Effects of different media on the enrichment of low numbers of Shiga toxin-producing Escherichia coli in mung bean sprouts and on the development of the sprout microbiome

Microbial Antagonism in Food-Enrichment Culture: Inhibition of Shiga Toxin-Producing Escherichia coli and Shigella Species

Bacterial pathogens, such as Shiga toxin-producing Escherichia coli (STEC) and Shigella spp., are important causes of foodborne illness internationally. Recovery of these organisms from foods is critical for food safety investigations to support attribution of illnesses to specific food commodities; however, isolation of bacterial cultures can be challenging. Methods for the isolation of STEC and Shigella spp. from foods typically require enrichment to amplify target organisms to detectable levels. Yet, during enrichment, target organisms can be outcompeted by other bacteria in food matrices due to faster growth rates, or through production of antimicrobial agents such as bacteriocins or bacteriophages. The purpose of this study was to evaluate the occurrence of Shigella and STEC inhibitors produced by food microbiota. The production of antimicrobial compounds in cell-free extracts from 200 bacterial strains and 332 food-enrichment broths was assessed. Cell-free extracts produced by 23 (11.5%) of the strains tested inhibited growth of at least one of the five Shigella and seven STEC indicator strains used in this study. Of the 332 enrichment broths tested, cell-free extracts from 25 (7.5%) samples inhibited growth of at least one of the indicator strains tested. Inhibition was most commonly associated with E. coli recovered from meat products. Most of the inhibiting compounds were determined to be proteinaceous (34 of the 48 positive samples, 71%; including 17 strains, 17 foods) based on inactivation by proteolytic enzymes, indicating presence of bacteriocins. The cell-free extracts from 13 samples (27%, eight strains, five foods) were determined to contain bacteriophages based on the observation of plaques in diluted extracts and/or resistance to proteolytic enzymes. These results indicate that the production of inhibitors by food microbiota may be an important challenge for the recovery of foodborne pathogens, particularly for Shigella sonnei. The performance of enrichment media for recovery of Shigella and STEC could be improved by mitigating the impact of inhibitors produced by food microbiota during the enrichment process.

Nitroso-hemoglobin-ginger conjugates effects on bacterial growth and color stability in a minced beef model

This study aims to enhance the color and microbiological qualities of a raw beef using natural ingredients. Nitroso-hemoglobin (NO-Hb) integrated with vitamin C (VC), calcium lactate, and ginger complexation were used as natural inhibitors against the growth of aerobic and pathogenic bacteria, namely (Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and Salmonella. NO-Hb inhibited E. coli, S. aureus, and Salmonella, and enhanced the color stability more than nitrite in the minced beef model. After the multiexponential analysis of relaxation decays, the water component (T2b) was analyzed using the low-field NMR. The results indicated that, at the 7th d of cold-storage the third component (T2) was detected. Significant correlations were observed between T21 and T22 relaxation times and water-holding capacity in minced beef, implying that the LF-NMR measurements could be an efficient method for the determination and prediction of beef freshness. NO-Hb- ginger mixture, as a novel ingredient, could be used instead of nitrite in terms of meat safety.

Development of an Effective Two-Step Enrichment Process to Enhance Bax System Detection of Healthy and Injured Salmonella Enteritidis in Liquid Whole Egg and Egg Yolk

ABSTRACT

The BAX system for pathogen detection has been highly accurate in a variety of food products. However, false-negative results have been reported for the detection of pathogens in liquid egg products because of failed pathogen resuscitation and the existence of inhibitory components. In this study, a short-time enrichment step was used to simultaneously resuscitate the target cells to the detection level and to dilute the inhibitory components to reduce detection interference. The MP medium (BAX system) enabled faster multiplication of healthy Salmonella cells than did buffered peptone water (BPW) in tested liquid whole egg and egg yolk. However, MP failed to resuscitate heat-injured cells even after 24 h of incubation. Therefore, MP was replaced with BPW as the enrichment broth for the BAX system. However, the use of BPW for a one-step enrichment was not effective for removal of PCR inhibitors in egg yolk, and unstable detection results were obtained. To improve detection accuracy, a second step of enrichment with brain heart infusion was added. This two-step enrichment process shortened the enrichment time to 14 h and greatly increased the number of samples in which the pathogen was detected during the same enrichment time, especially in the liquid egg yolk samples. The validation study revealed 100% diagnostic accuracy of the two-step enrichment process plus the BAX system. These results indicate that a two-step enrichment process added to the BAX system can improve the detection of pathogenic Salmonella in liquid egg products.

