Listeria Monocytogenes In Soft Cheese Research Articles

Occurrence of Listeria spp. in Soft Cheese and Ice Cream: Effect of Probiotic Bifidobacterium spp. on Survival of Listeria monocytogenes in Soft Cheese.

Listeria monocytogenes is one of the most important emerging foodborne pathogens. The objectives of this work were to investigate the incidence of Listeria spp. and L. monocytogenes in soft cheese and ice cream in Assiut city, Egypt, and to examine the effect of some probiotic Bifidobacterium spp. (Bifidobacterium breve, Bifidobacterium animalis, or a mixture of the two) on the viability of L. monocytogenes in soft cheese. The existence of Listeria spp. and L. monocytogenes was examined in 30 samples of soft cheese and 30 samples of ice cream. Bacteriological analyses and molecular identification (using 16S rRNA gene and hlyA gene for Listeria spp. and L. monocytogenes, respectively) were performed on those samples. Additionally, Bifidobacterium spp. were incorporated in the making of soft cheese to study their inhibitory impacts on L. monocytogenes. Out of 60 samples of soft cheese and ice cream, 25 samples showed Listeria spp., while L. monocytogenes was found in only 2 soft cheese samples. Approximately 37% of soft cheese samples (11 out of 30) had Listeria spp. with about 18.0% (2 out of 11) exhibiting L. monocytogenes. In ice cream samples, Listeria spp. was presented by 47% (14 out of 30), while L. monocytogenes was not exhibited. Moreover, the addition of B. animalis to soft cheese in a concentration of 5% or combined with B. breve with a concentration of 2.5% for each resulted in decreasing L. monocytogenes efficiently during the ripening of soft cheese for 28 d. Listeria spp. is widely found in milk products. Probiotic bacteria, such as Bifidobacterium spp., can be utilized as a natural antimicrobial to preserve food and dairy products.

Starch-halloysite nanocomposites containing nisin: Characterization and inhibition of Listeria monocytogenes in soft cheese

Abstract Starch/halloysite/nisin nanocomposite films were developed as active antimicrobial packaging. Nanocomposites without nisin and control films (containing starch and glycerol only) were also prepared. Scanning electron microscopy revealed that all samples were homogeneous, halloysite nanotubes (HNT) were dispersed in starch matrix, and film surfaces denoted aggregates as higher amount of nisin was added. X-ray diffraction spectra displayed alterations in typical starch peaks after HNT and nisin incorporation, indicating a decrease in polymer crystallization. Mechanical properties were improved when HNT was incorporated, but inferior values of Young modulus and tensile strength were obtained with nisin addition. Thermal stability decreased for bionanocomposites containing nisin, which showed T max values about 20 °C lower than films without nisin. The antimicrobial activity was tested against Listeria monocytogenes, Clostridium perfringens and Staphylococcus aureus in skimmed milk agar and all microorganisms were inhibited by nanocomposites containing nisin. These active films were applied on Minas Frescal cheese surface previously inoculated with L. monocytogenes . After 4 days, antimicrobial nanocomposite films with 2 g/100 g nisin significantly reduced the initial counts of the bacterium and those with 6 g/100 g nisin completely inhibited L. monocytogenes . Results showed that nisin supported in starch/halloysite films can be an active and useful barrier to control food contamination.

European validation of a real-time PCR-based method for detection of Listeria monocytogenes in soft cheese

