High-throughput screening for aroma production in food fermentations

A microtiter plate (MTP) method was developed to screen 1064 unique microorganisms-substrate fermentations for production of 68 target aroma compounds. Based on the number of hits identified by GC-MS, 50 fermentations were repeated at 50-mL scale in flasks. Comparison of GC-MS data showed that scaling up from MTP to flask did not generally result in large differences between the volatile profiles, even with a wide variety of substrates (juice, food slurry and food side-streams) and microorganisms (yeast, bacteria and fungi) used. From the screening results, Lactobacillus plantarum fermentation of chilli pepper was further studied as a high amount of phenols, especially guaiacol and 4-ethylphenol, was produced after fermentation. From HPLC-MS and sensory analysis, capsaicin was shown to be a probable precursor for these phenols and a potential mechanism was proposed. The protocol described herein to screen aroma compounds from fermentation of agri-food products and side streams can support development of clean label flavourful food ingredients.

Exploring the potential of Bacillus subtilis as cell factory for food ingredients and special chemicals

BACKGROUND

Bacillus subtilis has been established as model microorganism for fundamental research in the laboratory on protein production/secretion and sporulation and as model bacterium for controlling spoilage in the food industry. It has also been used for production of (commercial) enzymes and several secondary metabolites such as vitamins. However, this doesn't fully reflect the potential of B. subtilis as a cell-factory. Here, various strains of B. subtilis, including food-grade, spore-deficient strains and industrially used strains, were compared for their growth and metabolic potential. Industry-relevant parameters were analyzed for all strains under various aeration regimes, under anaerobic conditions, in various nutritious and nutrient-limited cultivation media, with and without organic nitrogen sources, and with and without sugar.

RESULTS

Practical experiments were conducted to compare industrial relevant properties like growth rates, intracellular components and extracellular metabolite profile of different B. subtilis strains. Based on growth flexibility in different media, we found that some strains like NCIB3610 and DSM1092 are adapted to inorganic or organic nitrogen source utilization, which is highly relevant when considering a biorefinery approach using various cheap and abundant waste/sidestreams. Secondly, spore-deficient strains such as 3NA, 168 S and PY79S, showed advantages in microbial protein and acetolactate pathway expression, which is associated with applications in food industry for protein supplement and diacetyl production. Lastly, WB800 and PY79S exhibited potential for fermentative production of dipicolinic acid, 2,3-butanediol and lactic acid that could serve as precursors for biopolymers.

CONCLUSION

This study demonstrates the broad potential for more extensive industrial use of Bacillus subtilis in the (bio-based) chemical industry for use of sidestreams, in the personal care industry, in the food industry for food additive production, and in the bio-sustainable industry for biofuel and bio-degradable plastic precursors production. In addition, selecting different B. subtilis strains for specific purposes makes full use of the diversity of this species and increases the potential of B. subtilis in its contribution to the bio-based economy.

Model-driven approach for the production of butyrate from CO2/H2 by a novel co-culture of C. autoethanogenum and C. beijerinckii

One-carbon (C1) compounds are promising feedstocks for the sustainable production of commodity chemicals. CO₂ is a particularly advantageous C1-feedstock since it is an unwanted industrial off-gas that can be converted into valuable products while reducing its atmospheric levels. Acetogens are microorganisms that can grow on CO₂/H₂ gas mixtures and syngas converting these substrates into ethanol and acetate. Co-cultivation of acetogens with other microbial species that can further process such products, can expand the variety of products to, for example, medium chain fatty acids (MCFA) and longer chain alcohols. Solventogens are microorganisms known to produce MCFA and alcohols via the acetone-butanol-ethanol (ABE) fermentation in which acetate is a key metabolite. Thus, co-cultivation of an acetogen and a solventogen in a consortium provides a potential platform to produce valuable chemicals from CO₂. In this study, metabolic modeling was implemented to design a new co-culture of an acetogen and a solventogen to produce butyrate from CO₂/H₂ mixtures. The model-driven approach suggested the ability of the studied solventogenic species to grow on lactate/glycerol with acetate as co-substrate. This ability was confirmed experimentally by cultivation of Clostridium beijerinckii on these substrates in batch serum bottles and subsequently in pH-controlled bioreactors. Community modeling also suggested that a novel microbial consortium consisting of the acetogen Clostridium autoethanogenum, and the solventogen C. beijerinckii would be feasible and stable. On the basis of this prediction, a co-culture was experimentally established. C. autoethanogenum grew on CO₂/H₂ producing acetate and traces of ethanol. Acetate was in turn, consumed by C. beijerinckii together with lactate, producing butyrate. These results show that community modeling of metabolism is a valuable tool to guide the design of microbial consortia for the tailored production of chemicals from renewable resources.

Sustainable Production of the Cyanophycin Biopolymer in Tobacco in the Greenhouse and Field

The production of biodegradable polymers as coproducts of other commercially relevant plant components can be a sustainable strategy to decrease the carbon footprint and increase the commercial value of a plant. The biodegradable polymer cyanophycin granular polypeptide (CGP) was expressed in the leaves of a commercial tobacco variety, whose seeds can serve as a source for biofuel and feed. In T0 generation in the greenhouse, up to 11% of the leaf dry weight corresponded to the CGP. In T1 generation, the maximum content decreased to approximately 4% dw, both in the greenhouse and first field trial. In the field, a maximum harvest of 4 g CGP/plant could be obtained. Independent of the CGP content, most transgenic plants exhibited a slight yield penalty in the leaf biomass, especially under stress conditions in greenhouse and field trials. After the harvest, the leaves were either Sun dried or ensiled. The resulting material was used to evaluate the extraction of CGP compared to that in the laboratory protocol. The farm-level analysis indicates that the extraction of CGP from tobacco plants can provide alternative income opportunities for tobacco farmers. The CGP yield/ha indicates that the CGP production in plants can be economically feasible depending on the cultivation and extraction costs. Moreover, we analyzed the consumer acceptance of potential applications associated with GM tobacco in four European countries (Germany, Finland, Italy and the Netherlands) and found unexpectedly high acceptance.

