °C. The strains exhibited reductions ranging from 1.9 to 7.1log₁₀ CFU/ml in tryptic soy broth with 6% yeast extract (TSBYE). The 3 most and the 3 least pressure-tolerant strains were further tested for their thermal resistance (based on their ability to survive at 55 °C), acid tolerance (based on their ability to survive in acidified TSBYE; pH 2.0) and for their nisin sensitivity. No correlation between pressure tolerance and heat, acid or nisin resistances was found. Nucleotide sequence analysis of the ctsR region in these 6 strains demonstrated that this gene codes for a CtsR protein with identical predicted amino acid sequences. The sequences of the 200-bp region located immediately upstream of the ctsR start codon of the different strains were virtually identical and it is therefore likely that differences in pressure tolerance are based on factors other than the stress gene regulator CtsR. The pressure sensitivity of a cocktail of the 2 most pressure-resistant strains and a cocktail of the 2 most-sensitive strains was investigated when the cocktails were inoculated into a real food system consisting of ground chicken meat. We demonstrated that the nature of the suspending substrate or the temperature did not change the expected pressure tolerance of the cocktails.     

Survival of acid-adapted or non-adapted Salmonella Enteritidis, Listeria monocytogenes and Escherichia coli O157:H7, in traditional Greek salads

The fate of three pathogens Salmonella Enteritidis, Listeria monocytogenes and Escherichia coli O157:H7 that were inoculated in fish roe salad and aubergine salad with or without preservatives after being adapted in acid environment or not, was determined. The salads were stored at 10  ° C and the pathogens population was counted at regular intervals. Parameters (lag time, death rates calculated with Baranyi equation) were used to compare the behaviour of the pathogens. In the absence of preservatives the pathogens survived during the 15 days of storage. A 1 log reduction was observed for Listeria and 2 logs reduction for Salmonella and E. coli in both salads. In most cases, acid adaptation decreased the death rate even in the presence of preservatives. The addition of sorbic and benzoic acid in the salads increased the death rate of the pathogens during storage significantly and they were not detected at 7–10 days for Salmonella , 8–12 days for Listeria and 5 days for E. coli . It is concluded that a well-studied combination of hurdles is appropriate to ensure safety of home-made traditional salads free of preservatives.     

Prevalence of Listeria monocytogenes in delicatessen food products in China

This study was to investigate the prevalence of Listeria monocytogenes in delicatessen food, raw materials and environmental samples in food processing chain. Two hundred and forty-five samples of delicatessen foods in markets, 98 samples of cooked foods from restaurants and hotels, 154 samples of food product contact surfaces such as counters, iceboxes and chopping blocks etc. from processing and selling sites, 51 environmental samples from the food-cooking rooms of restaurants and hotels were collected and detected for the prevalence of L. monocytogenes. The results showed that there were 32 strains isolated from delicatessen foods in the market and the average prevalence was 13.06% which is significantly higher (p < 0.01) than those isolated from cooked foods in hotels and restaurants (1.02%) suggesting that the delicatessen food products may be contaminated during the delivery from hotels and restaurants to the markets. Ten strains were isolated from 335 raw materials and 21 strains were isolated from 154 processing equipments in selling and processing sites. No strains were isolated from 51 samples of equipments in cooked food rooms of hotels and restaurants. The study shows that the prevalence of L. monocytogenes in delicatessen foods in the market was significantly higher than those in the cooked foods of hotels and restaurants, and therefore, the critical control points were: (1) to establish relative closed selling rooms; (2) to establish the sterilizing measures to keep the delicatessens from being contaminated with L. monocytogenes during the delivery from restaurants or hotels to the retail markets.