Expression of a heterologous glutamate dehydrogenase gene in Lactococcus lactis highly improves the conversion of amino acids to aroma compounds

Submerged macrophyte restoration enhanced microbial carbon utilization in shallow lakes

Submerged macrophytes are integral to the functioning of shallow lakes through their interaction with microorganisms. However, we have a limited understanding of how microbial communities in shallow lakes respond when macrophytes are restored after being historically extirpated. Here, we explored the interactions between prokaryotic communities and carbon utilization in two lakes where submerged macrophytes were restored. We found restoration reduced total carbon in sediment by 8.9 %-27.9 % and total organic carbon by 16.7 %-36.9 % relative to control treatment, but had no effects on carbon content in the overlying water. Sediment microbial communities were more sensitive to restoration than planktonic microbes and showed enhanced utilization of simple carbon substrates, such as Tween 40, after restoration. The increase in carbon utilization was attributed to declines in the relative abundance of some genera, such as Saccharicenans and Desertimonas, which were found weakly associated with the utilization of different carbon substrates. These genera likely competed with microbes with high carbon utilization in restored areas, such as Lubomirskia. Our findings highlight how restoring submerged macrophytes can enhance microbial carbon utilization and provide guidance to improve the carbon sequestration capacity of restored shallow lakes.

Microbial processing of fruit and vegetable wastes into potential biocommodities: a review

The review focuses on some of the high value-end biocommodities, such as fermented beverages, single-cell proteins, single-cell oils, biocolors, flavors, fragrances, polysaccharides, biopesticides, plant growth regulators, bioethanol, biogas and biohydrogen, developed from the microbial processing of fruit and vegetable wastes. Microbial detoxification of fruit and vegetable processing effluents is briefly described. The advances in genetic engineering of microorganisms for enhanced yield of the above-mentioned biocommodities are elucidated with selected examples. The bottleneck in commercialization, integrated approach for improved production, techno-economical feasibility and real-life uses of some of these biocommodities, as well as research gaps and future directions are discussed.

Isolation and antioxidative activity of amino acid derivatives produced by Leuconostoc mesenteroides

One new phenyl lactic acid derivative, N-(5-amino-2-hydroxy-1-oxopentyl)-tyrosine (2), and three known metabolites [(S)-8-hydroxy-4-hydroxy-phenylpropanoic acid (1), (S)-8-hydroxy-phenylpropanoic acid (3), and indole-(S)-3-lactic acid (4)] were isolated from the culture medium of Leuconostoc mesenteroides guided with DPPH radical-scavenging assay. The chemical structures of the isolated compounds were determined by NMR and electrospray ionization mass spectroscopy. The isolated four compounds were assumed to be metabolites formed by the transformation of amino acids. The DPPH radical-scavenging activities of 1 and 2 were significantly higher than those of 3 and 4. This result may provide meaningful information of antioxidative metabolites from lactic acid bacteria.

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.

Uptake of α-ketoglutarate by citrate transporter CitP drives transamination in Lactococcus lactis

Transamination is the first step in the conversion of amino acids into aroma compounds by lactic acid bacteria (LAB) used in food fermentations. The process is limited by the availability of α-ketoglutarate, which is the best α-keto donor for transaminases in LAB. Here, uptake of α-ketoglutarate by the citrate transporter CitP is reported. Cells of Lactococcus lactis IL1403 expressing CitP showed significant levels of transamination activity in the presence of α-ketoglutarate and one of the amino acids Ile, Leu, Val, Phe, or Met, while the same cells lacking CitP showed transamination activity only after permeabilization of the cell membrane. Moreover, the transamination activity of the cells followed the levels of CitP in a controlled expression system. The involvement of CitP in the uptake of the α-keto donor was further demonstrated by the increased consumption rate in the presence of L-lactate, which drives CitP in the fast exchange mode of transport. Transamination is the only active pathway for the conversion of α-ketoglutarate in IL1403; a stoichiometric conversion to glutamate and the corresponding α-keto acid from the amino acids was observed. The transamination activity by both the cells and the cytoplasmic fraction showed a remarkably flat pH profile over the range from pH 5 to pH 8, especially with the branched-chain amino acids. Further metabolism of the produced α-keto acids into α-hydroxy acids and other flavor compounds required the coupling of transamination to glycolysis. The results suggest a much broader role of the citrate transporter CitP in LAB than citrate uptake in the citrate fermentation pathway alone.

