Glutamate dehydrogenase activity can be transmitted naturally to Lactococcus lactis strains to stimulate amino acid conversion to aroma compounds

Pyruvate stimulates transamination of leucine into α-ketoisocaproic acid and supports 3-methylbutanal production by Lactococcus lactis

AIM

To investigate the effect of pyruvate and glucose on leucine transamination and 3-methylbutanal production by Lactococcus lactis, including the comparison with cells possessing glutamate dehydrogenase (GDH) activity.

METHODS AND RESULTS

Lactococcus lactis cells were incubated in chemically defined medium (CDM) with the pH controlled at 5.2 to mimic cheese conditions. Pyruvate supplementation stimulated the production of the key flavour compound 3-methylbutanal by 3-4 times after 72 h of incubation. Concurrently, alanine production increased, demonstrating the involvement of pyruvate in transamination reactions. Glucose-metabolizing cells excreted α-ketoisocaproic acid and produced even 3 times more 3-methylbutanal after 24 h than pyruvate-supplemented cells. Conjugal transfer technique was used to transfer the plasmid pGdh442 carrying the gdh gene encoding for GDH to L. lactis. Introducing GDH did not stimulate the excretion of α-ketoisocaproic acid and the production of 3-methylbutanal.

CONCLUSIONS

These results demonstrate that Lactococcus uses pyruvate to transaminate leucine into α-ketoisocaproic acid which supports 3-methylbutanal production. Surprisingly, GDH activity did not stimulate leucine transamination and 3-methylbutanal production.

Mechanism of carbohydrate and protein conversion during sourdough fermentation: An analysis based on representative Chinese sourdough microbiota

Sourdough fermentation is attracting growing attention because of its positive effects on properties of leavened baked good. However, the changes in dough features and the mechanisms behind them are not well understood, which limits its widespread use. In this study, we assessed the effects of representative lactic acid bacteria in sourdough monoculture or co-culture with yeasts on dough characteristics. Physicochemical analysis identified increased proteolysis and enhanced nutritional properties of co-culture groups. However, a reduction in organic acids contents of co-culture groups compared to monoculture was detected, and this effect was not limited by the yeast species. The RNA sequencing further demonstrated that the presence of yeast enhanced the protein metabolic activity of lactic acid bacteria, while decreased its organic acid biosynthetic activity. Moreover, the proteomic analysis revealed that endogenous metabolic proteins of flour, such as pyruvate kinase, glucosyltransferase and pyruvate dehydrogenase play a key role in carbohydrate metabolism during fermentation. This study uncovered the influence of typical microorganisms and endogenous enzymes on dough characteristics based on different aspects. Bacteria-mediated consumption of proteins and increased proteolysis in co-culture groups may underlie the improved digestibility and nutritional effects of sourdough fermented products, which provides an important basis for nutrient fortified bread making with multi-strain leavening agent.

Diversity of volatile organic compound production from leucine and citrate in Enterococcus faecium

Enterococcus faecium is frequently isolated from fermented food; in particular, they positively contribute to the aroma compound generation in traditional cheese. Citrate fermentation is a desirable property in these bacteria, but this feature is not uniformly distributed among E. faecium strains. In the present study, three selected E. faecium strains, IQ110 (cit^-), GM70 (cit⁺ type I), and Com12 (cit⁺ type II), were analyzed in their production of aroma compounds in milk. End products and volatile organic compounds (VOCs) were determined by solid-phase micro-extraction combined with gas chromatography mass spectrometry (SPME-GC-MS). Principal component analysis (PCA) of aroma compound profiles revealed a different VOC composition for the three strains. In addition, resting cell experiments of E. faecium performed in the presence of leucine, citrate, or pyruvate as aroma compound precursors allowed us to determine metabolic differences between the studied strains. GM70 (cit⁺ type I) showed an active citrate metabolism, with increased levels of diacetyl and acetoin generation relative to Com12 or to citrate defective IQ110 strains. In addition, in the experimental conditions tested, a defective citrate-fermenting phenotype for the Com12 strain was found, while its leucine degradation and pyruvate metabolism were conserved. In conclusion, rational selection of E. faecium strains could be performed based on genotypic and phenotypic analyses. This would result in a performing strain, such as GM70, that could positively contribute to flavor, with typical notes of diacetyl, acetoin, 3-methyl butanal, and 3-methyl butanol in an adjuvant culture.

Construction of a novel inducible expression vector for Lactococcus lactis M4 and Lactobacillus plantarum Pa21

A vector that drives the expression of the reporter gusA gene in both Lactobacillus plantarum and Lactococcus lactis was constructed in this study. This vector contained a newly characterized heat shock promoter (Phsp), amplified from an Enterococcus faecium plasmid, pAR6. Functionality and characterization of this promoter was initially performed by cloning Phsp into pNZ8008, a commercial lactococcal plasmid used for screening of putative promoters which utilizes gusA as a reporter. It was observed that Phsp was induced under heat, salinity and alkaline stresses or a combination of all three stresses. The newly characterized Phsp promoter was then used to construct a novel Lactobacillus vector, pAR1801 and its ability to express the gusA under stress-induced conditions was reproducible in both Lb. plantarum Pa21 and L. lactis M4 hosts.