The classical microbiological method for detection of Listeria monocytogenes requires around 7days for final confirmation, and due to perishable nature of RTE food products, there is a clear need for an alternative methodology for detection of this pathogen. This study presents an international (at European level) ISO 16140-based validation trial of a non-proprietary real-time PCR-based methodology that can generate final results in the following day of the analysis. This methodology is based on an ISO compatible enrichment coupled to a bacterial DNA extraction and a consolidated real-time PCR assay. Twelve laboratories from six European countries participated in this trial, and soft cheese was selected as food model since it can represent a difficult matrix for the bacterial DNA extraction and real-time PCR amplification. The limit of detection observed was down to 10CFU per 25 of sample, showing excellent concordance and accordance values between samples and laboratories (>75%). In addition, excellent values were obtained for relative accuracy, specificity and sensitivity (82.75%, 96.70% and 97.62%, respectively) when the results obtained for the real-time PCR-based methods were compared to those of the ISO 11290-1 standard method. An interesting observation was that the L. monocytogenes detection by the real-time PCR method was less affected in the presence of Listeria innocua in the contaminated samples, proving therefore to be more reliable than the reference method. The results of this international trial demonstrate that the evaluated real-time PCR-based method represents an excellent alterative to the ISO standard since it shows a higher performance as well as reduce the extent of the analytical process, and can be easily implemented routinely by the competent authorities and food industry laboratories.

Sensitivity Analysis for Critical Control Points Determination and Uncertainty Analysis to Link FSO and Process Criteria: Application to Listeria monocytogenes in Soft Cheese Made from Pasteurized Milk

Microbiological food safety is an important economic and health issue in the context of globalization and presents food business operators with new challenges in providing safe foods. The hazard analysis and critical control point approach involve identifying the main steps in food processing and the physical and chemical parameters that have an impact on the safety of foods. In the risk-based approach, as defined in the Codex Alimentarius, controlling these parameters in such a way that the final products meet a food safety objective (FSO), fixed by the competent authorities, is a big challenge and of great interest to the food business operators. Process risk models, issued from the quantitative microbiological risk assessment framework, provide useful tools in this respect. We propose a methodology, called multivariate factor mapping (MFM), for establishing a link between process parameters and compliance with a FSO. For a stochastic and dynamic process risk model of Listeriamonocytogenes in soft cheese made from pasteurized milk with many uncertain inputs, multivariate sensitivity analysis and MFM are combined to (i) identify the critical control points (CCPs) for L.monocytogenes throughout the food chain and (ii) compute the critical limits of the most influential process parameters, located at the CCPs, with regard to the specific process implemented in the model. Due to certain forms of interaction among parameters, the results show some new possibilities for the management of microbiological hazards when a FSO is specified.

Risk‐Based Approach for Microbiological Food Safety Management in the Dairy Industry: The Case of Listeria monocytogenes in Soft Cheese Made from Pasteurized Milk

According to Codex Alimentarius Commission recommendations, management options applied at the process production level should be based on good hygiene practices, HACCP system, and new risk management metrics such as the food safety objective. To follow this last recommendation, the use of quantitative microbiological risk assessment is an appealing approach to link new risk-based metrics to management options that may be applied by food operators. Through a specific case study, Listeria monocytogenes in soft cheese made from pasteurized milk, the objective of the present article is to practically show how quantitative risk assessment could be used to direct potential intervention strategies at different food processing steps. Based on many assumptions, the model developed estimates the risk of listeriosis at the moment of consumption taking into account the entire manufacturing process and potential sources of contamination. From pasteurization to consumption, the amplification of a primo-contamination event of the milk, the fresh cheese or the process environment is simulated, over time, space, and between products, accounting for the impact of management options, such as hygienic operations and sampling plans. A sensitivity analysis of the model will help orientating data to be collected prioritarily for the improvement and the validation of the model. What-if scenarios were simulated and allowed for the identification of major parameters contributing to the risk of listeriosis and the optimization of preventive and corrective measures.

A survey of soft cheeses in Greek retail outlets highlights a low prevalence of Listeria spp.