Recent research on 3-phenyllactic acid, a broad-spectrum antimicrobial compound

3-Phenyllactic acid (PLA), which is an organic acid widely existing in honey and lactic acid bacteria fermented food, can be produced by many microorganisms, especially lactic acid bacteria. It was proved as an ideal antimicrobial compound with broad and effective antimicrobial activity against both bacteria and fungi. In addition, it could be used as feed additives to replace antibiotics in livestock feeds. This article presented a review of recent studies on the existing resource, antimicrobial activity, and measurement of PLA. In addition, microorganism strains and dehydrogenases producing PLA were reviewed in detail, the metabolic pathway and regulation of PLA synthesis in LAB strains were discussed, and high-level bioproduction of PLA by microorganism fermentation was also summarized.

Effect of biosynthetic intermediates and citrate on the phenyllactic and hydroxyphenyllactic acids production by Lactobacillus plantarum CRL 778

AIM

To evaluate the influence of biosynthetic precursors, intermediates and electron acceptors on the production of antifungal compounds [phenyllactic acid (PLA) and hydroxyphenyllactic acid (OH-PLA)] by Lactobacillus plantarum CRL 778, a strain isolated from home-made sourdough.

METHODS AND RESULTS

Growth of fermentative activity and antifungal compounds production by Lact. plantarum CRL 778 were evaluated in a chemically defined medium (CDM) supplemented with biosynthetic precursors [phenylalanine (Phe), tyrosine (Tyr)], intermediates [glutamate (Glu), alpha-ketoglutarate (α-KG)] and electron acceptors [citrate (Cit)]. Results showed that the highest PLA production (0.26 mmol l(-1)), the main antifungal compound produced by Lact. plantarum CRL 778, occurred when greater concentrations of Phe than Tyr were present. Both PLA and OH-PLA yields were increased 2-folds when Cit was combined with α-KG instead of Glu at similar Tyr/Phe molar ratio. Similarly, glutamate dehydrogenase (GDH) activity was significantly (P < 0.01) stimulated by α-KG and Cit in Glu-free medium.

CONCLUSION

Phe was the major stimulant for PLA formation; however, Cit could increase both PLA and OH-PLA synthesis by Lact. plantarum CRL 778 probably due to an increase in oxidized NAD(+). This effect, as well as the GDH activity, was enhanced by α-KG and down regulated by Glu.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study where the role of Glu and GDH activity in the PLA and OH-PLA synthesis was evidenced in sourdough lactic acid bacteria (LAB) using a CDM. These results contribute to the knowledge on the antifungal compounds production by sourdough LAB with potential applications on the baked goods.

Arginine metabolism in sugar deprived Lactococcus lactis enhances survival and cellular activity, while supporting flavour production

Flavour development in cheese is affected by the integrity of Lactococcus lactis cells. Disintegrated cells enhance for instance the enzymatic degradation of casein to free amino acids, while integer cells are needed to produce specific flavour compounds from amino acids. The impact of the cellular activity of these integer cells on flavour production remains to be elucidated. In this study we investigated whether lactose-deprived L. lactis cells that use arginine as an alternative energy source can extend cellular activity and produce more specific flavours. In cheese experiments we demonstrated that arginine metabolising cells survived about 3 times longer than non-arginine metabolising cells, which suggests prolonged cellular activity. Cellular activity and flavour production of L. lactis was further studied in vitro to enable controlled arginine supplementation. Comparable with the results found in cheese, the survival rates of in vitro incubated cells improved when arginine was metabolised. Furthermore, elongated cellular activity was reflected in 3-4-fold increased activity of flavour generating enzymes. The observed prolonged cellular activity resulted in about 2-fold higher concentrations of typical Gouda cheese flavours. These findings provide new leads for composing starter cultures that will produce specific flavour compounds.