Transcriptome Analysis of a Spray Drying-Resistant Subpopulation Reveals a Zinc-Dependent Mechanism for Robustness in L. lactis SK11

The viability of starter cultures is essential for an adequate contribution to the fermentation process and end-product. Therefore, robustness during processing and storage is an important characteristic of starter culture strains. For instance, during spray drying cells are exposed to heat and oxidative stress, generally resulting in loss of viability. In this study, we exposed the industrially relevant but stress-sensitive Lactococcus lactis strain SK11 to two cycles of heat stress, with intermediate recovery and cultivation at moderate temperatures. After these two cycles of heat exposure, the abundance of robust derivatives was increased as compared with the original culture, which enabled isolation of heat-resistant subpopulations displaying up to 1,000-fold enhanced heat stress survival. Moreover, this heat-resistant subpopulation demonstrated an increased survival during spray drying. Derivatives from two independent lineages displayed different transcriptome changes as compared with the wild type strain, indicating that the increased robustness within these lineages was established by different adaptive strategies. Nevertheless, an overlap in differential gene expression in all five derivatives tested in both lineages included three genes in an operon involved in zinc transport. The link between zinc homeostasis and heat stress survival in L. lactis was experimentally established by culturing of the wild type strain SK11 in medium with various levels of zinc ions, which resulted in alterations in heat stress survival phenotypes. This study demonstrates that robust derivatives of a relatively sensitive L. lactis strain can be isolated by repeated exposure to heat stress. Moreover, this work demonstrates that transcriptome analysis of these robust derivatives can provide clues for improvement of the robustness of the original strain. This could boost the industrial application of strains with specific desirable traits but inadequate robustness characteristics.

Strain-Dependent Transcriptome Signatures for Robustness in Lactococcus lactis

Recently, we demonstrated that fermentation conditions have a strong impact on subsequent survival of Lactococcus lactis strain MG1363 during heat and oxidative stress, two important parameters during spray drying. Moreover, employment of a transcriptome-phenotype matching approach revealed groups of genes associated with robustness towards heat and/or oxidative stress. To investigate if other strains have similar or distinct transcriptome signatures for robustness, we applied an identical transcriptome-robustness phenotype matching approach on the L. lactis strains IL1403, KF147 and SK11, which have previously been demonstrated to display highly diverse robustness phenotypes. These strains were subjected to an identical fermentation regime as was performed earlier for strain MG1363 and consisted of twelve conditions, varying in the level of salt and/or oxygen, as well as fermentation temperature and pH. In the exponential phase of growth, cells were harvested for transcriptome analysis and assessment of heat and oxidative stress survival phenotypes. The variation in fermentation conditions resulted in differences in heat and oxidative stress survival of up to five 10-log units. Effects of the fermentation conditions on stress survival of the L. lactis strains were typically strain-dependent, although the fermentation conditions had mainly similar effects on the growth characteristics of the different strains. By association of the transcriptomes and robustness phenotypes highly strain-specific transcriptome signatures for robustness towards heat and oxidative stress were identified, indicating that multiple mechanisms exist to increase robustness and, as a consequence, robustness of each strain requires individual optimization. However, a relatively small overlap in the transcriptome responses of the strains was also identified and this generic transcriptome signature included genes previously associated with stress (ctsR and lplL) and novel genes, including nanE and genes encoding transport proteins. The transcript levels of these genes can function as indicators of robustness and could aid in selection of fermentation parameters, potentially resulting in more optimal robustness during spray drying.

Polyol production during heterofermentative growth of the plant isolate Lactobacillus florum 2F

AIMS

This study examined the fermentative growth and polyol production of Lactobacillus florum and other plant-associated lactic acid bacteria (LAB).

METHODS AND RESULTS

Sugar consumption and end-product production were measured for Lact. florum 2F in the presence of fructose, glucose and both sugars combined. The genome of Lact. florum was examined for genes required for mannitol and erythritol biosynthesis. The capacity for other plant-associated LAB to synthesize polyols was also assessed.

CONCLUSIONS

Lactobacillus florum exhibited higher growth rates and cell yields in the presence of both fructose and glucose. Lactobacillus florum 2F produced lactate, acetate and ethanol as well as erythritol and mannitol. Lactobacillus florum 2F synthesized mannitol during growth on fructose and erythritol during growth on glucose. Gene and protein homology searches identified a mannitol dehydrogenase in the Lact. florum 2F genome but not the genes responsible for erythritol biosynthesis. Lastly, we found that numerous other heterofermentative LAB species synthesize erythritol and/or mannitol.

SIGNIFICANCE AND IMPACT OF THE STUDY

Lactobacillus florum is a recently identified, plant-associated, fructophilic LAB species. Our results show that Lact. florum growth rates and heterofermentation end-products differ depending on the sugar substrates present and growth yields can be improved when combinations of sugars are provided. Lactobacillus florum 2F produces erythritol and mannitol, two polyols that are relevant to foods and potentially also in plant environments. The capacity for polyol biosynthesis appears to be common among plant-associated, LAB species.

Genome-scale reconstruction of the Streptococcus pyogenes M49 metabolic network reveals growth requirements and indicates potential drug targets

Genome-scale metabolic models comprise stoichiometric relations between metabolites, as well as associations between genes and metabolic reactions and facilitate the analysis of metabolism. We computationally reconstructed the metabolic network of the lactic acid bacterium Streptococcus pyogenes M49. Initially, we based the reconstruction on genome annotations and already existing and curated metabolic networks of Bacillus subtilis, Escherichia coli, Lactobacillus plantarum and Lactococcus lactis. This initial draft was manually curated with the final reconstruction accounting for 480 genes associated with 576 reactions and 558 metabolites. In order to constrain the model further, we performed growth experiments of wild type and arcA deletion strains of S. pyogenes M49 in a chemically defined medium and calculated nutrient uptake and production fluxes. We additionally performed amino acid auxotrophy experiments to test the consistency of the model. The established genome-scale model can be used to understand the growth requirements of the human pathogen S. pyogenes and define optimal and suboptimal conditions, but also to describe differences and similarities between S. pyogenes and related lactic acid bacteria such as L. lactis in order to find strategies to reduce the growth of the pathogen and propose drug targets.

Use of non-growing Lactococcus lactis cell suspensions for production of volatile metabolites with direct relevance for flavour formation during dairy fermentations

Background

Lactococcus lactis is a lactic acid bacterium that has been used for centuries in the production of a variety of cheeses, as these bacteria rapidly acidify milk and greatly contribute to the flavour of the fermentation end-products. After a short growth phase during cheese ripening L. lactis enters an extended non-growing state whilst still strongly contributing to amino acid-derived flavour formation. Here, a research approach is presented that allows investigation of strain- and amino acid-specific flavour formation during the non-growing state.

