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Nuts have been identified as a vector for salmonellosis. The objective of this project was to estimate the prevalence and contamination level of Salmonellain raw tree nuts (cashews, pecans, hazelnuts, macadamia nuts, pine nuts, and walnuts) at retail markets in the United States. A total of 3,656 samples of six types of tree nuts were collected from different types of retail stores and markets nationwide between October 2014 and October 2015. These samples were analyzed using a modified version of the Salmonellaculture method from the U.S. Food and Drug Administration's Bacteriological Analytical Manual. Of the 3,656 samples collected and tested, 32 were culturally confirmed as containing Salmonella. These isolates represented 25 serotypes. Salmonellawas not detected in pecans and in-shell hazelnuts. Salmonellaprevalence estimates (and 95% confidence intervals) in cashews, shelled hazelnuts, pine nuts, walnuts, and macadamia nuts were 0.55% [0.15, 1.40], 0.35% [0.04, 1.20], 0.48% [0.10, 1.40], 1.20% [0.53, 2.40], and 4.20% [2.40, 6.90], respectively. The rates of Salmonellaisolation from major or big chain supermarkets, small chain supermarkets, discount, variety, or drug stores, and online were 0.64% [0.38, 1.00], 1.60% [0.80, 2.90], 0.00% [0.00, 2.40], and 13.64% [2.90, 35.00], respectively (Cochran-Mantel-Haenszel test: P= 0.02). The rates of Salmonellaisolation for conventional and organic nuts were not significantly different. Of the samples containing Salmonella,60.7% had levels less than 0.003 most probable number (MPN)/g. The highest contamination level observed was 0.092 MPN/g. The prevalence and levels of Salmonellain these tree nut samples were comparable to those previously reported for similar foods.

The purpose of the present study was to determine the heat resistance of six non-O157 Shiga toxin-producing Escherichia coli(STEC) serotypes in comparison to E. coliO157:H7 in single-strength apple juice without pulp. The thermal parameters for stationary-phase and acid-adapted cells of E. colistrains from serogroups O26, O45, O103, O111, O121, O145, and O157:H7 were determined by using an immersed coil apparatus. The most heat-sensitive serotype in the present study was O26. Stationary- phase cells for serotypes O145, O121, and O45 had the highest D56°C-value among the six non-O157 serotypes studied, although all were significantly lower (P< 0.05) than that of E. coliO157:H7. At 60°C E. coliO157:H7 and O103 demonstrated the highest D-values (1.37 ± 0.23 and 1.07 ± 0.03 min, respectively). The D62°Cfor the most heat-resistant strain belonging to the serotype O145 was similar (P> 0.05) to that for the most resistant O157:H7 strain (0.61 ± 0.17 and 0.60 ± 0.09 min, respectively). The heat resistance for stationary-phase cells was generally equal to or higher than that of acid-adapted counterparts. Although E. coliO157:H7 revealed D-values similar to or higher than the individual six non-O157 STEC serotypes in apple juice, the z-values for most non-O157 STEC tested strains were greater than those of E. coliO157:H7. When data were used to calculate heat resistance parameters at a temperature recommended in U.S. Food and Drug Administration guidance to industry, the D71.1°Cfor E. coliO157:H7 and non-O157 STEC serotypes were not significantly different (P> 0.05).

The objective of this study was to evaluate the hypothesis that the dose-response relationship for Listeria monocytogenesin humans varies with genotypic lineage or subtype. The linkages between molecular subtyping data and enumeration data for L. monocytogenessubtypes in foods consumed by the at-risk population were examined to test this hypothesis. We applied a conditional probability model to conduct a subtype-specific dose-response analysis, with the focus on invasive listeriosis. L. monocytogenesdiffered not only in the molecular subtype and lineage but also in the contamination level when isolates of the pathogen occurred in retail samples of ready-to-eat foods. Using the exponential model parameter r-value as a measure (essentially the probability of a single cell causing illness), we found that the virulence varied among L. monocytogeneslineages by several orders of magnitude. Under the assumptions made, for L. monocytogeneslineages I and II the consumption of a single cell would result in listeriosis with log average probabilities of −7.88 (equivalent to once in 107.78times) and −10.3, respectively, as compared with −9.72 for L. monocytogenesindependent of subtype. A greater difference in r-values was found for selected ribotypes. The uncertainty about the r-value estimates was small compared with the large differences in the virulence parameters themselves. Thus, for L. monocytogenesboth subtype and the number of cells consumed matter, highlighting the usefulness of considering both exposure concentration and subtype prevalence in dose-response analysis. As advances are made in molecular subtyping and quantitative tools for dose-response analysis, further studies integrating genomic data into quantitative risk assessments will enable better attribution of disease risk to L. monocytogenessubtypes.