Ranking the significance of fermentation conditions on the volatile organic compounds of Tuber melanosporum fermentation system by combination of head-space solid phase microextraction and chromatographic fingerprint similarity analysis

Tuber melanosporum is highly appreciated in culinary contexts due to its unique and characteristic aroma. T. melanosporum fermentation has been established as a promising alternative for fruiting bodies to produce volatile organic compounds (VOCs). In this work, a technique using a combination of chromatographic fingerprint similarity analysis, head-space solid phase microextraction and gas chromatography was developed to rank the significance of fermentation conditions on the VOCs profile during T. melanosporum fermentation. Omission tests indicated that the absence of major carbon source (i.e., sucrose) in the fermentation media had the most significant effect on the profile of VOCs, followed by the absence of yeast extract or peptone. Consideration of the culture conditions revealed that VOCs produced was the most significantly affected by temperature. These results indicated that it is possible to adjust the aroma of truffles via fermentation process control.

Methionine metabolism: major pathways and enzymes involved and strategies for control and diversification of volatile sulfur compounds in cheese

For economical reasons and to accommodate current market trends, cheese manufacturers and product developers are increasingly interested in controlling cheese flavor formation and developing new flavors. Due to their low detection threshold and diversity, volatile sulfur compounds (VSCs) are of prime importance in the overall flavor of cheese and make a significant contribution to their typical flavors. Thus, the control of VSCs formation offers considerable potential for industrial applications. This paper gives an overview of the main VSCs found in cheese, along with the major pathways and key enzymes leading to the formation of methanethiol from methionine, which is subsequently converted into other sulfur-bearing compounds. As these compounds arise primarily from methionine, the metabolism of this amino acid and its regulation is presented. Attention is focused in the enzymatic potential of lactic acid bacteria (LAB) that are widely used as starter and adjunct cultures in cheese-making. In view of industrial applications, different strategies such as the enhancement of the abilities of LAB to produce high amounts and diversity of VSCs are highlighted as the principal future research trend.

Plasmids of raw milk cheese isolate Lactococcus lactis subsp. lactis biovar diacetylactis DPC3901 suggest a plant-based origin for the strain

The four-plasmid complement of the raw milk cheese isolate Lactococcus lactis subsp. lactis biovar diacetylactis DPC3901 was sequenced, and some genetic features were functionally analyzed. The complete sequences of pVF18 (18,977 bp), pVF21 (21,739 bp), pVF22 (22,166 bp), and pVF50 (53,876 bp) were obtained. Each plasmid contained genes not previously described for Lactococcus, in addition to genes associated with plant-derived lactococcal strains. Most of the novel genes were found on pVF18 and encoded functions typical of bacteria associated with plants, such as activities of plant cell wall modification (orf11 and orf25). In addition, a predicted high-affinity regulated system for the uptake of cobalt was identified (orf19 to orf21 [orf19-21]), which has a single database homolog on a plant-derived Leuconostoc plasmid and whose functionality was demonstrated following curing of pVF18. pVF21 and pVF22 encode additional metal transporters, which, along with orf19-21 of pVF18, could enhance host ability to uptake growth-limiting amounts of biologically essential ions within the soil. In addition, vast regions from pVF50 and pVF21 share significant homology with the plant-derived lactococcal plasmid pGdh442, which is indicative of extensive horizontal gene transfer and recombination between these plasmids and suggests a common plant niche for their hosts. Phenotypes associated with these regions include glutamate dehydrogenase activity and Na(+) and K(+) transport. The presence of numerous plant-associated markers in L. lactis DPC3901 suggests a plant origin for the raw milk cheese isolate and provides for the first time the genetic basis to support the concept of the plant-milk transition for Lactococcus strains.

alpha-Ketoglutarate biosynthesis in wild and industrial strains of Lactococcus lactis

AIMS

This study was carried out to explore the ability of wild and industrial strains of Lactococcus lactis to produce alpha-ketoglutarate (alpha-KG), which is essential during the conversion of amino acids to flavour compounds.

METHODS AND RESULTS

Two pathways in alpha-KG biosynthesis were explored in strains of L. lactis isolated from dairy products, vegetables and commercial dairy starter cultures. Half of the strains efficiently converted glutamine to glutamate (Glu) and grew in Glu-free medium. Strains did not present isocitrate dehydrogenase and aconitase activities. However, half of the strains presented glutamate dehydrogenase (GDH) activity.

CONCLUSIONS

The ability of L. lactis to synthesize either alpha-KG or Glu via GDH was confirmed. However, L. lactis strains were not able to biosynthesize alpha-KG by the citrate-isocitrate pathway. NADP-GDH activity was mainly found in strains isolated from vegetables, whereas NAD-GDH activity was mainly found in strains isolated from dairy products.