Results

Non-growing cells of five selected L. lactis strains were demonstrated to degrade amino acids into flavour compounds that are relevant in food fermentations and differs greatly from production of flavour compounds using growing cells. As observed earlier in other research set-ups and with other microorganisms, addition of NADH, α-ketoglutarate and pyridoxal-5-phosphate was demonstrated to be essential for optimal flavour formation, suggesting that intracellular pools of these substrates are too low for the significant production of the flavour compounds. Production of flavours during the non-growing phase strongly depends on the individual amino acids that were supplied, on the presence of other amino acids (mixtures versus single compounds), and on the strain used. Moreover, we observed that the plasmid-free model strains L. lactis MG1363 and IL1403 produce relatively low amounts of flavour components under the various conditions tested.

Conclusions

By using this simplified and rapid approach to study flavour formation by non-growing lactic acid bacteria, lengthy ripening periods are no longer required to assess the capacity of strains to produce flavours in the long, non-growing state of dairy fermentation. In addition, this method also provides insight into the conversion of single amino acids versus the conversion of a mixture of amino acids as produced during protein degradation. The generated results are complementary to earlier generated datasets using growing cells, allowing assessment of the full flavour forming potential of strains used as starter cultures in industrial food fermentation processes.

Fermentation-induced variation in heat and oxidative stress phenotypes of Lactococcus lactis MG1363 reveals transcriptome signatures for robustness

Background

Lactococcus lactis is industrially employed to manufacture various fermented dairy products. The most cost-effective method for the preservation of L. lactis starter cultures is spray drying, but during this process cultures encounter heat and oxidative stress, typically resulting in low survival rates. However, viability of starter cultures is essential for their adequate contribution to milk fermentation, supporting the ambition to better understand and improve their robustness phenotypes.

Results

This study describes a transcriptome-phenotype matching approach in which the starter L. lactis MG1363 was fermented under a variety of conditions that differed in the levels of oxygen and/or salt, as well as the fermentation pH and temperature. Samples derived from these fermentations in the exponential phase of bacterial growth were analyzed by full-genome transcriptomics and the assessment of heat and oxidative stress phenotypes. Variations in the fermentation conditions resulted in up to 1000-fold differences in survival during heat and oxidative stress. More specifically, aeration during fermentation induced protection against heat stress, whereas a relatively high fermentation temperature resulted in enhanced robustness towards oxidative stress. Concomitantly, oxygen levels and fermentation temperature induced differential expression of markedly more genes when compared with the other fermentation parameters. Correlation analysis of robustness phenotypes and gene expression levels revealed transcriptome signatures for oxidative and/or heat stress survival, including the metC-cysK operon involved in methionine and cysteine metabolism. To validate this transcriptome-phenotype association we grew L. lactis MG1363 in the absence of cysteine which led to enhanced robustness towards oxidative stress.

Conclusions

Overall, we demonstrated the importance of careful selection of fermentation parameters prior to industrial processing of starter cultures. Furthermore, established stress genes as well as novel genes were associated with robustness towards heat and/or oxidative stress. Assessment of the expression levels of this group of genes could function as an indicator for enhanced selection of fermentation parameters resulting in improved robustness during spray drying. The increased robustness after growth without cysteine appeared to confirm the role of expression of the metC-cysK operon as an indicator of robustness and suggests that sulfur amino acid metabolism plays a pivotal role in oxidative stress survival.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0148-6) contains supplementary material, which is available to authorized users.

Traditional biotechnology for new foods and beverages

The food and beverage industry is re-discovering fermentation as a crucial step in product innovation. Fermentation can provide various benefits such as unique flavor, health and nutrition, texture and safety (shelf life), while maintaining a 100% natural label. In this review several examples are presented on how fermentation is used to replace, modify or improve current, artificially produced, foods and beverages and how also fermentation can be used for completely novel consumer products.

Functional genomics for food fermentation processes

This review describes recent scientific and technological drivers of food fermentation research. In addition, a number of practical implications of the results of this development will be highlighted. The first part of the manuscript elaborates on the message that genome sequence information gives us an unprecedented view on the biodiversity of microbes in food fermentation. This information can be made applicable for tailoring relevant characteristics of food products through fermentation. The second part deals with the integration of genome sequence data into metabolic models and the use of these models for a number of topics that are relevant for food fermentation processes. The final part will be about metagenomics approaches to reveal the complexity and understand the functionality of undefined complex microbial consortia used in a diverse range of food fermentation processes.

Functional identification in Lactobacillus reuteri of a PocR-like transcription factor regulating glycerol utilization and vitamin B12 synthesis

Background

Lactobacillus reuteri harbors the genes responsible for glycerol utilization and vitamin B₁₂ synthesis within a genetic island phylogenetically related to gamma-Proteobacteria. Within this island, resides a gene (lreu_1750) that based on its genomic context has been suggested to encode the regulatory protein PocR and presumably control the expression of the neighboring loci. However, this functional assignment is not fully supported by sequence homology, and hitherto, completely lacks experimental confirmation.

Results

In this contribution, we have overexpressed and inactivated the gene encoding the putative PocR in L. reuteri. The comparison of these strains provided metabolic and transcriptional evidence that this regulatory protein controls the expression of the operons encoding glycerol utilization and vitamin B₁₂ synthesis.

Conclusions

We provide clear experimental evidence for assigning Lreu_1750 as PocR in Lactobacillus reuteri. Our genome-wide transcriptional analysis further identifies the loci contained in the PocR regulon. The findings reported here could be used to improve the production-yield of vitamin B₁₂, 1,3-propanediol and reuterin, all industrially relevant compounds.

Supplementation with engineered Lactococcus lactis improves the folate status in deficient rats

OBJECTIVE

The aim of this study was to establish the bioavailability of different folates produced by engineered Lactococcus lactis strains using a rodent depletion-repletion bioassay.

METHODS

Rats were fed a folate-deficient diet, which produces a reversible subclinical folate deficiency, supplemented with different L. lactis cultures that were added as the only source of folate. Three bacterial strains that overexpressed the folC, folKE, or folC +KE genes were used. These strains produce folates with different poly glutamyl tail lengths. The growth response of the rats and the concentration of folates in different organs and blood samples were monitored.

RESULTS

The folate produced by the engineered strains was able to compensate the folate depletion in the diet and showed similar bioavailability compared with commercial folic acid that is normally used for food fortification. Folate concentrations in organ and blood samples increased significantly in animals that received the folate-producing strains compared with those that did not receive bacterial supplementation. Hematologic studies also showed that administration of the L. lactis strains was able to revert a partial megaloblastic anemia caused by folate deficiency. No significant differences were observed in the bioavailability of folates containing different glutamyl tail lengths.

CONCLUSION

To our knowledge, this is the first study that demonstrated that folates produced by engineered lactic acid bacteria represent a bioavailable source of this essential vitamin.

