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Quantitative risk assessment model of human salmonellosis linked to the consumption of Camembert cheese made from raw milk.

Almabrouk Fares
Abstract : Salmonellae are one of the most important causes of foodborne illness associated with raw dairy products. The assessment of the real risk associated with the consumption of these products is needed and the most appropriate method to achieve this goal is the risk analysis process which links pathogens in food to the public health problem. The main aim of this thesis is to quantitatively assess the risk of human salmonellosis linked to the consumption of Camembert cheese made from raw milk. A data gap that is routinely identified in risk assessment is the lack of quantitative data on pathogens contaminated food. Therefore, as a first objective of this thesis, we developed a rapid, sensitive and reliable method for the quantification of Salmonella in artificially contaminated milk samples. The method combined the principles of most-probable-number (MPN) method with a real-time PCR assay. With this developed assay (MPN-real-time PCR) low levels of Salmonella (1-5 CFU/mL) in milk could be enumerated after 8 h of non-selective enrichment in buffered peptone water. All estimated MPN counts corresponded well to the estimated contamination level of Salmonella inoculated into milk samples. In order to evaluate the utility of this developed quantification assay, our second objective was to apply it to naturally contaminated bulk tank milk samples collected from dairy farms located in western France. Eight (2.68%) of 299 bulk tank milk samples were found positive, with estimated MPN values ranging from 3.7 to 79.2 MPN/mL of milk. Despite the PCR inhibitors that were apparently present in some raw bulk tank milk samples, the application of the MPN-real-time PCR assay for quantifying Salmonella in raw milk proved to be rapid, easy-to-perform and highly sensitive. In the assessment of potential risks associated with Salmonella in raw milk and raw milk products it was necessary to examine the ability of Salmonella to grow in milk. Therefore, we presented in this thesis as a third v objective, primary and secondary models describing mathematically the growth of two Salmonella strains (S. Typhimurium and S. Montevideo) in milk under constant temperatures during different incubation periods. The primary logistic-with-delay model was used to describe Salmonella growth as a function of time. The specific growth rates of S. Typhimurium and S. Montevideo varied according to serotype and temperature. The maximum growth rates were then modeled as function of temperature using the secondary cardinal Rosso model. The reported cardinal estimates obtained with S. Typhimurium and S. Montevideo were: Tmin 3.02, 3.40; Topt 38.44, 38.55 and Tmax 44.51, 46.97°C, respectively. At the optimum growth temperature (Topt) the maximum growth rates were 1.36 and 1.39 log10 CFU/h-1 for S. Typhimurium and S. Montevideo respectively. Both the primary and secondary models fitted growth data well with a high-pseudo R2 (0.97-99). Finally, a quantitative risk assessment of human salmonellosis linked to the consumption of Camembert cheese made from raw milk is presented. Different distributions were assumed for the parameters of the model and a Monte Carlo simulation was used to model the process and to quantify the risk associated with the consumption of 25 g serving of cheese. The 99th percentile of Salmonella cell numbers in servings of 25 g of cheese was 5 cells at the time of consumption, corresponding to 0.2 cells of Salmonella per gram. The risk of salmonellosis per 25 g serving varied from 0 to 1.2 × 10-7 with a median of 7.4 × 10-8. For 100 million servings of 25g, the expected number of cases of salmonellosis predicted by the model is in average of 7.4. When the prevalence was reduced in the model by a factor of 10, the number of cases per 100 million servings was reduced to less than 1 case. Despite the limitations and the data gap, we demonstrated the benefit of risk assessment not only as a risk evaluation tool but also as a helping device in the decision-making and the risk management.
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Submitted on : Thursday, February 28, 2008 - 8:00:00 AM
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Almabrouk Fares. Quantitative risk assessment model of human salmonellosis linked to the consumption of Camembert cheese made from raw milk.. Life Sciences [q-bio]. AgroParisTech, 2007. English. ⟨NNT : 2007AGPT0051⟩. ⟨pastel-00003463⟩

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