Microbial diversity changes in soybean sprouts treated with enterocin AS-48

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.

Impact of a Pilot-Scale Plasma-Assisted Washing Process on the Culturable Microbial Community Dynamics Related to Fresh-Cut Endive Lettuce

Cold plasma is described as a promising technique for the treatment of fresh food. In particular, the application of plasma-treated water gained interest in fresh-cut produce processing. This study aimed to evaluate the effectiveness of plasma-treated water (PTW) to decontaminate lettuce during washing on a pilot-scale level with special interest in the dynamics of the culturable microbial community in a first approach. PTW was used in pilot-scale washing at different processing steps, and the total viable count (TVC) of endive lettuce was determined after treatment and after storage (seven days, 2 °C). Microflora representatives were identified using MALDI-ToF MS. The highest reduction of TVC (1.8 log units) was achieved using PTW for washing whole lettuce before cutting. The microbial community structure showed high variations in the composition along the processing chain and during storage with a decrease in diversity after washing with PTW. PTW reduced the microbial load of endive lettuce; however, this was not clearly detectable at the end of storage, similar to other sanitizers used in comparable studies. To assure the safety of fresh products, detailed knowledge about the microbial load and the composition of the microbial community close to the end of shelf life is of high interest for optimized process design.

Biogenic amines analysis and microbial contribution in traditional fermented food of Douchi

Biogenic amines (BAs) have been reported to threaten the Douchi safety, while the BAs formation mechanism and corresponding control method have not been clarified for Douchi. The present study aims to investigate the microbial contribution to BAs in Douchi, and to find the beneficial strain for BAs control. Firstly, the BAs profiles of 15 Douchi samples were analyzed, and common 6 kinds of BAs were detected from different samples. All the samples showed the total BAs contents within the safe dosage range, while the histamine concentrations in 2 samples and β-phenethylamine in 6 samples were above the toxic level. Then, the bacterial and fungal communities were investigated by high-throughput sequencing analysis, and Bacillus and Candida were identified as the dominant bacteria and fungi genus, respectively. Furthermore, nineteen strains were selected from the dominant species of Douchi samples, including 14 Bacillus strains, 2 Staphylococcus strains, 1 Enterococcus strain and 2 Candida strains, and their BAs formation and degradation abilities were evaluated. B. subtilis HB-1 and S. pasteuri JX-2 showed no BAs producing ability, and B. subtilis GD-4 and Candida sp. JX-3 exhibited high BAs degradation ability. Finally, fermented soybean model analysis further verified that B. subtilis HB-1 and S. pasteuri JX-2 could significantly reduce BAs. This study not only contributed to understanding the BAs formation mechanism in Douchi, but also provided potential candidates to control the BAs in fermented soybean products.

Analysis of the effect of high hydrostatic pressure treatment and enterocin AS-48 addition on the bacterial communities of cherimoya pulp

In the present study, pulp obtained from cherimoya pulp (Annona cherimola) was inoculated with epiphytic microbiota collected from cherimoya fruits, and supplemented or not with the circular bacteriocin enterocin AS-48 (50μg/g) and then packed under vacuum. Samples supplemented or not with enterocin were treated by high hydrostatic pressure (600MPa, 8min) and then stored at 5°C for 30days. The single AS-48 treatment only delayed microbial growth non-significantly (p>0.05). HHP treatment reduced microbial counts by five log cycles, but it did not prevent further growth of survivors by day 7. The combined treatment (AS-48+HHP) was the most effective, keeping bacterial cell densities at ≤1.5 log CFU/g for up to 15days. 16S rRNA gene pyrosequencing analysis was done on amplicon libraries from the growth on TSA plates seeded with ten-fold dilutions of pulp suspensions and incubated at 22°C for 24h. The results obtained are limited by the experimental conditions used in the study, and only concern the bacterial fraction that was selected by the TSA and growth conditions used. Pantoea (Pantoea agglomerans, Pantoea vagans) were the operational taxonomic units (OTUs) detected at highest relative abundance in bacterial biomass grown from control samples for the first 7days of storage, followed by Enterococcus gallinarum and Leuconostoc mesenteroides during late storage. The single HHP treatment significantly reduced the relative abundance of OTUs belonging to Pantoea and strongly increased that of endosporeformers (mainly Bacillus firmus and Bacillus stratosphericus) early after treatment, although Pantoea became again the predominant OTUs during storage. Samples singly treated with enterocin AS-48 revealed a strong inhibition of E. gallinarum as well as an early decrease in the relative abundance of Pantoea and an increased relative abundance of OTUs belonging to other Gram-negative species (mainly from genera Serratia and Pseudomonas). The strong microbial inactivation achieved by the combined treatment with enterocin and HHP reduced the levels of viable cells below detectable limits at days 0 and 1, and survivors recovered on TSA at day 7 were represented in >99% by B. firmus OTU. OTUs from endosporeformers were no longer detected during prolonged incubation, displaced by Pantoea spp., Erwinia billingiae and leuconostocs. Results from the present study indicate that HHP in combination with enterocin AS-48 is more effective in preserving the microbiological quality of cherimoya pulp during storage than the single HHP treatment.