Understanding the physiology of Lactobacillus plantarum at zero growth

The physiology of Lactobacillus plantarum at extremely low growth rates, through cultivation in retentostats, is much closer to carbon-limited growth than to stationary phase, as evidenced from transcriptomics data, metabolic fluxes, and biomass composition and viability.

Using a genome-scale metabolic model and constraint-based computational analyses, amino-acid fluxes—in particular, the rather paradoxical excretion of Asp, Arg, Met, and Ala—could be rationalized as a means to allow extensive metabolism of other amino acids, that is, that of branched-chain and aromatic amino acids.

Catabolic products from aromatic amino acids are known to have putative plant-hormone action. The metabolism of amino acids, as well as transcription data, strongly suggested a plant environment-like response in slow-growing L. plantarum, which was confirmed by significant effects of fermented medium on plant root formation.

Natural ecosystems are usually characterized by extremely low and fluctuating nutrient availability. Hence, microorganisms in these environments live a ‘feast-and-famine' existence, with famine the most habitual state. As a result, extremely slow or no growth is the most common state of bacteria, and maintenance processes dominate their life.

In the present study, Lactobacillus plantarum was used as a model microorganism to investigate the physiology of slow growth. Besides fermented foods, this microorganism can be observed in a variety of environmental niches, including plants and lakes, in which nutrient supply is limited. To mimic these conditions, L. plantarum was grown in a glucose-limited chemostat with complete biomass retention (retentostat). During cultivation, biomass progressively accumulated, resulting in steadily decreasing specific substrate availability. Less energy was thus available for growth, and the specific growth rate decreased accordingly, with a final calculated doubling time greater than one year. Detailed measurements of metabolic fluxes were used as constraints in a genome-scale metabolic model to precisely calculate the amount of energy used for net biomass synthesis and for maintenance purposes: at the lowest growth rate investigated (μ=0.0002 h⁻¹), maintenance accounted for 94% of all energy expenses.

Genome-scale metabolic analysis was used in combination with transcriptomics to study the adaptation of L. plantarum to extremely slow growth under limited carbon and energy supply. Importantly, slow growth as investigated here was fundamentally different from the widely studied carbon starvation-induced stationary phase: non-growing cells in retentostat conditions were glucose limited rather than starved, and the transition from a growing to a non-growing state under retentostat conditions was progressive, in contrast with the abrupt transition in batch cultures. These differences were reflected in various aspects of the cell physiology.

The metabolic behavior was remarkably stable during adaptation to slow growth. Although carbon catabolite repression was clearly relieved, as indicated by the upregulation of genes for the utilization of alternative carbohydrates, the metabolism remained largely based on the conversion of glucose to lactate.

Stress resistance mechanisms were also not massively induced. In particular, analysis of the biomass composition—which remained similar to fast-growing cells even under virtually non-growing conditions—and of the gene expression profile, failed to reveal clear stringent or general stress responses, which are generally triggered in glucose-starved cells. The observation that genes involved in growth-associated processes were not downregulated suggested that active synthesis of biomass components (RNA, proteins, and membranes) was required to account for the observed stable biomass and that turnover of macromolecules was high in slow-growing cells. Biomass viability or morphology was also not affected, compared with faster growth conditions. The only typical stress response was the induction of an SOS response—in particular, the upregulation of the two error-prone DNA polymerases—suggesting an increased potential for genetic diversity under adverse conditions. Although diversity was not apparent under the conditions studied here, such mechanisms of increased rates of mutagenesis are likely to have an important role in the adaptation of L. plantarum to slow growth.

A surprising response of L. plantarum during adaptation to slow growth was the production of several amino acids (Arg, Asp, Met, and Ala). A priori, this metabolic behavior seemed inefficient in a context of energy limitation. However, reduced cost analysis using the genome-scale metabolic model indicated that it had a positive effect on energy generation. In-depth analysis of metabolic flux distributions showed that biosynthesis of these amino acids was connected to the catabolism of branched-chain and aromatic amino acids (BCAAs and AAAs), under conditions of limited ammonium efflux. At a fixed ammonium efflux—fixed at the measured value—flux balance analysis indicated that BCAAs and AAAs were expensive to metabolize, because the regeneration of 2-ketoglutarate through glutamate dehydrogenase was limited by ammonium dissipation. Therefore, alternative pathways had to be active to supply the necessary pool of 2-ketoglutarate. At low growth rates, amino-acid production (Arg, Asp, Ala, and Met) accounted for most of the 2-ketoglutarate regeneration. Although it came at the expense of ATP, this metabolic alternative to glutamate dehydrogenase was less energy costly than other solutions such as purine biosynthesis. This is thus an excellent example in which precise, quantitative modeling results in new insights in physiology that intuition would never have achieved. It also shows that flux balance analysis can be used to accurately predict energetically inefficient metabolism, provided the appropriate fluxes are constrained (here, ammonium efflux).

The observation that BCAAs and AAAs were catabolized at the expense of energy was intriguing. However, several end products of these catabolic pathways can serve as signaling molecules for interactions with other organisms. In particular, precursors of plant hormones were predicted as possible end products in the model simulations. Accordingly, the production of compounds interfering with plant root development was demonstrated in slow-growing L. plantarum. The metabolic analysis thus suggested that slow-growing L. plantarum produced plant hormones—or precursors thereof—as a strategy to divert the plant metabolism towards its own interest. In support of this view, transcriptome analysis indicated the upregulation of genes involved in the catabolism of β-glucosides—typical sugars from plant cell wall—as well as a very high induction of six gene clusters encoding cell-surface protein complexes predicted to have a role in the utilization of plant polysaccharides (csc clusters). In such a plant context, limited ammonium production would also make sense, because of the well-documented toxicity of ammonium for plants: production of amino acids could represent an alternative to ammonium excretion while keeping both parties satisfied.

In conclusion, the physiology of L. plantarum at extremely low growth rates, as studied by genome-scale metabolic modeling and transcriptomics, is fundamentally different from that of starvation-induced stationary phase cells. Excitingly, these conditions seem to trigger responses that favor interactions with the environment, more specifically with plants. The reported observations were made in the absence of any plant-derived material, suggesting that this response might constitute a hardwired behavior.