Recently, the European Food Safety Authority proposed that each EU member state should conduct a survey of Listeria monocytogenes in soft cheeses for sale in retail outlets. Hence, retail samples of soft cheeses in Greece (n = 137) were analyzed qualitatively and quantitatively for both L. monocytogenes and other Listeria spp., as well as for pH and a w. None of the cheeses analyzed were found positive for L. monocytogenes (0%; 95% confidence interval (CI) = 0.0–2.2%). Three samples (2.2%; 95% CI = 0.5–6.3%) were positive for other Listeria spp., with populations ranging from <5 to 4.5 × 102 CFU g−1. Although soft cheeses are regarded as foods that permit the proliferation of L. monocytogenes, 15.4% of the whey cheeses tested had pH values ≤4.4 and 29% of the interior mold-ripened cheeses had a w values ≤0.92. Such low pH values for whey cheeses are unexpected, based on their manufacturing technology, and are associated with lower quality and reduced shelf life. Nonetheless, in the present survey, the percentage of whey cheeses in the Greek retail market exhibiting unusually low pH values or testing positive for Listeria spp. was much lower compared to the respective percentages reported from studies conducted in the 1990s. The absence of L. monocytogenes in the tested cheeses undoubtedly constitutes an encouraging result with respect to the safety of soft cheeses in the Greek retail market. However, the presence of other Listeria spp. in three of the tested samples implies that L. monocytogenes would, most likely, also be present in the food-processing or retail-handling environments of these products. Therefore, cheese manufacturers and retail handlers should continue or even intensify the application of all necessary measures with the aim of preventing food contamination.

The international risk governance council framework and its application to Listeria monocytogenes in soft cheese made from unpasteurised milk

The International Risk Governance Council (IRGC) developed a Risk Governance Framework whose purpose is to help policy makers, regulators and risk managers both understand the concept of risk governance and apply it to their handling of risks. The Framework allows input from the scientific and public health perspective as well as the less-easily measurable side of freedom of choice. For food, the public wants many choices and less regulation, but at the same time safe products to consume. These ideals are at times in conflict. One example of this is the demand for soft cheese made from unpasteurised milk which may be contaminated with L. monocytogenes or other pathogens. Distinguishing between simple, complex, uncertain and ambiguous risks can help to design a risk management strategy. Public health advocates typically see management of soft cheese made from unpasteurised milk as a simple risk (protection of health), whereas for a small but vociferous proportion of the public sees it more of an uncertain or ambiguous one (choice trumps the slight risk of illness). The implications of five management options for the cheese are discussed. The new Codex Guidelines tend to put these cheeses into Option 1, a strict control on the presence of the pathogen, which will be ignored by the cheese aficionados. Other options are worth exploring to give some more choice but at low risk.

Detection of Listeria monocytogenes in Soft-Cheese and Sweet Yogurt Produced Locally in Baghdad

In order to investigate the presence of Listeria monocytogenes in soft cheese and sweet yogurt producedlocally, this study was done by random collection of samples from different areas in Baghdad province and itssurrounding from different animals (Cows and Buffalos) as 55 total samples (47 soft –cheese samples and 8sweet-yogurt samples) from October 2007 till January 2008 and processed according to standard protocols ofListeria monocytogenes .The results showed isolation of 6 isolates from 55 samples(10.9%) as 5 isolates from 47 soft – cheese samples(10.638%) and 1 isolated from 8 sweet yogurt samples (12.5%),these revealed significant differences (P≤0.05) inisolation percentages between Cows and Buffalos .Samples of raw –dairy products from local Buffaloes recordedhighest isolation percentage of Listeria monocytogenes as 73.5%(23.5% represented 4 isolates from 17 soft –cheese samples and 50% represented 1 isolate from 2 yogurt samples) highest percentage of isolation of Listeriamonocytogenes from soft cheese samples occurred in January (2008) as 18.2% (2 isolates from 11samples)especially from Buffalos in Al-Taji region as 33.4% (1 isolate from 3 samples ) while ,highest isolationpercentage of Listeria monocytogenes from yogurt occurred in December (2007)as 50%,1 isolate from 2 samplesespecially from Buffaloes at Al-Fadhyllea region.We concluded from this study by contamination of soft –cheese and sweet-yoghurt samples produced locallyin Baghdad by Listeria monocytogenes .