SIGNIFICANCE AND IMPORTANCE OF THE STUDY

The origin of isolation highly influenced NAD or NADP-GDH activities. These enzymatic activities may be correlated to the flavour production capacity of the different strains.

Transcriptional analysis of the gdhA gene in Streptococcus thermophilus

AIMS

To study the transcriptional analysis of glutamate dehydrogenase gene, involved in the amino acid conversion to aroma compound in Streptococcus thermophilus.

METHODS AND RESULTS

Analysis of the gdhA gene nucleotide sequence of S. thermophilus CNRZ1066 revealed that the coding region is 1353 nucleotides long. The deduced amino acids sequence exhibits the putative GDH active site and some conserved domains characteristic of family I of hexameric GDHs. Phylogenetic analysis revealed that the gdh gene of S. thermophilus clustered with the orthologues of other streptococci such as Streptococcus mutans, Streptococcus agalactiae and Streptococcus infantarius. Studying the structural organization of the gdhA locus the amino acid similarity of GDHs was higher than 87%, but the locus organization was not conserved. A dominant transcript of approximately 1.4 kbp was revealed by Northern blot hybridization, suggesting that gdhA mRNA is monocystronic. Primer extension showed that transcription start point of gdhA was localized 43 bp upstream of the potential start codon (ATG).

CONCLUSIONS

The gdhA represents a monocistronic operon highly conserved in phylogenetic-related bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

A deeper knowledge of gdh transcriptional mechanisms could lead to develop S. thermophilus industrial starter cultures with optimized aromatic properties.

Branched chain aldehydes: production and breakdown pathways and relevance for flavour in foods

Branched aldehydes, such as 2-methyl propanal and 2- and 3-methyl butanal, are important flavour compounds in many food products, both fermented and non-fermented (heat-treated) products. The production and degradation of these aldehydes from amino acids is described and reviewed extensively in literature. This paper reviews aspects influencing the formation of these aldehydes at the level of metabolic conversions, microbial and food composition. Special emphasis was on 3-methyl butanal and its presence in various food products. Knowledge gained about the generation pathways of these flavour compounds is essential for being able to control the formation of desired levels of these aldehydes.

Surface physicochemical analysis of natural Lactococcus lactis strains reveals the existence of hydrophobic and low charged strains with altered adhesive properties

The cell surface physicochemical properties of 50 Lactococcus lactis strains of different subspecies and isolated from different origins (dairy, vegetal and animal) were examined. Cell surface hydrophobicity and Lewis acid-base properties were evaluated by affinity measurements to solvents in a partitioning test, while the global electrical charge of the cells was assessed by micro-electrophoresis using a laser zeta-meter. A global multivariate analysis of the results revealed a high natural diversity of L. lactis cell surface properties. While 52% of the strains present a hydrophilic and electronegative cell wall surface, a group of strikingly hydrophobic strains (12% of the strains) and a group of strains with unusual low charged surface (18%) were identified. Adhesion on polystyrene microtitre plates was evaluated for twelve strains selected from the multivariate analysis as representatives of the various observed cell wall surface physicochemical patterns. A significant correlation between adhesion, hydrophobicity and low electronegativity was observed when adhesion was performed in a low ionic strength suspending medium. The most adhesive strains were hydrophobic or low charged. The presence of repulsive electrostatic interactions led to a decrease in adhesion of the most negatively charged hydrophilic strains. The present study highlights the diversity of L. lactis cell surface physicochemical properties, diversity that could not be connected to the origin or to the subspecies of the strains.

Sequence analysis of the mobilizable lactococcal plasmid pGdh442 encoding glutamate dehydrogenase activity

A novel plasmid named pGdh442 had previously been isolated from a plant Lactococcus lactis strain. This plasmid encodes two interesting properties with applications in the dairy industry: a glutamate dehydrogenase activity that stimulates amino acid conversion to aroma compounds, and cadmium/zinc resistance that can be used as a selectable marker. Moreover, this plasmid can be transferred naturally to other strains, but appears to be incompatible with certain other lactococcal plasmids. During this study, the complete sequence of pGdh442 (68 319 bp) was determined and analysed. This plasmid contains 67 ORFs that include 20 IS elements that may have mediated transfer events between L. lactis and other genera living in the same biotope, such as Streptococcus, Pediococcus and Lactobacillus. Even though it is a low-copy-number plasmid, it is relatively stable due to a theta replication mode and the presence of two genes involved in its maintenance system. However, pGdh442 is incompatible with pSK08-derived protease/lactose plasmids because both possess the same replication and partition system. pGdh442 is not self-transmissible, but can be naturally transmitted via mobilization by conjugative elements carried by the chromosome or by other plasmids, such as the 712-type sex factor, which is widely distributed in L. lactis. In addition to several genes already found on other L. lactis plasmids, such as the oligopeptide transport and utilization genes, pGdh442 also carries several genes not yet identified in L. lactis. Finally, it does not carry genes that would trigger concern over its presence in human food.