NADP-glutamate dehydrogenase activity in nonstarter lactic acid bacteria: effects of temperature, pH and NaCl on enzyme activity and expression

AIMS

To screen the glutamate dehydrogenase (GDH) activity of nonstarter lactic acid bacteria (NSLAB) and to determine the effects of temperature, pH and NaCl values used for cheese ripening on enzyme activity and expression of GDH gene.

METHODS AND RESULTS

A subcellular fractionation protocol and specific enzyme assays were used. The effect of temperature, pH and NaCl on enzyme activity was evaluated. The expression of GDH gene was monitored by real-time PCR. One selected strain was also used as adjunct starter for cheese making to evaluate the catabolism of free amino acids and the production of volatile organic compounds (VOC) during cheese ripening. The cytoplasm fraction of all strains showed in vitro NADP-dependent GDH activity. NADP-GDH activity was markedly strain dependent and varied according to the interactions between temperature, pH and NaCl. Lactobacillus plantarum DPPMA49 showed the highest NADP-GDH activity under temperature, pH and NaCl values found during cheese ripening. RT-PCR analysis revealed that GDH expression of Lact. plantarum DPPMA49 was down-expressed by low temperature (<13°C) and over-expressed by NaCl (1·87-5·62%). According to NADP-GDH activity, the highest level of VOC (alcohols, aldehydes, miscellaneous and carboxylic acids) was found in cheeses made with DPPMA49.

CONCLUSIONS

The results of this study may be considered as an example of the influence of temperature, pH and NaCl on enzyme activity and expression of functional genes, such as GDH, in cheese-related bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

It focuses on the phenotypic and molecular characterization of the NADP-GDH in lactobacilli under cheese-ripening conditions. The findings of this study contribute to the knowledge about enzymes involved in the catabolism of amino acids, to be used as an important selection trait for cheese 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.

Amino Acid Catabolic Pathways of Lactic Acid Bacteria

Lactic acid bacteria (LAB) constitute a diverse group of Gram positive obligately fermentative microorganisms which include both beneficial and pathogenic strains. LAB generally have complex nutritional requirements and therefore they are usually associated with nutrient-rich environments such as animal bodies, plants and foodstuffs. Amino acids represent an important resource for LAB and their utilization serves a number of physiological roles such as intracellular pH control, generation of metabolic energy or redox power, and resistance to stress. As a consequence, the regulation of amino acid catabolism involves a wide set of both general and specific regulators and shows significant differences among LAB. Moreover, due to their fermentative metabolism, LAB amino acid catabolic pathways in some cases differ significantly from those described in best studied prokaryotic model organisms such as Escherichia coli or Bacillus subtilis. Thus, LAB amino acid catabolism constitutes an interesting case for the study of metabolic pathways. Furthermore, LAB are involved in the production of a great variety of fermented products so that the products of amino acid catabolism are also relevant for the safety and the quality of fermented products.

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.

Glutamate dehydrogenase activity in lactobacilli and the use of glutamate dehydrogenase-producing adjunct Lactobacillus spp. cultures in the manufacture of cheddar cheese

AIMS

The study was undertaken to investigate the occurrence of glutamate dehydrogenase activity in different species of lactobacilli, and to determine, in a series of cheese-making trials, the effects of glutamate dehydrogenase-producing adjunct cultures on sensory attribute development during the maturation of cheddar cheese.

METHODS AND RESULTS

The presence of dehydrogenase activity with glutamate as substrate was monitored in cell lysates of >100 strains from 30 different species of lactobacilli using a qualitative colorimetric plate screening assay. Activity was detectable in 25 of the 29 representative species obtained from culture collections and in 12 of the 13 non-starter species isolated from cheese. There were pronounced interspecies and strain differences in the occurrence, level and pyridine nucleotide specificity of the glutamate dehydrogenase activity detected. Among the non-starter lactobacilli the highest frequency of enzyme occurrence and activity was detected in the Lactobacillus plantarum isolates. The establishment of glutamate dehydrogenase-producing adjunct strains in the predominant population of lactobacilli in the cheese curd affected the formation of a number of volatile compounds in ripening cheddar cheese, while the presence of Lact. plantarum strains, in particular, was associated with an intensification and acceleration of aroma and flavour development during the maturation period.

