Transcriptome response ofLactobacillus sakeito meat protein environment

Adaptive mechanism of Lactobacillus amylolyticus L6 in soymilk environment based on metabolism of nutrients and related gene-expression profiles

Lactobacillus amylolyticus L6 isolated from naturally fermented tofu-whey was characterized as potential probiotics. To give insight into the adaptive mechanism of L. amylolyticus L6 in soymilk, the gene-expression profiles of this strain and changes of chemical components in fermented soymilk were investigated. The viable counts of L. amylolyticus L6 in soymilk reached 10¹² CFU/mL in the stationary phase (10 hr). The main sugars reduced gradually while the acidity value significantly increased from 45.33° to 95.88° during fermentation. About 50 genes involved in sugar metabolization and lactic acid production were highly induced during soymilk fermentation. The concentration of total amino acid increased to 668.38 mg/L in the logarithmic phase, and 45 differentially expressed genes (DEGs) in terms of nitrogen metabolism and biosynthesis of amino acid were detected. Other genes related to lipid metabolism, inorganic ion transport, and stress response were also highly induced. Besides, the concentration of isoflavone aglycones with high bioactivity increased from 14.51 mg/L to 36.09 mg/L during the fermentation, and the expression of 6-phospho-β-glucosidase gene was also synchronously induced. This study revealed the adaptive mechanism of L. amylolyticus L6 in the soymilk-based ecosystem, which gives the theoretical guidance for the application of this strain in other soybean-derived products.

Genomic and metabolic features of Lactobacillus sakei as revealed by its pan-genome and the metatranscriptome of kimchi fermentation

The genomic and metabolic features of Lactobacillus sakei were investigated using its pan-genome and by analyzing the metatranscriptome of kimchi fermentation. In the genome-based relatedness analysis, the strains were divided into the Lb. sakei ssp. sakei and Lb. sakei ssp. carnosus lineage groups. Genomic and metabolic pathway analysis revealed that all Lb. sakei strains have the capability of producing d/l-lactate, ethanol, acetate, CO₂, formate, l-malate, diacetyl, acetoin, and 2,3-butanediol from d-glucose, d-fructose, d-galactose, sucrose, d-lactose, l-arabinose, cellobiose, d-mannose, d-gluconate, and d-ribose through homolactic and heterolactic fermentation, whereas their capability of d-maltose, d-xylose, l-xylulose, d-galacturonate, and d-glucuronate metabolism is strain-specific. All strains carry genes for the biosynthesis of folate and thiamine, whereas genes for biogenic amine and toxin production, hemolysis, and antibiotic resistance were not identified. The metatranscriptomic analysis showed that the expression of Lb. sakei transcripts involved in carbohydrate metabolism increased as kimchi fermentation progressed, suggesting that Lb. sakei is more competitive during late fermentation stage. Homolactic fermentation pathway was highly expressed and generally constant during kimchi fermentation, whereas expression of heterolactic fermentation pathway increased gradually as fermentation progressed. l-Lactate dehydrogenase was more highly expressed than d-lactate dehydrogenase, suggesting that l-lactate is the major lactate metabolized by Lb. sakei.

Phenotypic Diversity of Lactobacillus sakei Strains

Lactobacillus sakei is a lactic acid bacteria (LAB) species highly adapted to the meat environment. For this reason, selected strains are often used as starter culture in the production of fermented sausages, especially in Mediterranean countries. It often represents the dominant species in these products and can maintain its viability during all the ripening period, which can take also some months. This ability is guaranteed by the possibility of the species to obtain energy through pathways active even when hexoses are depleted. This species is characterized by a relevant genetic and phenotypic diversity and its metabolism can be further affected by the growth condition applied. In this work we investigate the metabolic responses of six different L. sakei in a synthetic medium (DM) containing defined amounts of amino acids in relation to temperature and NaCl concentration. In addition, the activities of cells pre-grown in presence of glucose o ribose were tested. Arginine was efficiently up-taken with the exception of the type strain DSMZ 20017^t. Other amino acids (i.e., serine, asparagine, cysteine, and methionine) were metabolized through potentially energetic pathways which start from pyruvate accumulation, as demonstrated by the organic acid accumulation trend in the condition tested, especially in DM without sugar added. The presence of excesses of pyruvate deriving from amino acids lead to the accumulation of diacetyl and acetoin by all the strains when sugars were added. This approach allowed a deeper insight into the phenotypic variability of the species and improved the comprehension of the metabolic pathways adopted by L. sakei to survive and grow in restrictive conditions such as those found in fermented sausages during fermentations. Thus, the results obtained are useful information for improving and optimizing the use of such strains as starter culture for these products.