Situations of extremely low substrate availability, resulting in slow growth, are common in natural environments. To mimic these conditions, Lactobacillus plantarum was grown in a carbon-limited retentostat with complete biomass retention. The physiology of extremely slow-growing L. plantarum—as studied by genome-scale modeling and transcriptomics—was fundamentally different from that of stationary-phase cells. Stress resistance mechanisms were not massively induced during transition to extremely slow growth. The energy-generating metabolism was remarkably stable and remained largely based on the conversion of glucose to lactate. The combination of metabolic and transcriptomic analyses revealed behaviors involved in interactions with the environment, more particularly with plants: production of plant hormones or precursors thereof, and preparedness for the utilization of plant-derived substrates. Accordingly, the production of compounds interfering with plant root development was demonstrated in slow-growing L. plantarum. Thus, conditions of slow growth and limited substrate availability seem to trigger a plant environment-like response, even in the absence of plant-derived material, suggesting that this might constitute an intrinsic behavior in L. plantarum.

Selection of Protease-Positive and Protease-Negative Variants of Streptococcus cremoris

Protease-negative variants were shown to outcompete the wild-type strains of Streptococcus cremoris E(8), HP, and Wg(2) at pH values higher than 6.0 in milk. For S. cremoris E(8) this process was studied in more detail. At lower pH values the wild type had a selective advantage. This pH-dependent selection was not found in all media tested. The poor growth of the protease-negative variant at low pH was not due to lower internal pH values. By growing S. cremoris E(8) and Wg(2) in acidified milk (pH 5.9) the proteolytic activity of the cultures could be stabilized. In continuous cultures under amino acid limitation the wild type S. cremoris E(8) and HP strains had a selective advantage over the protease-negative variants at low dilution rates (D < 0.2) at all pH values of the medium. This was apparently due to a lower affinity-constant (K(s)) of the protease-positive variants for amino acids. Finally, a high fraction of protease-positive variants could be maintained in continuous cultures by using a growth medium with low concentrations of casein as a nitrogen source. At high dilution rates nearly all cells were protease positive.

Citrate Fermentation by Lactococcus and Leuconostoc spp

Citrate and lactose fermentation are subject to the same metabolic regulation. In both processes, pyruvate is the key intermediate. Lactococcus lactis subsp. lactis biovar diacetylactis homofermentatively converted pyruvate to lactate at high dilution (growth) rates, low pH, and high lactose concentrations. Mixed-acid fermentation with formate, ethanol, and acetate as products was observed under conditions of lactose limitation in continuous culture at pH values above 6.0. An acetoin/butanediol fermentation with alpha-acetolactate as an intermediate was found upon mild aeration in continuous culture and under conditions of excess pyruvate production from citrate. Leuconostoc spp. showed a limited metabolic flexibility. A typical heterofermentative conversion of lactose was observed under all conditions in both continuous and batch cultures. The pyruvate produced from either lactose or citrate was converted to d-lactate. Citrate utilization was pH dependent in both L. lactis and Leuconostoc spp., with maximum rates observed between pH 5.5 and 6.0. The maximum specific growth rate was slightly stimulated by citrate, in L. lactis and greatly stimulated by citrate in Leuconostoc spp., and the conversion of citrate resulted in increased growth yields on lactose for both L. lactis and Leuconostoc spp. This indicates that energy is conserved during the metabolism of citrate.

High-level expression of Lactobacillus beta-galactosidases in Lactococcus lactis using the food-grade, nisin-controlled expression system NICE

In this work the overlapping genes (lacL and lacM) encoding heterodimeric beta-galactosidases from Lactobacillus reuteri , Lb. acidophilus , Lb. sakei , and Lb. plantarum were cloned into two different nisin-controlled expression (NICE) vectors and expressed using Lactococcus lactis NZ9000 and NZ3900 as hosts. The lacL gene, encoding the large subunit of the beta-galactosidases, was fused translationally downstream of the nisin-inducible promoter nisA. Chloramphenicol was employed as selection marker for the standard system using L. lactis NZ9000, whereas lactose utilization based on the complementation of the lacF gene was used as a dominant selection marker for the food-grade system employing L. lactis NZ3900. Comparison of the standard and the food-grade expression system, differing only in their selection markers, gave considerable differences in volumetric beta-galactosidase activity, ranging from 1.17 to 14 kU/L of fermentation broth, depending on both the origin of the lacLM genes and the selection marker used. The occurrence of codons less frequently used by L. lactis especially at the beginning of the lacL gene could be an explanation for the significant differences between the expression levels of lacLM from different origins, while plasmid stability might cause the difference obtained when employing the different selection markers.

Heme and menaquinone induced electron transport in lactic acid bacteria

BACKGROUND

For some lactic acid bacteria higher biomass production as a result of aerobic respiration has been reported upon supplementation with heme and menaquinone. In this report, we have studied a large number of species among lactic acid bacteria for the existence of this trait.

RESULTS

Heme- (and menaquinone) stimulated aerobic growth was observed for several species and genera of lactic acid bacteria. These include Lactobacillus plantarum, Lactobacillus rhamnosus, Lactobacilllus brevis, Lactobacillus paralimentarius, Streptococcus entericus and Lactococcus garviae. The increased biomass production without further acidification, which are respiration associated traits, are suitable for high-throughput screening as demonstrated by the screening of 8000 Lactococcus lactis insertion mutants. Respiration-negative insertion-mutants were found with noxA, bd-type cytochrome and menaquinol biosynthesis gene-disruptions. Phenotypic screening and in silico genome analysis suggest that respiration can be considered characteristic for certain species.

CONCLUSION

We propose that the cyd-genes were present in the common ancestor of lactic acid bacteria, and that multiple gene-loss events best explains the observed distribution of these genes among the species.

Electron transport chains of lactic acid bacteria - walking on crutches is part of their lifestyle

A variety of lactic acid bacteria contain rudimentary electron transport chains that can be reconstituted by the addition of heme and menaquinone to the growth medium. These activated electron transport chains lead to higher biomass production and increased robustness, which is beneficial for industrial applications, but a major concern when dealing with pathogenic lactic acid bacteria.