CONCLUSIONS

Glutamate dehydrogenase formation by lactobacilli is a strain-dependent metabolic attribute, and adjunct cultures expressing the activity that are able to proliferate during cheese ripening have a positive impact on the rate of development and the intensity of cheddar cheese aroma and flavour development.

SIGNIFICANCE AND IMPACT OF THE STUDY

It has been demonstrated that some strains of glutamate dehydrogenase-producing lactobacilli have potential use as adjunct cultures to accelerate and intensify aroma and flavour formation during the manufacture of cheddar and, by analogy, other similar varieties of cheese. The importance of phenotypic discriminative monitoring of the dominant lactobacilli present during ripening to confirm adjunct establishment and population complexity was highlighted as was the requirement to establish the metabolic attributes of the non-starter population in uninoculated control cheeses in comparative trials.

Reduction of benzenoid synthesis in petunia flowers reveals multiple pathways to benzoic acid and enhancement in auxin transport

In plants, benzoic acid (BA) is believed to be synthesized from Phe through shortening of the propyl side chain by two carbons. It is hypothesized that this chain shortening occurs via either a beta-oxidative or non-beta-oxidative pathway. Previous in vivo isotope labeling and metabolic flux analysis of the benzenoid network in petunia (Petunia hybrida) flowers revealed that both pathways yield benzenoid compounds and that benzylbenzoate is an intermediate between L-Phe and BA. To test this hypothesis, we generated transgenic petunia plants in which the expression of BPBT, the gene encoding the enzyme that uses benzoyl-CoA and benzyl alcohol to make benzylbenzoate, was reduced or eliminated. Elimination of benzylbenzoate formation decreased the endogenous pool of BA and methylbenzoate emission but increased emission of benzyl alcohol and benzylaldehyde, confirming the contribution of benzylbenzoate to BA formation. Labeling experiments with 2H5-Phe revealed a dilution of isotopic abundance in most measured compounds in the dark, suggesting an alternative pathway from a precursor other than Phe, possibly phenylpyruvate. Suppression of BPBT activity also affected the overall morphology of petunia plants, resulting in larger flowers and leaves, thicker stems, and longer internodes, which was consistent with the increased auxin transport in transgenic plants. This suggests that BPBT is involved in metabolic processes in vegetative tissues as well.

Carbohydrate, peptide and lipid metabolism of lactic acid bacteria in sourdough

The metabolic pathways of lactic acid bacteria that influence bread quality are coupled to the central carbon flux by the availability of cofactors influencing the cellular and environmental redox potential. Homo- and heterofermentative metabolism differ fundamentally with respect to the requirement for regeneration of reduced cofactors, NADH or NADPH. The utilization of co-substrates such as oxygen or fructose as electron acceptors by obligate heterofermentative lactobacilli is coupled to an increased production of acetate in dough. Recently, several oxidoreductases involved in cofactor regeneration were characterized and glutathione and short-chain aldehydes derived from lipid oxidation were identified as substrates for cofactor regeneration by Lactobacillus sanfranciscensis. Based on the different metabolic requirements for cofactor regeneration, homo- and heterofermentative lactobacilli exert divergent effects on redox-reactions in sourdough that influence bread quality beyond the formation of acetate. Proteolysis, followed by peptide or amino acid metabolism by LAB is one of the key routes of flavour formation in bread flavour, and enables the strain-specific formation of antifungal metabolites. Peptide metabolism as well as the metabolism of cysteine, arginine, and phenylalanine in Lactobacillus plantarum, L. sanfranciscensis, and Lactobacillus pontis is increasingly understood and these insights provide new opportunities for the directed application of sourdough LAB for improved bread quality.