Non starter lactic acid bacteria biofilms: A means to control the growth of Listeria monocytogenes in soft cheese

The possibility to consider non starter lactic acid bacteria (NSLAB) biofilms as a means to control the growth of Listeria monocytogenes in soft cheeses was evaluated. In particular, the aptitude to form biofilm of four NSLAB strains ( Lactobacillus plantarum DSM1055, Lactobacillus casei DSM20011, Lactobacillus curvatus DSM20019 and Lactobacillus paracasei DSM20207) was investigated. All tested strains were able to form biofilm on stainless steel and the highest quantities were produced when NSLAB were present simultaneously (about 6 Log CFU cm −2). Then, experimental cheeses were made in presence of chips containing 7-days NSLAB biofilms and inoculated with L. monocytogenes (about 2 Log CFU g −1). Results demonstrated that NSLAB biofilms can be considered a useful means to delay the growth of L. monocytogenes in soft cheeses: the maximum cell load attained at the stationary phase was about 6 Log CFU g −1 in experimental cheeses against about 7 Log CFU g −1 observed in control samples.

Development of a membrane filtration method for enumeration of Listeria monocytogenes from soft cheese

A new and simple analytical method has been developed to quantify low levels (≤50 cfu g−1) of Listeria monocytogenes in soft cheese. The technique allows the analysis of 1 g of cheese instead of 0·1 g or 0·01 g using the ISO 11290-2 standard method. The analysis protocol combines filtration of the decimally diluted cheese suspension through a 0·45-μm pore size cellulose ester membrane, and a culture of the filter on a Palcam agar. A tween 80–trypsin treatment used to increase filterability of cheese did not reduce L. monocytogenes counts. The tested method provided more precise results (nearer to the true value) compared to the ISO 11290-2 standard method in the enumeration of L. monocytogenes from artificially contaminated cheese. However, it improves neither repeatability nor reproducibility since the selected medium, Palcam, does not allow distinction between the different Listeria species.

Enhanced sensitivity in PCR detection of Listeria monocytogenes in soft cheese through use of an aqueous two-phase system as a sample preparation method.

A sample treatment method based on an aqueous two-phase system containing polyethylene glycol and dextran was developed for enhancing sensitivity in the detection of Listeria monocytogenes in soft cheese with PCR. The results suggest that the improved detection sensitivity following partitioning of the cheese homogenate in an aqueous two-phase system may be due to partitioning of the PCR inhibitors to the polyethylene glycol phase.

Use of the polymerase chain reaction for direct detection of Listeria monocytogenes in soft cheese

The polymerase chain reaction (PCR) amplification technique was investigated as a tool for direct detection of Listeria monocytogenes in soft cheeses. Different sets of oligonucleotide primers were used, and parts of the L. monocytogenes Dth 18-gene could be amplified specifically when either a plasmid vector carrying the cloned gene or chromosomal DNA was used a template. The detection limit for L. monocytogenes in dilutions of pure cultures was between 1 and 10 colony-forming units. In extracts from soft cheeses containing L. monocytogenes DNA, the amplification was strongly inhibited. This inhibition could be reduced by an additional purification step. Despite this the detection limit showed a large variation, depending on the brand of cheese used. In some cheeses 10(3) cfu/0.5g could be visualized whereas in others the presence of 10(8) cfu/0.5 g did not yield a detectable quantity of amplified product.

The occurrence of Listeria monocytogenes in soft cheeses and raw milk and its resistance to heat

Listeria monocytogenes was detected by a colony count procedure in 7 out of 69 samples of imported soft cheese; numbers varied between 10 3 and 10 6/g. The organism could be detected by enrichment in another 3 samples. All samples containing L. monocytogenes had been prepared from raw milk. L. monocytogenes was detected in 6 out of 137 samples of raw milk obtained in The Netherlands. Heat resistance experiments carried out in open tubes suggested that L. monocytogenes could survive heating at 80°C for 1 min. These results could not be confirmed in heat resistance experiments with small impermeable plastic bags, enabling total submersion of the sample in the water bath. These experiments showed that L. monocytogenes inoculated at a level of 1.8 × 10 4/ml did not survive heating at 67°C for 20 s or more. Since these latter results were confirmed in experiments with a plate pasteurizer and the validity of results of open tube tests is questionable, it is concluded that normal pasteurization of milk would prevent the contamination of (soft) cheeses with Listeria spp., provided that recontamination during production and ripening is prevented.