HIGHLIGHTS

Dynamics of bacterial communities in alfalfa and mung bean sprouts during refrigerated conditions

Sprouts are considered a healthy ready-to-eat food and has gained popularity in recent years. The objective of the present study was to determine the dynamics of sprouts' microbiome during cold storage to the end of their shelf-life at home. The microbiological quality of fresh alfalfa (Medicago sativa) and mung bean (Vigna radiata) sprouts from two commercial brands was tested and the number of APC ranges from 5.0 to 8.7 log CFU/g in alfalfa and 6.7 to 9.3 log CFU/g in mung bean sprouts. In the case of alfalfa, but not mung beans, there were differences in the mean numbers of APC between the two brands. The number of coliform bacteria ranges from 4.3 to 7.7 log CFU/g in alfalfa and 4.1 to 8.1 log CFU/g in mung bean sprouts. Four independent batches of sprouts were used for DNA preparation and were sampled immediately after purchase and once a week during subsequent storage in refrigerator until the end of their shelf-life. Microbial population of the sprouts was determined using next generation sequencing of 16S rRNA amplicons. Alfalfa sprouts were dominated by Pseudomonas throughout the storage time with relative abundance of >60% at 3 weeks. Fresh mung bean sprouts were dominated by both Pseudomonas and Pantoea, but Pantoea became the dominant taxa after 2 weeks of storage, with >46% of relative abundance. The bacterial communities associated with sprouts were largely dependent on the sprout type, and less dependent on the brand. The species richness and diversity declined during storage and the development of spoilage. Among the 160 genera identified on sprouts, 23 were reported to contain known spoilage-associated species and 30 genera comprise potential human pathogenic species. This study provides new insight into the microbiome dynamics of alfalfa and mung bean sprouts during cold storage.

Application of high-throughput pyrosequencing in the analysis of microbiota of food commodities procured from small and large retail outlets in a U.S. metropolitan area - A pilot study

With the advent of high-throughput sequencing technologies, it is possible to comprehensively analyze the microbial community of foods without culturing them in the laboratory. The estimation of all microbes inhabiting a food commodity (food microbiota) therefore may shed light on the microbial quality and safety of foods. In this study, we utilized high-throughput pyrosequencing of 16S rRNA genes as well as traditional microbiological methods to evaluate the bacterial diversity and the predicted metabolic pathways associated with the bacterial communities of selected foods (romaine lettuce, cabbage, deli meat, and chicken legs, total 200 samples) procured from small and large retail outlets located in Memphis-Shelby County, Tennessee, USA. For high-throughput sequencing, microbial genomic DNA was directly extracted from the food products and subjected to genetic sequencing. Aerobic plate count of all food samples was also performed. Foods from small stores (such as corner stores) were found to contain higher bacterial counts as compared to large stores (such as supermarkets). High-throughput pyrosequencing in tandem with bioinformatics analyses revealed a comprehensive picture of the bacterial ecology of foods at different taxonomic levels. Firmicutes and Proteobacteria were the most abundant phyla across all products. At the genus level, Enterobacter and Pantoea in vegetables, and Bacillus and Aeromonas in animal products were found to be the most abundant. The bacterial predicted metabolic pathways such as inosine-5'-phosphate biosynthesis I, methylglyoxal (MG) degradation pathways, urea cycle, dTDP-l-rhamnose biosynthesis I, and mevalonate pathway I differed in foods procured from small stores as compared to large groceries or supermarkets. The results from this study revealed that the bacterial ecology (both in terms of numbers and types of bacteria) of food commodities might differ based on the vending outlet type (large vs. small) of retail stores. The overall estimation bacterial communities in foods by high-throughput sequencing method may be useful to identify potential taxa responsible for food spoilage. Moreover, the data from pyrosequencing of 16S rRNA genes can also be applied to infer major metabolic pathways in bacteria inhabiting different foods. This may reflect the role of these pathways in food-bacteria interaction and adaptation.