Influence of peptide supply and cosubstrates on phenylalanine metabolism of Lactobacillus sanfranciscensis DSM20451(T) and Lactobacillus plantarum TMW1.468

Bread spoilage is mainly due to the growth of filamentous fungi, and metabolites produced during sourdough fermentation by lactobacilli can inhibit fungal growth. One of these metabolites is phenyllactic acid (PLA), which is a catabolite from phenylalanine. In this work, the influence of peptide supply and cosubstrates was determined on PLA formation from phenylalanine by Lactobacillus plantarum TMW1.468 and Lactobacillus sanfranciscensis DSM20451(T). Transport of single amino acids is not efficient in lactobacilli, and only 1% of the offered phenylalanine was converted to PLA. PLA yields were increased 2-4-fold when peptides instead of single amino acids were used as a substrate. The accumulation of phenylalanine after peptide addition indicated that, after transport, transamination was the second limiting factor. In L. plantarum TMW1.468, PLA yields were increased from 5 to >30% upon the addition of alpha-ketoglutarate. In L. sanfranciscensis DSM20451, a combination of both citric acid and alpha-ketoglutarate increased PLA formation. The combined effect of citric acid and alpha-ketoglutarate can be attributed to changes in the NAD/NADH ratio.

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

Amino acid conversion to aroma compounds by Lactococcus lactis is limited by the low production of alpha-ketoglutarate that is necessary for the first step of conversion. Recently, glutamate dehydrogenase (GDH) activity that catalyzes the reversible glutamate deamination to alpha-ketoglutarate was detected in L. lactis strains isolated from a vegetal source, and the gene responsible for the activity in L. lactis NCDO1867 was identified and characterized. The gene is located on a 70-kb plasmid also encoding cadmium resistance. In this study, gdh gene inactivation and overexpression confirmed the direct impact of GDH activity of L. lactis on amino acid catabolism in a reaction medium at pH 5.5, the pH of cheese. By using cadmium resistance as a selectable marker, the plasmid carrying gdh was naturally transmitted to another L. lactis strain by a mating procedure. The transfer conferred to the host strain GDH activity and the ability to catabolize amino acids in the presence of glutamate in the reaction medium. However, the plasmid appeared unstable in a strain also containing the protease lactose plasmid pLP712, indicating an incompatibility between these two plasmids.

Ability of thermophilic lactic acid bacteria to produce aroma compounds from amino acids

Although a large number of key odorants of Swiss-type cheese result from amino acid catabolism, the amino acid catabolic pathways in the bacteria present in these cheeses are not well known. In this study, we compared the in vitro abilities of Lactobacillus delbrueckii subsp. lactis, Lactobacillus helveticus, and Streptococcus thermophilus to produce aroma compounds from three amino acids, leucine, phenylalanine, and methionine, under mid-pH conditions of cheese ripening (pH 5.5), and we investigated the catabolic pathways used by these bacteria. In the three lactic acid bacterial species, amino acid catabolism was initiated by a transamination step, which requires the presence of an alpha-keto acid such as alpha-ketoglutarate (alpha-KG) as the amino group acceptor, and produced alpha-keto acids. Only S. thermophilus exhibited glutamate dehydrogenase activity, which produces alpha-KG from glutamate, and consequently only S. thermophilus was capable of catabolizing amino acids in the reaction medium without alpha-KG addition. In the presence of alpha-KG, lactobacilli produced much more varied aroma compounds such as acids, aldehydes, and alcohols than S. thermophilus, which mainly produced alpha-keto acids and a small amount of hydroxy acids and acids. L. helveticus mainly produced acids from phenylalanine and leucine, while L. delbrueckii subsp. lactis produced larger amounts of alcohols and/or aldehydes. Formation of aldehydes, alcohols, and acids from alpha-keto acids by L. delbrueckii subsp. lactis mainly results from the action of an alpha-keto acid decarboxylase, which produces aldehydes that are then oxidized or reduced to acids or alcohols. In contrast, the enzyme involved in the alpha-keto acid conversion to acids in L. helveticus and S. thermophilus is an alpha-keto acid dehydrogenase that produces acyl coenzymes A.

