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There is a continued need to develop improved rapid methods for detection of foodborne pathogens. The aim of this project was to evaluate the 3M Molecular Detection System (3M MDS), which uses isothermal DNA amplification, and the 3M Molecular Detection Assay Listeriausing environmental samples obtained from retail delicatessens and meat, seafood, and dairy processing plants. Environmental sponge samples were tested for Listeriawith the 3M MDS after 22 and 48 h of enrichment in 3M Modified ListeriaRecovery Broth (3M mLRB); enrichments were also used for cultural detection of Listeriaspp. Among 391 samples tested for Listeria, 74 were positive by both the 3M MDS and the cultural method, 310 were negative by both methods, 2 were positive by the 3M MDS and negative by the cultural method, and one sample was negative by the 3M MDS and positive by the cultural method. Four samples were removed from the sample set, prior to statistical analyses, due to potential cross-contamination during testing. Listeriaisolates from positive samples represented L. monocytogenes, L. innocua, L. welshimeri, and L. seeligeri. Overall, the 3M MDS and culture-based detection after enrichment in 3M mLRB did not differ significantly (P> 0.05) with regard to the number of positive samples, when chi-square analyses were performed for (i) number of positive samples after 22 h, (ii) number of positive samples after 48 h, and (iii) number of positive samples after 22 and/or 48 h of enrichment in 3M mLRB. Among 288 sampling sites that were tested with duplicate sponges, 67 each tested positive with the 3M MDS and the traditional U.S. Food and Drug Administration Bacteriological Analytical Manualmethod, further supporting that the 3M MDS performs equivalently to traditional methods when used with environmental sponge samples.

Despite growing concerns about cross-contamination of ready-to-eat foods with Listeria monocytogenes, our knowledge about the ecology and transmission of L. monocytogenesin retail establishments has remained limited. We conducted a cross-sectional study to characterize the prevalence, distribution, and subtype diversity of L. monocytogenesin 120 New York State retail deli establishments that were hypothesized to present an increased risk for environmental L. monocytogenescontamination (i.e., small establishments and establishments with a history of failed New York State Agriculture and Markets inspections). Analysis of these data along with previously reported data for 121 predominantly larger retail establishments in New York State identified establishment size, geographic location, and inspection history as significant predictors of L. monocytogenespresence and prevalence. The odds of an establishment being L. monocytogenespositive were approximately twice as high for large establishments, establishments located in New York City, or establishments with poor inspection history (as compared with establishments without these attributes), even though correlation between location and inspection history complicated interpretation of results. Within an establishment, L. monocytogeneswas significantly more prevalent on nonfood contact surfaces than on food contact surfaces; prevalence was particularly high for floors and in floor drains, sinks, the dairy case, and milk crates. L. monocytogenessubtype diversity differed between sites, with lineage I isolates significantly associated with nonfood contact surfaces and lineage II isolates significantly associated with food contact surfaces. Isolates belonging to the same ribotype were often found dispersed across multiple sites within an operation.

AbstractListeria monocytogenescan survive and grow under wide-ranging environmental stress conditions encountered both in foods and in the host. The ability of certain L. monocytogenessubtypes to thrive under stress conditions present in specific niches was hypothesized to reflect genetic characteristics and phenotypic capabilities conserved among strains within a subtype. To quantify variations in salt stress phenotypes among 40 strains selected to represent the diversity of the three major L. monocytogenesgenetic lineages and to determine if salt stress phenotypes were associated with genetic relatedness, we measured growth under salt stress at both 7°C and 37°C. At 7°C, in brain–heart infusion with 6% NaCl, average growth rates among the lineages were similar. A comparison of doubling times after exposure to salt stress at 7°C or 37°C indicated that growth at 7°C provided crossprotection to subsequent salt stress for strains in lineages I and II. At 37°C, in brain–heart infusion with 6% NaCl, lineage I and III strains grew significantly faster (p< 0.0001) than lineage II strains. Under salt stress at 37°C, differences in growth parameters were significantly (p< 0.005) associated with genetic relatedness of the strains. Compatible solute uptake is part of the L. monocytogenessalt stress response, but growth differences between the lineages were not related to differences in transcript levels of osmolyte transporter-encoding genes betL, gbuA, oppA, and opuCA.The combination of phylogenetic and phenotypic data suggests that L. monocytogeneslineage I and III strains, which are most commonly associated with human and animal disease, may be better adapted to osmotic stress at 37°C, conditions that are present in the host gastrointestinal tract.