The complete coenzyme B12 biosynthesis gene cluster of Lactobacillus reuteri CRL1098

The coenzyme B(12) production pathway in Lactobacillus reuteri has been deduced using a combination of genetic, biochemical and bioinformatics approaches. The coenzyme B(12) gene cluster of Lb. reuteri CRL1098 has the unique feature of clustering together the cbi, cob and hem genes. It consists of 29 ORFs encoding the complete enzymic machinery necessary for de novo biosynthesis. Transcriptional analysis showed it to be expressed as two tandem transcripts of approximately 22 and 4 kb, carrying cobD, cbiABCDETFGHJ, cobA/hemD, cbiKLMNQOP, sirA, hemACBL, and cobUSC, hemD, cobT, respectively. Both transcripts appear to be similarly regulated, and under the conditions assayed are induced in the late-exponential growth phase. Evidence for a regulatory mechanism of negative feedback inhibition by vitamin B(12) itself was observed. Comparative genomics analysis of the coding sequences showed them to be most similar to those coding for the anaerobic coenzyme B(12) pathways previously characterized in a few representatives of the genera Listeria and Salmonella. This contrasts with the trusted species phylogeny and suggests horizontal gene transfer of the B(12) biosynthesis genes. G+C content and codon adaptation index analysis is suggestive that the postulated transfer of these genes was not a recent event. Additional comparative genomics and transcriptional analysis of the sequences acquired during this study suggests a functional link between coenzyme B(12) biosynthesis and reuterin production, which might be implicated in Lb. reuteri's success in colonizing the gastrointestinal tract. This information on gene organization, gene transcription and gene acquisition is relevant for the development of (fermented) foods and probiotics enriched in B(12).

Folate biofortification in food plants

Folate deficiency is a global health problem affecting many people in the developing and developed world. Current interventions (industrial food fortification and supplementation by folic acid pills) are effective if they can be used but might not be possible in less developed countries. Recent advances demonstrate that folate biofortification of food crops is now a feasible complementary strategy to fight folate deficiency worldwide. The genes and enzymes of folate synthesis are sufficiently understood to enable metabolic engineering of the pathway, and results from pilot engineering studies in plants (and bacteria) are encouraging. Here, we review the current status of investigations in the field of folate enhancement on the eve of a new era in food fortification.

Diversity analysis of dairy and nondairy Lactococcus lactis isolates, using a novel multilocus sequence analysis scheme and (GTG)5-PCR fingerprinting

The diversity of a collection of 102 lactococcus isolates including 91 Lactococcus lactis isolates of dairy and nondairy origin was explored using partial small subunit rRNA gene sequence analysis and limited phenotypic analyses. A subset of 89 strains of L. lactis subsp. cremoris and L. lactis subsp. lactis isolates was further analyzed by (GTG)(5)-PCR fingerprinting and a novel multilocus sequence analysis (MLSA) scheme. Two major genomic lineages within L. lactis were found. The L. lactis subsp. cremoris type-strain-like genotype lineage included both L. lactis subsp. cremoris and L. lactis subsp. lactis isolates. The other major lineage, with a L. lactis subsp. lactis type-strain-like genotype, comprised L. lactis subsp. lactis isolates only. A novel third genomic lineage represented two L. lactis subsp. lactis isolates of nondairy origin. The genomic lineages deviate from the subspecific classification of L. lactis that is based on a few phenotypic traits only. MLSA of six partial genes (atpA, encoding ATP synthase alpha subunit; pheS, encoding phenylalanine tRNA synthetase; rpoA, encoding RNA polymerase alpha chain; bcaT, encoding branched chain amino acid aminotransferase; pepN, encoding aminopeptidase N; and pepX, encoding X-prolyl dipeptidyl peptidase) revealed 363 polymorphic sites (total length, 1,970 bases) among 89 L. lactis subsp. cremoris and L. lactis subsp. lactis isolates with unique sequence types for most isolates. This allowed high-resolution cluster analysis in which dairy isolates form subclusters of limited diversity within the genomic lineages. The pheS DNA sequence analysis yielded two genetic groups dissimilar to the other genotyping analysis-based lineages, indicating a disparate acquisition route for this gene.

Pseudovitamin B12 is the corrinoid produced byLactobacillus reuteriCRL1098 under anaerobic conditions

We have reported previously on the ability of Lactobacillus reuteri to produce a compound with vitamin B(12) activity. Here we report on the chemical characterisation of this corrinoid-like molecule. High performance liquid chromatography coupled to an ultraviolet diode array detector, mass spectrometry and nuclear magnetic resonance spectroscopy has enabled us to identify the compound as Coalpha-[alpha-(7-adenyl)]-Cobeta-cyanocobamide or pseudovitamin B(12). This molecule differs from cobalamin in the alpha-ligand, where it has adenine instead of 5,6-dimethylbenzimidazole bound in a alpha-glycosidic linkage to C-1 of ribose. L. reuteri is the first lactic acid bacterium in which the production of a cobalamin-like molecule has been identified and the first microorganism reported to produce exclusively pseudo-B(12).

Glutathione protects Lactococcus lactis against acid stress

Previously we showed that glutathione (GSH) can protect Lactococcus lactis against oxidative stress (Y. Li et al., Appl. Environ. Microbiol. 69:5739-5745, 2003). In the present study, we show that the GSH imported by L. lactis subsp. cremoris SK11 or produced by engineered L. lactis subsp. cremoris NZ9000 can protect both strains against a long-term mild acid challenge (pH 4.0) and a short-term severe acid challenge (pH 2.5). This shows for the first time that GSH can protect a gram-positive bacterium against acid stress. During acid challenge, strain SK11 containing imported GSH and strain NZ9000 containing self-produced GSH exhibited significantly higher intracellular pHs than the control. Furthermore, strain SK11 containing imported GSH had a significantly higher activity of glyceraldehyde-3-phosphate dehydrogenase than the control. These results suggest that the acid stress resistance of starter culture can be improved by selecting L. lactis strains capable of producing or importing GSH.

Generation of a membrane potential by Lactococcus lactis through aerobic electron transport

Lactococcus lactis, a facultative anaerobic lactic acid bacterium, is known to have an increased growth yield when grown aerobically in the presence of heme. We have now established the presence of a functional, proton motive force-generating electron transfer chain (ETC) in L. lactis under these conditions. Proton motive force generation in whole cells was measured using a fluorescent probe (3',3'-dipropylthiadicarbocyanine), which is sensitive to changes in membrane potential (Delta psi). Wild-type cells, grown aerobically in the presence of heme, generated a Delta psi even in the presence of the F(1)-F(o) ATPase inhibitor N,N'-dicyclohexylcarbodiimide, while a cytochrome bd-negative mutant strain (CydA Delta) did not. We also observed high oxygen consumption rates by membrane vesicles prepared from heme-grown cells, compared to CydA Delta cells, upon the addition of NADH. This demonstrates that NADH is an electron donor for the L. lactis ETC and demonstrates the presence of a membrane-bound NADH-dehydrogenase. Furthermore, we show that the functional respiratory chain is present throughout the exponential and late phases of growth.

