Metataxonomic Profiling of Native and Starter Microbiota During Ripening of Gouda Cheese Made With Listeria monocytogenes-Contaminated Unpasteurized Milk.

Unpasteurized milk is used to produce aged artisanal cheeses, which presents a safety concern due to possible contamination with foodborne pathogens, especially Listeria monocytogenes. The objective of this study was to examine the composition of the bacterial community in unpasteurized milk used to prepare Gouda cheese artificially contaminated with L. monocytogenes (~1 log CFU/ml) and assess the community dynamics and their potential interaction with L. monocytogenes during a 90-day ripening period using targeted 16S rRNA sequencing. The diversity of bacterial taxa in three batches of unpasteurized milk was not significantly different, and the microbiomes were dominated by species of Lactococcus, Streptomyces, Staphylococcus, and Pseudomonas. The highest relative abundances were observed for Pseudomonas fluorescens (31.84-78.80%) and unidentified operational taxonomic units (OTUs) of Pseudomonas (7.56-45.27%). After manufacture, both with and without L. monocytogenes-contaminated unpasteurized milk, Gouda cheese was dominated by starter culture bacteria (including Lactococcus lactis subsp. cremoris, lactis, lactis bv. diacetylactis, and Streptococcus thermophilus), in addition to unassigned members in the taxa L. lactis and Streptococcus. During ripening there was an overall decrease in L. lactis abundance and an increase in the number of taxa with relative abundances >0.1%. After 90-day ripening, a total of 82 and 81 taxa were identified in the Gouda cheese with and without L. monocytogenes, respectively. Of the identified taxa after ripening, 31 (Gouda cheese with L. monocytogenes) and 56 (Gouda cheese without L. monocytogenes) taxa had relative abundances >0.1%; 31 were shared between the two types of Gouda cheese, and 25 were unique to the Gouda cheese without added L. monocytogenes. No unique taxa were identified in the Gouda cheese with the added L. monocytogenes. This study provides information on the dynamics of the bacterial community in Gouda cheese during ripening, both with and without the addition of L. monocytogenes.

Listeria monocytogenes growth kinetics in refrigerated ready-to-eat dips and dip components.

Refrigerated ready-to-eat (RTE) dips often have pH and water activity combinations conducive to the proliferation of foodborne pathogens, including Listeria monocytogenes. This study conducted product assessments of five refrigerated RTE dips: baba ghanoush, guacamole, hummus, pesto, and tahini, along with individual dip components including avocado, basil, chickpeas, cilantro, eggplant, garlic, and jalapeno pepper. Dips and dip components were inoculated with 2 log CFU/g of L. monocytogenes and stored at 10°C for 28 days. The pathogen was enumerated throughout storage and growth rates were determined using the DMFit program to compute the time required for L. monocytogenes to achieve a 1 log CFU/g increase in population. Survival and growth rates varied significantly between the refrigerated RTE dips and dip components assessed in this study. For dips, L. monocytogenes progressively decreased in baba ghanoush, pesto, and tahini. In contrast, the pathogen proliferated in both hummus and guacamole and the highest growth rate was observed in guacamole (0.34±0.05 log CFU/g per day) resulting in a 1 log CFU/g increase in population in 7.8 days. L. monocytogenes proliferated in all dip components with the exception of eggplant and garlic. The pathogen achieved the highest growth rate in chickpeas (2.22±1.75 log CFU/g per day) resulting in a computed 1 log CFU/g increase in only 0.5 days. Results from this study can aid in understanding how L. monocytogenes behaves in refrigerated RTE dips and dip components and data can be utilized in understanding product formulations and in risk assessments.

Population Dynamics of Listeria monocytogenes, Escherichia coli O157:H7, and Native Microflora During Manufacture and Aging of Gouda Cheese Made with Unpasteurized Milk.

ABSTRACT: Cheeses made with unpasteurized milk are a safety concern due to possible contamination with foodborne pathogens. Listeria monocytogenes and Escherichia coli O157:H7 have been implicated in several outbreaks and recalls linked to Gouda cheese made with unpasteurized milk. The U.S. Food and Drug Administration Code of Federal Regulations requires cheeses made with unpasteurized milk to be aged at a minimum of 1.7°C for at least 60 days before entering interstate commerce. The goal of this study was (i) to assess the population dynamics of L. monocytogenes and E. coli O157:H7 during aging of Gouda cheese when the pathogens were inoculated into the unpasteurized milk used for manufacture and (ii) to compare the native microbial populations throughout manufacture and aging. Unpasteurized milk was inoculated with L. monocytogenes at 1 or 3 log CFU/mL or with E. coli O157:H7 at 1 log CFU/mL, and Gouda cheese was manufactured in laboratory-scale or pilot plant-scale settings. Cheeses were stored at 10°C for at least 90 days, and some cheeses were stored up to 163 days. Initial native microflora populations in unpasteurized milk did not differ significantly for laboratory-scale or pilot plant-scale trials, and population dynamics trended similarly throughout cheese manufacture and aging. During manufacture, approximately 81% of the total L. monocytogenes and E. coli O157:H7 populations was found in the curd samples. At an inoculation level of 1 log CFU/mL, L. monocytogenes survived in the cheese beyond 60 days in four of five trials. In contrast, E. coli O157:H7 was detected beyond 60 days in only one trial. At the higher 3-log inoculation level, the population of L. monocytogenes increased significantly from 3.96 ± 0.07 log CFU/g at the beginning of aging to 6.00 ± 0.73 log CFU/g after 150 days, corresponding to a growth rate of 0.04 ± 0.02 log CFU/g/day. The types of native microflora assessed included Enterobacteriaceae, lactic acid bacteria, mesophilic bacteria, and yeasts and molds. Generally, lactic acid and mesophilic bacterial populations remained consistent at approximately 8 to 9 log CFU/g during aging, whereas yeast and mold populations steadily increased. The data from this study will contribute to knowledge about survival of these pathogens during Gouda cheese production and will help researchers assess the risks of illness from consumption of Gouda cheese made with unpasteurized milk.