AbstractObjective:Listeria monocytogenesis a foodborne pathogen found in a wide variety of environments. Subtype characterization of L. monocytogenesisolates from listeriosis outbreaks that have occurred over the last three decades has suggested that a number of outbreaks were caused by a small number of L. monocytogenesepidemic clones (ECs). In this study we compared the prevalence, ecology, and phylogenetic position of outbreak-associated isolates and non-outbreak–associated isolates to probe the evolutionary and ecological characteristics of outbreak-associated L. monocytogenessubtypes, including those representing previously described ECs.Methods:Multilocus sequence typing data for isolates from 15 listeriosis outbreaks in Europe and North America were generated and compared, using a phylogenetic framework, with 180 isolates representing a local sampling of diverse sources, including human sporadic cases.Results:Isolates from 15 listeriosis outbreaks represented eight sequence types (STs). STs corresponding to previously designated ECI (ST1 and ST93) and ECIa (ST29) represented isolates from eight outbreaks. ST17 (corresponding to ECII) was involved in two outbreaks in the United States (1998 and 2002). No other STs were involved in multiple outbreaks. While ST1 was the most common ST among sporadic human cases and non-human listeriosis–related isolates, ST29 was rare among non-human listeriosis–related isolates and was significantly overrepresented among isolates from human listeriosis outbreaks and sporadic cases as compared to isolates from other sources in our local sampling.Conclusions:STs associated with outbreaks (and representing previously designated ECs) appear to differ in their ecology. While association of ECI with multiple human listeriosis outbreaks appears to reflect strain abundance across environments, ECIa seems to represent an L. monocytogenesEC that appears to be overrepresented among outbreaks and sporadic cases and thus may have increased transmission potential.

As our understanding of Listeria monocytogenestransmission in retail and deli operations is limited, we conducted a cross-sectional study of L. monocytogenescontamination patterns in 121 retail establishments, using testing of food and environmental samples and subtype analysis (ribotyping) of L. monocytogenesisolates. Seventy-three (60%) establishments had at least one sample that tested positive for L. monocytogenes; 5 (2.7%) of the 183 food and 151 (13.0%) of the 1,161 environmental samples tested positive for L. monocytogenes, including 125 (16.7%) and 26 (6.3%) of non–food contact and food contact surface samples, respectively. Thirty-two EcoRI ribotypes were identified among the 156 L. monocytogenesisolated. Twenty-seven establishments had two or more L. monocytogeneswith the same ribotype within a given establishment, including 9 establishments where isolates from 3 to 5 samples had the same ribotype. In 5 of 7 establishments where follow-up sampling was conducted 8 to 19 months after the initial sampling, isolates with the same ribotype were obtained in both samplings; persistence of a given strain was also confirmed by pulsed-field gel electrophoresis. Our data indicate that (i) L. monocytogenesis regularly found in some retail environments; (ii) L. monocytogenesstrains are often widely distributed in retail, indicating cross-contamination and dispersal; (iii) L. monocytogenescan persist in retail environments for more than 1 year; and (iv) a number of L. monocytogenessubtypes isolated at retail are common among human listeriosis cases. We also identified specific contamination patterns in retail establishments, providing critical information for the development of L. monocytogenescontrol strategies.

While there is considerable information available regarding Listeria monocytogenescontamination patterns in food processing plants, our understanding of L. monocytogenescontamination and transmission in retail operations is limited. We characterized 125 food, 40 environmental, and 342 human clinical L. monocytogenesisolates collected in New York State from 1997 to 2002 using automated ribotyping and hlyallelic variation. All environmental isolates were obtained from retail establishments and the majority of food isolates (98 isolates) were obtained from foods that were prepared or handled at retail. Overall, food and/or environmental isolates from 50 different retail establishments were characterized. The 125 food and 40 environmental isolates were differentiated into 29 and 10 ribotypes, respectively. For 16 retail establishments, we found evidence for persistence of one or more specific L. monocytogenesstrains as indicated by isolation of the same EcoRI ribotype from food or environmental samples collected in a given establishment on different days. The human isolates were differentiated into 48 ribotypes. Statistical analyses showed that two ribotypes were significantly (P< 0.0001) more common among food isolates as compared with human isolates. However, a total of 17 ribotypes found among the human clinical isolates were also found among the food and environmental isolates. We conclude that L. monocytogenes, including subtypes that have been linked to human disease, can persist in retail environments. Implementation of Listeriacontrol procedures in retail operations, which process and handle products that permit the growth of L. monocytogenes, are thus a critical component of a farm-to-table L. monocytogenescontrol program.