Current intervention strategies for the microbial safety of sprouts

Sprouts have gained popularity worldwide due to their nutritional values and health benefits. The fact that their consumption has been associated with numerous outbreaks of foodborne illness threatens the $250 million market that this industry has established in the United States. Therefore, sprout manufacturers have utilized the U.S. Food and Drug Administration recommended application of 20,000 ppm of calcium hypochlorite solution to seeds before germination as a preventative method. Concentrations of up to 200 ppm of chlorine wash are also commonly used on sprouts. However, chlorine-based treatment achieves on average only 1- to 3-log reductions in bacteria and is associated with negative health and environmental issues. The search for alternative strategies has been widespread, involving chemical, biological, physical, and hurdle processes that can achieve up to 7-log reductions in bacteria in some cases. The compilation here of the current scientific data related to these techniques is used to compare their efficacy for ensuring the microbial safety of sprouts and their practicality for commercial producers. Of specific importance for alternative seed and sprout treatments is maintaining the industry-accepted germination rate of 95% and the sensorial attributes of the final product. This review provides an evaluation of suggested decontamination technologies for seeds and sprouts before, during, and after germination and concludes that thermal inactivation of seeds and irradiation of sprouts are the most practical stand-alone microbial safety interventions for sprout production.

LAB-Secretome: a genome-scale comparative analysis of the predicted extracellular and surface-associated proteins of Lactic Acid Bacteria

Background

In Lactic Acid Bacteria (LAB), the extracellular and surface-associated proteins can be involved in processes such as cell wall metabolism, degradation and uptake of nutrients, communication and binding to substrates or hosts. A genome-scale comparative study of these proteins (secretomes) can provide vast information towards the understanding of the molecular evolution, diversity, function and adaptation of LAB to their specific environmental niches.

Results

We have performed an extensive prediction and comparison of the secretomes from 26 sequenced LAB genomes. A new approach to detect homolog clusters of secretome proteins (LaCOGs) was designed by integrating protein subcellular location prediction and homology clustering methods. The initial clusters were further adjusted semi-manually based on multiple sequence alignments, domain compositions, pseudogene analysis and biological function of the proteins. Ubiquitous protein families were identified, as well as species-specific, strain-specific, and niche-specific LaCOGs. Comparative analysis of protein subfamilies has shown that the distribution and functional specificity of LaCOGs could be used to explain many niche-specific phenotypes.

A comprehensive and user-friendly database LAB-Secretome was constructed to store, visualize and update the extracellular proteins and LaCOGs http://www.cmbi.ru.nl/lab_secretome/. This database will be updated regularly when new bacterial genomes become available.

Conclusions

The LAB-Secretome database could be used to understand the evolution and adaptation of lactic acid bacteria to their environmental niches, to improve protein functional annotation and to serve as basis for targeted experimental studies.

Biochemical characterization of a bacteriocin-like inhibitory substance produced by Enterococcus faecium MXVK29, isolated from Mexican traditional sausage

BACKGROUND

Enterococci are lactic acid bacteria that can produce bacteriocins, which may offer an additional hurdle to control the growth of food-borne pathogens; moreover, these bacteriocins may have great potential as natural biopreservatives. The aim of this work was to characterize a bacteriocin-like inhibitory substance (BLIS) with antilisterial activity produced by an enterococcal strain.

RESULTS

The bacteriogenic strain was isolated from Mexican fermented sausages and identified as Enterococcus faecium with 99% sequence similarity. Maximal activity was detected at 16 h, where bacterial growth was in middle of the stationary phase. The producer strain was not inhibited by its own antimicrobial peptide. BLIS showed a strong anti-Listeria activity and was inactivated by proteinase K. Heating (121 °C for 15 min) induced some inactivation, but thermotolerance was higher at acid pH values. The yield obtained with a pH-mediated purification process was 32.7%, showing a band with an estimated molecular weight of 3.5 kDa. Automated N-terminal Edman degradation showed the following sequence: YYGNGVTCGSHHCSVD.

CONCLUSION

Biochemical characteristics of BLIS produced by E. faecium MXVK29 suggested that it belongs to Class IIa of the Klaenhammer classification and could be considered as a natural food preservative, although further studies need to be performed.