High-level production of the low-calorie sugar sorbitol by Lactobacillus plantarum through metabolic engineering

Sorbitol is a low-calorie sugar alcohol that is largely used as an ingredient in the food industry, based on its sweetness and its high solubility. Here, we investigated the capacity of Lactobacillus plantarum, a lactic acid bacterium found in many fermented food products and in the gastrointestinal tract of mammals, to produce sorbitol from fructose-6-phosphate by reverting the sorbitol catabolic pathway in a mutant strain deficient for both l- and d-lactate dehydrogenase activities. The two sorbitol-6-phosphate dehydrogenase (Stl6PDH) genes (srlD1 and srlD2) identified in the genome sequence were constitutively expressed at a high level in this mutant strain. Both Stl6PDH enzymes were shown to be active, and high specific activity could be detected in the overexpressing strains. Using resting cells under pH control with glucose as a substrate, both Stl6PDHs were capable of rerouting the glycolytic flux from fructose-6-phosphate toward sorbitol production with a remarkably high efficiency (61 to 65% glucose conversion), which is close to the maximal theoretical value of 67%. Mannitol production was also detected, albeit at a lower level than the control strain (9 to 13% glucose conversion), indicating competition for fructose-6-phosphate rerouting by natively expressed mannitol-1-phosphate dehydrogenase. By analogy, low levels of this enzyme were detected in both the wild-type and the lactate dehydrogenase-deficient strain backgrounds. After optimization, 25% of sugar conversion into sorbitol was achieved with cells grown under pH control. The role of intracellular NADH pools in the determination of the maximal sorbitol production is discussed.

Introducing glutathione biosynthetic capability into Lactococcus lactis subsp. cremoris NZ9000 improves the oxidative-stress resistance of the host

This study describes how a metabolic engineering approach can be used to improve bacterial stress resistance. Some Lactococcus lactis strains are capable of taking up glutathione, and the imported glutathione protects this organism against H(2)O(2)-induced oxidative stress. L. lactis subsp. cremoris NZ9000, a model organism of this species that is widely used in the study of metabolic engineering, can neither synthesize nor take up glutathione. The study described here aimed to improve the oxidative-stress resistance of strain NZ9000 by introducing a glutathione biosynthetic capability. We show that the glutathione produced by strain NZ9000 conferred stronger resistance on the host following exposure to H(2)O(2) (150 mM) and a superoxide generator, menadione (30 microM). To explore whether glutathione can complement the existing oxidative-stress defense systems, we constructed a superoxide dismutase deficient mutant of strain NZ9000, designated as NZ4504, which is more sensitive to oxidative stress, and introduced the glutathione biosynthetic capability into this strain. Glutathione produced by strain NZ4504(pNZ3203) significantly shortens the lag phase of the host when grown aerobically, especially in the presence of menadione. In addition, cells of NZ4504(pNZ3203) capable of producing glutathione restored the resistance of the host to H(2)O(2)-induced oxidative stress, back to the wild-type level. We conclude that the resistance of L. lactis subsp. cremoris NZ9000 to oxidative stress can be increased in engineered cells with glutathione producing capability.

Uncoupling of growth and exopolysaccharide production by Lactococcus lactis subsp. cremoris NIZO B40 and optimization of its synthesis

Exopolysaccharide (EPS) production by Lactococcus lactis subsp. cremoris NIZO B40 was found to be most efficient with glucose as a substrate. The optimal temperature and pH for EPS synthesis were 25 degrees C and pH 5.8, respectively. EPS production could not be identified as a stress response: increased oxygen tension and reduced water activity negatively affected both growth and EPS synthesis. It is often assumed that there is a competition between growth and EPS formation. Within the range of 0.5 and 0.1 h(-1), reducing the growth rate resulted indeed in an increase of the specific EPS production but the polymer formation decreased again at even lower growth rates. Most of the applied fermentation conditions influenced both growth and EPS formation. As the growth rate itself also influenced EPS formation, we studied the linking between growth and EPS synthesis. Interestingly, EPS production was not strictly coupled to growth. Significant de novo synthesis of EPS was observed in non-growing cultures. Consequently, the influence of different culture conditions on EPS production could be studied independent of growth.

Overproduction of heterologous mannitol 1-phosphatase: a key factor for engineering mannitol production by Lactococcus lactis

To achieve high mannitol production by Lactococcus lactis, the mannitol 1-phosphatase gene of Eimeria tenella and the mannitol 1-phosphate dehydrogenase gene mtlD of Lactobacillus plantarum were cloned in the nisin-dependent L. lactis NICE overexpression system. As predicted by a kinetic L. lactis glycolysis model, increase in mannitol 1-phosphate dehydrogenase and mannitol 1-phosphatase activities resulted in increased mannitol production. Overexpression of both genes in growing cells resulted in glucose-mannitol conversions of 11, 21, and 27% by the L. lactis parental strain, a strain with reduced phosphofructokinase activity, and a lactate dehydrogenase-deficient strain, respectively. Improved induction conditions and increased substrate concentrations resulted in an even higher glucose-to-mannitol conversion of 50% by the lactate dehydrogenase-deficient L. lactis strain, close to the theoretical mannitol yield of 67%. Moreover, a clear correlation between mannitol 1-phosphatase activity and mannitol production was shown, demonstrating the usefulness of this metabolic engineering approach.

Functional ingredient production: application of global metabolic models

The biotechnology industry continuously explores new ways to improve the performance of microbial strains in fermentation processes. Recent focus has been on new genome-wide modelling approaches in functional genomics, which aim to take full advantage of genome sequence data, transcription profiling, proteomics and metabolite profiling for strain improvement. The integration of global metabolic models with genetic and regulatory models will be essential for the practice of metabolic engineering for strain improvement to move forward, simply because we cannot rely on our intuition to grasp the complexity of the biological systems involved.

A nudix enzyme removes pyrophosphate from dihydroneopterin triphosphate in the folate synthesis pathway of bacteria and plants

Removal of pyrophosphate from dihydroneopterin triphosphate (DHNTP) is the second step in the pterin branch of the folate synthesis pathway. There has been controversy over whether this reaction requires a specific pyrophosphohydrolase or is a metal ion-dependent chemical process. The genome of Lactococcus lactis has a multicistronic folate synthesis operon that includes an open reading frame (ylgG) specifying a putative Nudix hydrolase. Because many Nudix enzymes are pyrophosphohydrolases, YlgG was expressed in Escherichia coli and characterized. The recombinant protein showed high DHNTP pyrophosphohydrolase activity with a K(m) value of 2 microM, had no detectable activity against deoxynucleoside triphosphates or other typical Nudix hydrolase substrates, required a physiological level (approximately 1 mM) of Mg(2+), and was active as a monomer. Essentially no reaction occurred without enzyme at 1 mM Mg(2+). Inactivation of ylgG in L. lactis resulted in DHNTP accumulation and folate depletion, confirming that YlgG functions in folate biosynthesis. We therefore propose that ylgG be redesignated as folQ. The closest Arabidopsis homolog of YlgG (encoded by Nudix gene At1g68760) was expressed in E. coli and shown to have Mg(2+)-dependent DHNTP pyrophosphohydrolase activity. This protein (AtNUDT1) was reported previously to have NADH pyrophosphatase activity in the presence of 5 mM Mn(2+) (Dobrzanska, M., Szurmak, B., Wyslouch-Cieszynska, A., and Kraszewska, E. (2002) J. Biol. Chem. 277, 50482-50486). However, we found that this activity is negligible at physiological levels of Mn(2+) and that, with 1 mM Mg(2+), AtNUDT1 prefers DHNTP and (deoxy) nucleoside triphosphates.