Four ready-to-eat smoked fish plants were monitored for 2 years to study Listeriacontamination patterns and the impact of plant-specific Listeriacontrol strategies, including employee training and targeted sanitation procedures, on Listeriacontamination patterns. Samples from the processing plant environment and from raw and finished product were collected monthly and tested for Listeriaspp. and Listeria monocytogenes. Before implementation of intervention strategies, 19.2% of raw product samples (n= 276), 8.7% of finished product samples (n= 275), and 26.1% of environmental samples (n= 617) tested positive for Listeriaspp. During and after implementation of Listeriacontrol strategies, 19.0% of raw product samples (n= 242), 7.0% of finished product samples (n= 244), and 19.5% of environmental samples (n= 527) were positive for Listeriaspp. In one of the four fish plants (plant 4), no environmental samples were positive for L. monocytogenes, and this plant was thus excluded from statistical analyses. Based on data pooled from plants 1, 2, and 3, environmental Listeriaspp. prevalence was significantly lower (P< 0.05) for nonfood contact surfaces and the finished product area and for the overall core environmental samples after implementation of control strategies. Listeriaprevalence for floor drains was similar before and after implementation of controls (49.6 and 54.2%, respectively). Regression analysis revealed a significant positive relationship (P< 0.05) between L. monocytogenesprevalence in the environment and in finished products before implementation of control strategies; however, this relationship was absolved by implementation of Listeriacontrol strategies. Molecular subtyping (EcoRI ribotyping) revealed that specific L. monocytogenesribotypes persisted in three processing plants over time. These persistent ribotypes were responsible for all six finished product contamination events detected in plant 1. Ribotype data also indicated that incoming raw material is only rarely a direct source of finished product contamination. While these data indicate that plant-specific Listeriacontrol strategies can reduce cross-contamination and prevalence of Listeriaspp. and L. monocytogenesin the plant environment, elimination of persistent L. monocytogenesstrains remains a considerable challenge.

Four smoked fish processing plants were used as a model system to characterize Listeria monocytogenescontamination patterns in ready-to-eat food production environments. Each of the four plants was sampled monthly for approximately 1 year. At each sampling, four to six raw fish and four to six finished product samples were collected from corresponding lots. In addition, 12 to 14 environmental sponge samples were collected several hours after the start of production at sites selected as being likely contamination sources. A total of 234 raw fish, 233 finished products, and 553 environmental samples were tested. Presumptive Listeriaspp. were isolated from 16.7% of the raw fish samples, 9.0% of the finished product samples, and 27.3% of the environmental samples. L. monocytogeneswas isolated from 3.8% of the raw fish samples (0 to 10%, depending on the plant), 1.3% of the finished product samples (0 to 3.3%), and 12.8% of the environmental samples (0 to 29.8%). Among the environmental samples, L. monocytogeneswas found in 23.7% of the samples taken from drains, 4.8% of the samples taken from food contact surfaces, 10.4% of the samples taken from employee contact surfaces (aprons, hands, and door handles), and 12.3% of the samples taken from other nonfood contact surfaces. Listeriaspp. were isolated from environmental samples in each of the four plants, whereas L. monocytogeneswas not found in any of the environmental samples from one plant. Overall, the L. monocytogenesprevalence in the plant environment showed a statistically significant (P< 0.0001) positive relationship with the prevalence of this organism in finished product samples. Automated EcoRI ribotyping differentiated 15 ribotypes among the 83 L. monocytogenesisolates. For each of the three plants with L. monocytogenes–positive environmental samples, one or two ribotypes seemed to persist in the plant environment during the study period. In one plant, a specific L. monocytogenesribotype represented 44% of the L. monocytogenes–positive environmental samples and was also responsible for one of the two finished product positives found in this plant. In another plant, a specific L. monocytogenesribotype persisted in the raw fish handling area. However, this ribotype was never isolated from the finished product area in this plant, indicating that this operation has achieved effective separation of raw and finished product areas. Molecular subtyping methods can help identify plant-specific L. monocytogenescontamination routes and thus provide the knowledge needed to implement improved L. monocytogenescontrol strategies.

The purpose of this study was to develop data on the risk of listeriosis to support a science-based strategy for addressing Listeria monocytogenesin foods in the United States. Eight categories of ready-to-eat foods were collected over 14 to 23 months from retail markets at Maryland and northern California FoodNet sites. The product categories included luncheon meats, deli salads, fresh soft “Hispanic-style” cheeses, bagged salads, blue-veined and soft mold-ripened cheeses, smoked seafood, and seafood salads. The presence and levels of L. monocytogenesin the samples were determined by rapid DNA-based assays in combination with culture methods. Of 31,705 samples tested, 577 were positive. The overall prevalence was 1.82%, with prevalences ranging from 0.17 to 4.7% among the product categories. L. monocytogeneslevels in the positive samples varied from <0.3 MPN (most probable number) per g to 1.5 × 105CFU/g, with 402 samples having levels of <0.3 MPN/g, 21 samples having levels of >102CFU/g, and the rest of the samples having intermediate levels. No obvious trends with respect to seasonality were observed. Significant differences (P< 0.05) between the sampling sites were found, with higher prevalences for threes categories in northern California and for two categories in Maryland. Significantly (P< 0.001) higher prevalences were found for in-store-packaged samples than for manufacturer-packaged samples of luncheon meats, deli salads, and seafood salads, while 16 of the 21 samples with higher counts were manufacturer packaged. The data collected in this study help to fill gaps in the knowledge about the occurrence of L. monocytogenesin foods, and this new information should be useful in the assessment of the risk posed by L. monocytogenesto consumers.