Mark R. Powell

Dose-response envelope for Escherichia coli O157:H7.

Escherichia coli O157:H7 is an emerging food and waterborne pathogen in the U.S. and internationally. The objective of this work was to develop a dose-response model for illness by this organism that bounds the uncertainty in the dose-response relationship. No human clinical trial data are available for E. coli O157:H7, but such data are available for two surrogate pathogens: enteropathogenic E. coli (EPEC) and Shigella dysenteriae. E. coli O157:H7 outbreak data provide an initial estimate of the most likely value of the dose-response relationship within the bounds of an envelope defined by beta-Poisson dose-response models fit to the EPEC and S. dysenteriae data. The most likely value of the median effective dose for E. coli O157:H7 is estimated to be approximately 190[emsp4 ]000 colony forming units (cfu). At a dose level of 100[emsp4 ]cfu, the median response predicted by the model is six percent.

Considering the complexity of microbial community dynamics in food safety risk assessment

The potential for competitive inhibition to limit the growth of microbial pathogens in food raises questions about the external validity of typical predictive microbiology studies and suggests the need to consider microbial community dynamics in food safety risk assessment. Ecological theory indicates, however, that community dynamics are highly complex and may be very sensitive to initial conditions and random variation. Seemingly incongruous empirical results for Escherichia coli O157:H7 in ground beef are shown to be consistent with a simple theoretical model of interspecific competition. A potential means of incorporating community-level microbial dynamics into the food safety risk assessment process is explored.

Modeling the Response of the Mediterranean Fruit Fly (Diptera:Tephritidae) to Cold Treatment

Recent interceptions of live Mediterranean fruit fly, Ceratitis capitata (Wiedemann), larvae in fruit that had been cold-treated during transit from abroad led to a reevaluation of the scientific basis for the relevant regulatory treatment schedules. A time-temperature response surface model based on the original experimental data from 1916 was developed and evaluated based on subsequent experimental trials and recent surveillance data collected from shipping operations. The resultant model is reasonably robust and supports the conclusion that the previous treatment schedule falls short of the intended probit nine level of security. Given the vintage of the data, methodological inconsistencies among studies, and the potential consequences of new introductions, additional research is warranted. Quantitative analysis of the currently available data suggests that future studies regarding the efficacy of cold storage should focus on low-temperature, short-duration treatments, where uncertainty about performance appears greatest. The analysis of subsequent experiments also demonstrates that for cold treatment trials most often resulting in zero survivors, Bayesian statistical methods applied to a series of replicated trials of more manageable size offers a feasible alternative to conducting impracticably large trials.

Quantifying the robustness of a broth-based Escherichia coli O157:H7 growth model in ground beef

The robustness of a microbial growth model must be assessed before the model can be applied to new food matrices; therefore, a methodology for quantifying robustness was developed. A robustness index (RI) was computed as the ratio of the standard error of prediction to the standard error of calibration for a given model, where the standard error of calibration was defined as the root mean square error of the growth model against the data (log CFU per gram versus time) used to parameterize the model and the standard error of prediction was defined as the root mean square error of the model against an independent data set. This technique was used to evaluate the robustness of a broth-based model for aerobic growth of Escherichia coli 0157:H7 (in the U.S Department of Agriculture Agricultural Research Service Pathogen Modeling Program) in predicting growth in ground beef under different conditions. Comparison against previously published data (132 data sets with 1,178 total data points) from experiments in ground beef at various experimental conditions (4.8 to 45 degrees C and pH 5.5 to 5.9) yielded RI values ranging from 0.11 to 2.99. The estimated overall RI was 1.13. At temperatures between 15 and 40 degrees C, the RI was close to and smaller than 1, indicating that the growth model is relatively robust in that temperature range. However, the RI also was related (P < 0.05) to temperature. By quantifying the predictive accuracy relative to the expected accuracy, the RI could be a useful tool for comparing various models under different conditions.

Trends in Reported Foodborne Illness in the United States; 1996-2013.

Retrospective review is a key to designing effective food safety measures. The analysis examines trends in the reported incidence of illness due to bacterial pathogens commonly transmitted by food in the United States during 1996-2013 with and without specifying a model form for trend. The findings indicate early declines in reported incidence followed by a period of no significant trend for Campylobacter, Listeria, Shiga toxin-producing Escherichia coli O157, and Yersinia. The results are inconclusive about whether there is no trend or an increasing trend for Salmonella. While Shigella exhibits a continuous decline, Vibrio exhibits a continuous increase. Overall, the findings indicate a lack of evidence for continuous reduction in illness due to bacterial pathogens commonly transmitted by food in the United States during 1996-2013.

Risk Assessment for Invasive Plant Species1

Abstract Various domestic and international initiatives have elevated invasive species issues onto the policy agenda. In the invasive plants arena, risk assessment efforts have focused on classifying the invasive potential of nonindigenous plants. Currently, however, the field of risk assessment for invasive species is in an early stage of development, and there is a lack of broad scientific principles or reliable procedures for identifying the invasive potential of plants in new geographic ranges. Furthermore, identifying potential hazards may be just the first step in a more comprehensive risk assessment. At least for those regulatory decisions that may be disputed internationally or domestically, scientifically ambitious risk assessment for invasive plants is not optional. Therefore, there is a pressing need to formulate adaptable, biologically plausible methods and approaches in this emerging field that strike an appropriate balance between the demand for accuracy and precision in predicting risks and the constraints of limited information, time, and other resources. Additional index words: Regulatory analysis, Sanitary and Phytosanitary Agreement.

Analyzing the power and error of Listeria monocytogenes growth challenge studies

Domestic and international food safety policy developments have spurred interest in the design and interpretation of experimental growth challenge studies to determine whether ready-to-eat (RTE) foods are able to support growth of Listeria monocytogenes. Existing challenge study protocols and those under development differ markedly in terms of experimental design and the acceptance criteria under which a RTE food is determined not to support L. monocytogenes growth. Consequently, the protocols differ substantially with respect to the probability of incorrectly determining that growth occurs and the statistical power to detect growth if it does occur. Applying a fixed acceptance criteria exceedance value (e.g., less than a 0.5 log(10) or 1 log(10) increase) to distinguish real growth from quantitative measurement uncertainty over different experimental designs and/or measurement uncertainty values implies highly inconsistent type I error (alpha) probabilities. None of the L. monocytogenes growth challenge study designs currently being considered are likely to provide an F-test with alpha=0.05 and power >or=0.8 to detect a 1 log(10) increase in mean concentration over the entire range of measurement uncertainty values for enumeration of L. monocytogenes reported in food samples in a validation study of ISO Method 11290-2.