CodY-regulated aminotransferases AraT and BcaT play a major role in the growth of Lactococcus lactis in milk by regulating the intracellular pool of amino acids

Aminotransferases, which catalyze the last step of biosynthesis of most amino acids and the first step of their catabolism, may be involved in the growth of Lactococcus lactis in milk. Previously, we isolated two aminotransferases from L. lactis, AraT and BcaT, which are responsible for the transamination of aromatic amino acids, branched-chain amino acids, and methionine. In this study, we demonstrated that double inactivation of AraT and BcaT strongly reduced the growth of L. lactis in milk. Supplementation of milk with amino acids and keto acids that are substrates of both aminotransferases did not improve the growth of the double mutant. On the contrary, supplementation of milk with isoleucine or a dipeptide containing isoleucine almost totally inhibited the growth of the double mutant, while it did not affect or only slightly affected the growth of the wild-type strain. These results suggest that AraT and BcaT play a major role in the growth of L. lactis in milk by degrading the intracellular excess isoleucine, which is responsible for the growth inhibition. The growth inhibition by isoleucine is likely to be due to CodY repression of the proteolytic system, which is necessary for maximal growth of L. lactis in milk, since the growth of the CodY mutant was not affected by addition of isoleucine to milk. Moreover, we demonstrated that AraT and BcaT are part of the CodY regulon and therefore are regulated by nutritional factors, such as the carbohydrate and nitrogen sources.

Dual influence of the carbon source and L-methionine on the synthesis of sulphur compounds in the cheese-ripening yeast Geotrichum candidum

The effect of the carbon source and l-methionine on the ability of Geotrichum candidum to produce volatile sulphur compounds (VSC) was studied. This yeast was cultivated in a synthetic medium supplemented with various carbon sources and l-methionine at different concentrations. Both glycerol and glucose significantly increased VSC production by G. candidum. Unlike the effect on the l-methionine- and 4-methylthio-2-oxobutyric acid-demethiolating activities, the supply of a carbon source had a dramatic effect on the activity of aminotransferase, a key enzyme in l-methionine catabolism. An increase in the initial concentration of l-methionine resulted in a rise in the production of sulphur compounds (VSC, 4-methylthio-2-oxobutyric acid) but had limited effect on l-methionine-catabolising enzyme activities. Evidence for the existence of a dual effect of the carbon source and l-methionine on VSC biosynthesis was obtained in this study.

Cooperation between Lactococcus lactis and nonstarter lactobacilli in the formation of cheese aroma from amino acids

In Gouda and Cheddar type cheeses the amino acid conversion to aroma compounds, which is a major process for aroma formation, is essentially due to lactic acid bacteria (LAB). In order to evaluate the respective role of starter and nonstarter LAB and their interactions in cheese flavor formation, we compared the catabolism of phenylalanine, leucine, and methionine by single strains and strain mixtures of Lactococcus lactis subsp. cremoris NCDO763 and three mesophilic lactobacilli. Amino acid catabolism was studied in vitro at pH 5.5, by using radiolabeled amino acids as tracers. In the presence of alpha-ketoglutarate, which is essential for amino acid transamination, the lactobacillus strains degraded less amino acids than L. lactis subsp. cremoris NCDO763, and produced mainly nonaromatic metabolites. L. lactis subsp. cremoris NCDO763 produced mainly the carboxylic acids, which are important compounds for cheese aroma. However, in the reaction mixture containing glutamate, only two lactobacillus strains degraded amino acids significantly. This was due to their glutamate dehydrogenase (GDH) activity, which produced alpha-ketoglutarate from glutamate. The combination of each of the GDH-positive lactobacilli with L. lactis subsp. cremoris NCDO763 had a beneficial effect on the aroma formation. Lactobacilli initiated the conversion of amino acids by transforming them mainly to keto and hydroxy acids, which subsequently were converted to carboxylic acids by the Lactococcus strain. Therefore, we think that such cooperation between starter L. lactis and GDH-positive lactobacilli can stimulate flavor development in cheese.

