Marie Laure Delignette-Muller

Accuracy of microbial growth predictions with square root and polynomial models.

The results of growth predictions using square root and polynomial models published in 14 papers were studied. Errors on quantities of practical interest such as lag time, generation time or the time required to reach a given increase in number of cells, are analyzed. The distribution of these errors was examined with the perspective of the practical use of predictive models in food industry. Highly unsafe predictions and significant average errors were observed in some cases. A good knowledge of predictive models accuracy seems essential for their efficient and safe use, for example to predict the shelf life of a product. Yet, authors generally gave no pragmatic information on such things as the average relative error or the range of errors on predicted variables. Problems of robustness of models when tested in different conditions were noticed, which corroborates the necessity of a systematic validation of models on new data.

What Is the Relevance of Obtaining Multiple Blood Samples for Culture? A Comprehensive Model to Optimize the Strategy for Diagnosing Bacteremia

Through a heuristic and probabilistic approach, we evaluated blood culture operating characteristics (sensitivity, specificity, and predictive values) as a function of several pretest parameters, together with their variability. On the basis of a meta-analysis of quantitative data from the literature, a model was developed and an estimation of the operating characteristics through numerical simulations (Monte Carlo method) was performed. The model evaluates the influence of ordering and drawing parameters on the ability of blood culture to distinguish bacteremic from nonbacteremic patients, regardless of the causative species. By considering the total blood volume to be cultured (six 5-10-mL bottles), results were found to confirm the current guidelines. On the basis of this hypothesis, the results, together with an analysis of the literature, failed to show any benefit of a strategy that involves obtaining multiple samples. The best strategy when performing blood culture is to obtain blood for 6 bottles (for a total volume of 35-42 mL), preferably at the same time.

Growth and survival of non‐O157:H7 Shiga‐toxin‐producing Escherichia coli in cow manure

Aims:  The main objective of this study was to evaluate the behaviour of non‐O157:H7 Shiga‐toxin‐producing Escherichia coli (STEC) strains in cow manure.

Biological variability and exposure assessment.

Predictive models are now commonly used for exposure assessment, with growth parameters defined for each microbial species. In this study, we tried to take into account microbial growth variability among strains of a single species. Bacillus cereus in pasteurized milk was chosen to illustrate the influence of the biological variability on the outcome of exposure assessment. Each parameter of the exposure assessment (growth parameters, shelf-life conditions) was characterized by a probability distribution describing variability and/or uncertainty. The impact of the intra-species variability on the result of the exposure assessment was then quantified and discussed. Two simple domestic shelf life conditions were tested. The results confirm that the biological variability has a great impact on the accuracy of the result and should not be systematically neglected.

Relation between the generation time and the lag time of bacterial growth kinetics.

In predictive microbiology, the relation between the lag time (Lag) and the generation time (Tg) is commonly assumed to be proportional, as long as the pre-incubation environmental conditions remain constant. This relation was statistically examined in nine published datasets. For every dataset, it was roughly proportional. However, a more advanced study showed that the ratio Lag/Tg was not totally independent of the environmental conditions. In particular, a significant negative effect of the pH on this ratio was observed in five of the nine datasets. For modeling the environmental dependence of microbial growth parameters, some authors independently deal with Lag and Tg. Other authors only model the environmental dependence of Tg, assuming Lag/Tg to be constant. These two modeling methods were statistically compared for the nine datasets under study. Results differed from one dataset to another. For some, the model developed with a constant ratio Lag/Tg sufficed to describe the data, whereas for the others, an independent modeling of Lag and Tg was more satisfactory.

Supplementation of enrichment broths by novobiocin for detecting Shiga toxin‐producing Escherichia coli from food: a controversial use

Aims:  To investigate the assumption that usage of novobiocin (20 mg l−1) in Shiga toxin‐producing Escherichia coli (STEC) enrichment broths could achieve false‐negative results.