Survival kinetics of Listeria monocytogenes on chickpeas, sesame seeds, pine nuts, and black pepper as affected by relative humidity storage conditions.

Nuts and seeds have been increasingly associated with recalls due to contamination with Listeria monocytogenes. Storage of these food commodities occurs at various relative humidity (RH) conditions for months or years. The objective of this study was to assess L. monocytogenes survival on four commodities representing dried legumes, seeds, and spices categories: chickpeas, sesame seeds, pine nuts, and black pepper kernels. Inoculated products at 10 log CFU/g were stored for 180 days (6 months) at 25°C and different relative humidity (RH) levels: 25% (low), 45% (ambient), and 75% (high). After 180 days at 25% RH, L. monocytogenes populations decreased to 2.67-6.59 log CFU/g; the highest survival of the pathogen was observed on pine nuts and sesame seeds with decay rates of -0.014± 0.001 log CFU/g per d. Significantly greater population reductions on all products were observed during storage at 45 and 75% RH. At 45% RH, L. monocytogenes levels decreased to 1.90-6.36 log CFU/g. On chickpeas and black pepper stored at 75% RH, the pathogen population decreased to below the limit of enumeration (1 log CFU/g) yet were still detected via enrichments. The lowest survival of L. monocytogenes occurred at 75% RH on black pepper with a decay rate of -0.058±0.003 log CFU/g per d. Overall, regardless of RH level, the ability of the products to support survival of the pathogen may be expressed in the following order: pine nuts > sesame seeds > chickpeas > black pepper. The results of this study can aid in understanding how L. monocytogenes survives on dried legumes, seeds, and spices, and the data can contribute to the risk assessment of this pathogen.

Fate of Listeria monocytogenes in Ready-to-Eat Refrigerated Dips Treated with High Pressure Processing.

Various outbreaks and recalls have been associated with Listeria monocytogenes contamination of ready-to-eat (RTE) food products, including dips. High pressure processing (HPP) is useful for reducing levels of bacteria in many RTE food products, but its efficacy for reduction of pathogens in RTE dips is not well understood. In this study, laboratory-prepared hummus, tahini, baba ghanoush, guacamole, and pesto were initially treated with HPP at 350 MPa for up to 240 s to assess L. monocytogenes inactivation and determine D-values. D350 MPa-values in hummus, guacamole, and baba ghanoush were 105.3, 71.3, and 34.0 s, respectively. No significant reduction in L. monocytogenes levels was observed in tahini or pesto at 350 MPa for 240 s or after additional treatment for up to 600 s at 600 MPa (P > 0.05). Overall, the results of this study highlight the efficacy of HPP for reducing L. monocytogenes levels in certain RTE dips and but not in others.

Complete Genome Sequence of Listeria monocytogenes DFPST0073, Isolated from Imported Mexican Soft Cheese.

The genome of Listeria monocytogenes strain DFPST0073, isolated from imported fresh Mexican soft cheese in 2003, was sequenced using the Illumina MiSeq platform. Reads were assembled using SPAdes, and genome annotation was performed using the NCBI Prokaryotic Genome Annotation Pipeline.

Survival of Salmonella during Production of Partially Sprouted Pumpkin, Sunflower, and Chia Seeds Dried for Direct Consumption.

Ready-to-eat foods based on dried partially sprouted seeds have been associated with foodborne salmonellosis. Whereas research has focused on the potential for Salmonella initially present in or on seeds to grow and survive during fresh sprout production, little is known about the potential for growth and survival of Salmonella associated with seeds that have been partially sprouted and dried. The goal of this study was to determine the growth of Salmonella during soaking for partial germination of pumpkin, sunflower, and chia seeds and subsequent survival during drying and storage. Pumpkin, sunflower, and chia seeds were inoculated with a four-serotype Salmonella cocktail by the dry transfer method and were soaked in sterile water at 25 or 37°C for 24 h. During the soaking period, Salmonella exhibited growth rates of 0.37 ± 0.26, 0.27 ± 0.12, and 0.45 ± 0.19 log CFU/h at 25°C and 0.94 ± 0.44, 1.04 ± 0.84, and 0.73 ± 0.36 log CFU/h at 37°C for chia, pumpkin, and sunflower seeds, respectively. Soaked seeds were drained and dried at 25, 51, and 60°C. Drying resulted in >5 log CFU/g loss at both 51 and 60°C and ∼3 log CFU/g loss at 25°C on partially sprouted pumpkin and sunflower seeds. There was no decrease in Salmonella during drying of chia seeds at 25°C, and only drying at 60°C provided losses >5 log CFU/g. Dried seeds were stored at 37 and 45°C at 15 and 76% relative humidity (RH) levels. The combination of temperature and RH exerted a stronger effect than either factor alone, such that rates at which Salmonella decreased generally followed this order: 37°C at 15% RH < 45°C at 15% RH < 37°C at 76% RH < 45°C at 76% RH for all seeds tested. Rates differed based on seed type, with chia seeds and chia seed powder having the smallest rate of Salmonella decrease, followed by sunflower and pumpkin seeds. Drying at higher temperatures (50 and 61°C) or storing at elevated temperature and humidity (45°C and 76% RH) resulted in significantly different rates of Salmonella decrease.