Shotgun Metagenomics and Volatilome Profile of the Microbiota of Fermented Sausages

Changes in the microbial gene content and abundance can be analyzed to detect shifts in the microbiota composition due to the use of a starter culture in the food fermentation process, with the consequent shift of key metabolic pathways directly connected with product acceptance. Meat fermentation is a complex process involving microbes that metabolize the main components in meat. The breakdown of carbohydrates, proteins, and lipids can lead to the formation of volatile organic compounds (VOCs) that can drastically affect the organoleptic characteristics of the final products. The present meta-analysis, performed with the shotgun DNA metagenomic approach, focuses on studying the microbiota and its gene content in an Italian fermented sausage produced by using a commercial starter culture (a mix of Lactobacillus sakei and Staphylococcus xylosus), with the aim to discover the connections between the microbiota, microbiome, and the release of volatile metabolites during ripening. The inoculated fermentation with the starter culture limited the development of Enterobacteriaceae and reduced the microbial diversity compared to that from spontaneous fermentation. KEGG database genes associated with the reduction of acetaldehyde to ethanol (EC 1.1.1.1), acetyl phosphate to acetate (EC 2.7.2.1), and 2,3-butanediol to acetoin (EC 1.1.1.4) were most abundant in inoculated samples (I) compared to those in spontaneous fermentation samples (S). The volatilome profiles were highly consistent with the abundance of the genes; elevated acetic acid (1,173.85 μg/kg), ethyl acetate (251.58 μg/kg), and acetoin (1,100.19 μg/kg) were observed in the presence of the starters at the end of fermentation. Significant differences were found in the liking of samples based on flavor and odor, suggesting a higher preference by consumers for the spontaneous fermentation samples. Inoculated samples exhibited the lowest scores for the liking data, which were clearly associated with the highest concentration of acetic acid.IMPORTANCE We present an advance in the understanding of meat fermentation by coupling DNA sequencing metagenomics and metabolomics approaches to describe the microbial function during this process. Very few studies using this global approach have been dedicated to food, and none have examined sausage fermentation, underlying the originality of the study. The starter culture drastically affected the organoleptic properties of the products. This finding underlines the importance of starter culture selection that takes into consideration the functional characteristics of the microorganism to optimize production efficiency and product quality.

Lactobacillus sakei: A Starter for Sausage Fermentation, a Protective Culture for Meat Products

Among lactic acid bacteria of meat products, Lactobacillus sakei is certainly the most studied species due to its role in the fermentation of sausage and its prevalence during cold storage of raw meat products. Consequently, the physiology of this bacterium regarding functions involved in growth, survival, and metabolism during meat storage and processing are well known. This species exhibits a wide genomic diversity that can be observed when studying different strains and on which probably rely its multiple facets in meat products: starter, spoiler, or protective culture. The emerging exploration of the microbial ecology of meat products also revealed the multiplicity of bacterial interactions L. sakei has to face and their various consequences on microbial quality and safety at the end of storage.

The Use of Starter Cultures in Traditional Meat Products

Starter cultures could play an essential role in the manufacture of traditional cured meat products. In order to achieve objectives related to meat products’ quality and safety improvement, the selection of particular strains constituting a starter culture should be carried out in the context of its application, since its functionality will depend on the type of sausage and process conditions. Also, strain selection should comply with particular requirements to warrant safety. The aim of the current review is to update the knowledge on the use of starter cultures in traditional meat products, with focus on dry-fermented products. In this manuscript, we will try to give answers to some relevant questions: Which starter cultures are used and why? Why are LAB used? What are their role and their specific mode of action? Which other groups of microorganisms (bacteria and fungi) are used as starter cultures and how do they act? A particular revision of omics approach regarding starter cultures is made since the use of these techniques allows rapid screening of promising wild strains with desirable functional characteristics, enabling the development of starter cultures better adapted to the meat matrix.