Quantitative Risk Assessment of Listeria monocytogenes in French Cold-Smoked Salmon: II. Risk Characterization

A model for the assessment of exposure to Listeria monocytogenes from cold-smoked salmon consumption in France was presented in the first of this pair of articles (Pouillot et al., 2007, Risk Analysis, 27:683-700). In the present study, the exposure model output was combined with an internationally accepted hazard characterization model, adapted to the French situation, to assess the risk of invasive listeriosis from cold-smoked salmon consumption in France in a second-order Monte Carlo simulation framework. The annual number of cases of invasive listeriosis due to cold-smoked salmon consumption in France is estimated to be 307, with a very large credible interval ([10; 12,453]), reflecting data uncertainty. This uncertainty is mainly associated with the dose-response model. Despite the significant uncertainty associated with the predictions, this model provides a scientific base for risk managers and food business operators to manage the risk linked to cold-smoked salmon contaminated with L. monocytogenes. Under the modeling assumptions, risk would be efficiently reduced through a decrease in the prevalence of L. monocytogenes or better control of the last steps of the cold chain (shorter and/or colder storage during the consumer step), whereas reduction of the initial contamination levels of the contaminated products and improvement in the first steps of the cold chain do not seem to be promising strategies. An attempt to apply the recent risk-based concept of FSO (food safety objective) on this example underlines the ambiguity in practical implementation of the risk management metrics and the need for further elaboration on these concepts.

Quantitative risk assessment for Escherichia coli O157:H7 in frozen ground beef patties consumed by young children in French households

A quantitative risk assessment for Escherichia coli O157:H7 in frozen ground beef patties consumed by children under 10 years of age in French households was conducted by a national study group describing an outbreak which occurred in France in 2005. Our exposure assessment model incorporates results from French surveys on consumption frequency of ground beef patties, serving size and consumption preference, microbial destruction experiments and microbial counts on patties sampled from the industrial batch which were responsible for the outbreak. Two different exposure models were proposed, respectively for children under the age of 5 and for children between 5 and 10 years. For each of these two age groups, a single-hit dose-response model was proposed to describe the probability of hemolytic and uremic syndrome (HUS) as a function of the ingested dose. For each group, the single parameter of this model was estimated by Bayesian inference, using the results of the exposure assessment and the epidemiological data collected during the outbreak. Results show that children under 5 years of age are roughly 5 times more susceptible to the pathogen than children over 5 years. Exposure and dose-response models were used in a scenario analysis in order to validate the use of the model and to propose appropriate guidelines in order to prevent new outbreaks. The impact of the cooking preference was evaluated, showing that only a well-done cooking notably reduces the HUS risk, without annulling it. For each age group, a relation between the mean individual HUS risk per serving and the contamination level in a ground beef batch was proposed, as a tool to help French risk managers.

Modeling and predicting the simultaneous growth of Escherichia coli O157:H7 and ground beef background microflora for various enrichment protocols.

ABSTRACT The simultaneous growth of Escherichia coli O157:H7 (O157) and the ground beef background microflora (BM) was described in order to characterize the effects of enrichment factors on the growth of these organisms. The different enrichment factors studied were basal medium (Trypticase soy broth and E. coli broth), the presence of novobiocin in the broth, and the incubation temperature (37°C or 40°C). BM and O157 kinetics were simultaneously fitted by using a competitive growth model. The simple competition between the two microfloras implied that O157 growth stopped as soon as the maximal bacterial density in the BM was reached. The present study shows that the enrichment protocol factors had little impact on the simultaneous growth of BM and O157. The selective factors (i.e., bile salts and novobiocin) and the higher incubation temperature (40°C) did not inhibit BM growth, and incubation at 40°C only slightly improved O157 growth. The results also emphasize that when the level of O157 contamination in ground beef is low, the 6-h enrichment step recommended in the immunomagnetic separation protocol (ISO EN 16654) is not sufficient to detect O157 by screening methods. In this case, prior enrichment for approximately 10 h appears to be the optimal duration for enrichment. However, more experiments must be carried out with ground beef packaged in different ways in order to confirm the results obtained in the present study for non-vacuum- and non-modified-atmosphere-packed ground beef.

Optimal growth temperature of O157 and non-O157 Escherichia coli strains.

Aims: There are several biological characteristics that differ between Escherichia coli O157:H7, a dangerous food‐borne pathogen, and the other serotypes of E. coli.