Riboflavin production in Lactococcus lactis: potential for in situ production of vitamin-enriched foods

This study describes the genetic analysis of the riboflavin (vitamin B(2)) biosynthetic (rib) operon in the lactic acid bacterium Lactococcus lactis subsp. cremoris strain NZ9000. Functional analysis of the genes of the L. lactis rib operon was performed by using complementation studies, as well as by deletion analysis. In addition, gene-specific genetic engineering was used to examine which genes of the rib operon need to be overexpressed in order to effect riboflavin overproduction. Transcriptional regulation of the L. lactis riboflavin biosynthetic process was investigated by using Northern hybridization and primer extension, as well as the analysis of roseoflavin-induced riboflavin-overproducing L. lactis isolates. The latter analysis revealed the presence of both nucleotide replacements and deletions in the regulatory region of the rib operon. The results presented here are an important step toward the development of fermented foods containing increased levels of riboflavin, produced in situ, thus negating the need for vitamin fortification.

Multivitamin production in Lactococcus lactis using metabolic engineering

The dairy starter bacterium Lactococcus lactis has the potential to synthesize both folate (vitamin B11) and riboflavin (vitamin B2). By directed mutagenesis followed by selection and metabolic engineering we have modified two complicated biosynthetic pathways in L. lactis resulting in simultaneous overproduction of both folate and riboflavin: Following exposure to the riboflavin analogue roseoflavin we have isolated a spontaneous mutant of L. lactis strain NZ9000 that was changed from a riboflavin consumer into a riboflavin producer. This mutant contained a single base change in the regulatory region upstream of the riboflavin biosynthetic genes. By the constitutive overproduction of GTP cyclohydrolase I in this riboflavin-producing strain, the production of folate was increased as well. Novel foods, enriched through fermentation using these multivitamin-producing starters, could compensate the B-vitamin-deficiencies that are common even in highly developed countries and could specifically be used in dietary foods for the large fraction of the Caucasian people (10-15%) with mutations in the methylene tetrahydrofolate reductase (MTHFR).

Using Lactococcus lactis for glutathione overproduction

Glutathione and gamma-glutamylcysteine were produced in Lactococcus lactis using a controlled expression system and the genes gshA and gshB from Escherichia coli encoding the enzymes gamma-glutamylcysteine synthetase and glutathione synthetase. High levels of gamma-glutamylcysteine were found in strains growing on chemically defined medium and expressing either gshA alone or both gshA and gshB. As anticipated, glutathione was found in a strain expressing gshA and gshB. The level of glutathione production could be increased by addition of the precursor amino acid cysteine to the medium. The addition of cysteine led to an increased activity of glutathione synthetase, which is remarkable because the amino acid is not a substrate of this enzyme. The final intracellular glutathione concentration attained was 358 nmol mg(-1) protein, which is the highest concentration reported for a bacterium, demonstrating the suitability of engineered L. lactis for fine-chemical production and as a model for studies of the impact of glutathione on flavour formation and other properties of food.

Metabolic engineering of mannitol production in Lactococcus lactis: influence of overexpression of mannitol 1-phosphate dehydrogenase in different genetic backgrounds

To obtain a mannitol-producing Lactococcus lactis strain, the mannitol 1-phosphate dehydrogenase gene (mtlD) from Lactobacillus plantarum was overexpressed in a wild-type strain, a lactate dehydrogenase(LDH)-deficient strain, and a strain with reduced phosphofructokinase activity. High-performance liquid chromatography and (13)C nuclear magnetic resonance analysis revealed that small amounts (<1%) of mannitol were formed by growing cells of mtlD-overexpressing LDH-deficient and phosphofructokinase-reduced strains, whereas resting cells of the LDH-deficient transformant converted 25% of glucose into mannitol. Moreover, the formed mannitol was not reutilized upon glucose depletion. Of the metabolic-engineering strategies investigated in this work, mtlD-overexpressing LDH-deficient L. lactis seemed to be the most promising strain for mannitol production.

Transformation of folate-consuming Lactobacillus gasseri into a folate producer

Five genes essential for folate biosynthesis in Lactococcus lactis were cloned on a broad-host-range lactococcal vector and were transferred to the folate auxotroph Lactobacillus gasseri. As a result L. gasseri changed from a folate consumer to a folate producer. This principle can be used to increase folate levels in many fermented food products.

Enhancement of trehalose production in dairy propionibacteria through manipulation of environmental conditions

We have shown that the ability to produce trehalose is widespread within the genus Propionibacterium. Eighteen strains isolated from dairy sources were screened for trehalose synthesis; the effect of environmental conditions on trehalose production was evaluated in Propionibacterium freudenreichii ssp. shermanii NIZO B365, a strain that accumulated high amounts of this disaccharide. Lactose was the best carbohydrate source for trehalose production, whereas lactate, the substrate that led to the highest specific growth rate, was a poor precursor. Trehalose was consumed after exhaustion of the carbon source in the medium, suggesting its role as a reserve compound. The production of trehalose was not affected by lowering the growth temperature from 30 to 20 degrees C. On the other hand, the maximum trehalose accumulation increased from about 200 to 400 mg of trehalose/g of cell protein upon decreasing the pH from 7.0 to 4.7, by increasing the concentration of NaCl to 2% (w/v), or during growth under aerobic conditions (50% air saturation, 24 microM O(2), pH 7.0). In the absence of NaCl, trehalose accumulated concomitantly with growth, but an increase in salinity triggered a high trehalose production already in the early exponential growth phase. The data provide evidence for a dual function of trehalose as a reserve compound and as a stress-response metabolite. Moreover, P. freudenreichii ssp. shermanii NIZO B365 was able to produce high levels of trehalose in skim milk, which is promising for the implementation of fermented dairy products.