Genetically modified lactic acid bacteria: applications to food or health and risk assessment

Lactic acid bacteria have a long history of use in fermented food products. Progress in gene technology allows their modification by introducing new genes or by modifying their metabolic functions. These modifications may lead to improvements in food technology (bacteria better fitted to technological processes, leading to improved organoleptic properties em leader ), or to new applications including bacteria producing therapeutic molecules that could be delivered by mouth. Examples in these two fields will be discussed, at the same time evaluating their potential benefit to society and the possible risks associated with their use. Risk assessment and expected benefits will determine the future use of modified bacteria in the domains of food technology and health.

Recombinant Lactococcus starters as a potential source of additional peptidolytic activity in cheese ripening

AIMS

The aim of this study was to modulate the lactococcal proteolytic system for enhancement of the cheese ripening process.

METHODS AND RESULTS

The genes encoding PepN, PepC, PepX and PepI peptidases of a highly proteolytic Lactobacillus helveticus strain were transferred into Lactococcus lactis in a food-grade cloning system. A comparison of the relative peptidase activities from the transformants with those from the untransformed host, determined in the conditions of maturing cheese, showed that an increase in peptidase activity could be achieved by introducing a selected peptidase gene from Lact. helveticus into L. lactis.

CONCLUSIONS

Recombinant L. lactis starter strains, carrying a peptidase gene from Lact. helveticus, may have an important contribution to the proteolysis of maturing cheese by producing an additional peptidolytic enzyme activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results will be of importance in shortening the ripening period and production of special cheeses (e.g. reduced-fat cheeses) with improved characteristics.

Current and future benefits from the use of GM technology in food production

For the current generation of genetically modified (GM) crops the improvement of agronomic traits (e.g. herbicide tolerance, insect resistance) has been a major objective. The lack of obvious and direct benefits for the consumer has been a main point of criticism. Future trends will increasingly encompass the modification of quality traits, such as the improvement of sensory and especially nutritional properties. Some of the ongoing developments try to meet the desire of consumers for 'healthy' or 'high-tech' foods in developed countries. Others are intended to assist in adjusting the nutritional status of foods to the needs of consumers in developing countries. Considering the increasing world population and the limited amount of arable land, GM technology may also become a valuable tool to ensure food security. The major prerequisite for the applicability of the technique is the safety of the resulting products. The increasing complexity of modifications intended might require adjustments and improvements of the strategies applied to the safety assessment of GM foods. Present research activities try to meet these new challenges.

Conversion of L-leucine to isovaleric acid by Propionibacterium freudenreichii TL 34 and ITGP23

Several branched-chain volatile compounds are involved in the flavor of Swiss cheese. These compounds are probably produced by enzymatic conversion of branched-chain amino acids, but the flora and the pathways involved remain hypothetical. Our aim was to determine the ability of Propionibacterium freudenreichii, which is one of the main components of the secondary flora of Swiss cheese, to produce flavor compounds during leucine catabolism. Cell extracts and resting cells of two strains were incubated in the presence of L-leucine, alpha-ketoglutaric acid, and cofactors, and the metabolites produced were determined by high-performance liquid chromatography and gas chromatography. The first step of leucine catabolism was a transamination that produced alpha-ketoisocaproic acid, which was enzymatically converted to isovaleric acid. Both reactions were faster at pH 8.0 than at acidic pHs. Cell extracts catalyzed only the transamination step under our experimental conditions. Small amounts of 3-methylbutanol were also produced by resting cells, but neither 3-methylbutanal noralpha-hydroxyisocaproic acid was detected. L-Isoleucine and L-valine were also converted to the corresponding acids and alcohols. Isovaleric acid was produced by both strains during growth in a complex medium, even under conditions simulating Swiss cheese conditions (2.1% NaCl, pH 5.4, 24 degrees C). Our results show that P. frendenreichii could play a significant role in the formation of isovaleric acid during ripening.