Metabolic characterization of Lactococcus lactis deficient in lactate dehydrogenase using in vivo 13C-NMR

Analysis of ldh genes in Lactobacillus casei BL23: role on lactic acid production

Lactobacillus casei is a lactic acid bacterium that produces L-lactate as the main product of sugar fermentation via L-lactate dehydrogenase (Ldh1) activity. In addition, small amounts of the D-lactate isomer are produced by the activity of a D-hydroxycaproate dehydrogenase (HicD). Ldh1 is the main L-lactate producing enzyme, but mutation of its gene does not eliminate L-lactate synthesis. A survey of the L. casei BL23 draft genome sequence revealed the presence of three additional genes encoding Ldh paralogs. In order to study the contribution of these genes to the global lactate production in this organism, individual, as well as double mutants (ldh1 ldh2, ldh1 ldh3, ldh1 ldh4 and ldh1 hicD) were constructed and lactic acid production was assessed in culture supernatants. ldh2, ldh3 and ldh4 genes play a minor role in lactate production, as their single mutation or a mutation in combination with an ldh1 deletion had a low impact on L-lactate synthesis. A Deltaldh1 mutant displayed an increased production of D-lactate, which was probably synthesized via the activity of HicD, as it was abolished in a Deltaldh1 hicD double mutant. Contrarily to HicD, no Ldh1, Ldh2, Ldh3 or Ldh4 activities could be detected by zymogram assays. In addition, these assays revealed the presence of extra bands exhibiting D-/L-lactate dehydrogenase activity, which could not be attributed to any of the described genes. These results suggest that L. casei BL23 possesses a complex enzymatic system able to reduce pyruvic to lactic acid.

Contribution of citrate metabolism to the growth of Lactococcus lactis CRL264 at low pH

Lactococcus lactis subsp. lactis biovar diacetylactis CRL264 is a natural strain isolated from cheese (F. Sesma, D. Gardiol, A. P. de Ruiz Holgado, and D. de Mendoza, Appl. Environ. Microbiol. 56:2099-2103, 1990). The effect of citrate on the growth parameters at a very acidic pH value was studied with this strain and with derivatives whose citrate uptake capacity was genetically manipulated. The culture pH was maintained at 4.5 to prevent alkalinization of the medium, a well-known effect of citrate metabolism. In the presence of citrate, the maximum specific growth rate and the specific glucose consumption rate were stimulated. Moreover, a more efficient energy metabolism was revealed by analysis of the biomass yields relative to glucose consumption or ATP production. Thus, it was shown that the beneficial effect of citrate on growth under acid stress conditions is not primarily due to the concomitant alkalinization of the medium but stems from less expenditure of ATP, derived from glucose catabolism, to achieve pH homeostasis. After citrate depletion, a deleterious effect on the final biomass was apparent due to organic acid accumulation, particularly acetic acid. On the other hand, citrate metabolism endowed cells with extra ability to counteract lactic and acetic acid toxicity. In vivo 13C nuclear magnetic resonance provided strong evidence for the operation of a citrate/lactate exchanger. Interestingly, the greater capacity for citrate transport correlated positively with the final biomass and growth rates of the citrate-utilizing strains. We propose that increasing the citrate transport capacity of CRL264 could be a useful strategy to improve further the ability of this strain to cope with strongly acidic conditions.

The lactate dehydrogenases encoded by the ldh and ldhB genes in Lactococcus lactis exhibit distinct regulation and catalytic properties - comparative modeling to probe the molecular basis

Lactococcus lactis FI9078, a construct carrying a disruption of the ldh gene, converted approximately 90% of glucose into lactic acid, like the parental strain MG1363. This unexpected lactate dehydrogenase activity was purified, and ldhB was identified as the gene encoding this protein. The activation of ldhB was explained by the insertion of an IS905-like element that created a hybrid promoter in the intergenic region upstream of ldhB. The biochemical and kinetic properties of this alternative lactate dehydrogenase (LDHB) were compared to those of the ldh-encoded enzyme (LDH), purified from the parental strain. In contrast to LDH, the affinity of LDHB for NADH and the activation constant for fructose 1,6-bisphosphate were strongly dependent on pH. The activation constant increased 700-fold, whereas the K(m) for NADH increased more than 10-fold, in the pH range 5.5-7.2. The two enzymes also exhibited different pH profiles for maximal activity. Moreover, inorganic phosphate acted as a strong activator of LDHB. The impact of replacing LDH by LDHB on the physiology of L. lactis was assessed by monitoring the evolution of the pools of glycolytic intermediates and cofactors during the metabolism of glucose by in vivo NMR. Structural analysis by comparative modeling of the two proteins showed that LDH has a slightly larger negative charge than LDHB and a greater concentration of positive charges at the interface between monomers. The calculated pH titration curves of the catalytic histidine residues explain why LDH maintains its activity at low pH as compared to LDHB, the histidines in LDH showing larger pH titration ranges.

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.

The α-Phosphoglucomutase of Lactococcus lactis Is Unrelated to the α-d-Phosphohexomutase Superfamily and Is Encoded by the Essential Gene pgmH

alpha-Phosphoglucomutase (alpha-PGM) plays an important role in carbohydrate metabolism by catalyzing the reversible conversion of alpha-glucose 1-phosphate to glucose 6-phosphate. Isolation of alpha-PGM activity from cell extracts of Lactococcus lactis strain MG1363 led to the conclusion that this activity is encoded by yfgH, herein renamed pgmH. Its gene product has no sequence homology to proteins in the alpha-d-phosphohexomutase superfamily and is instead related to the eukaryotic phosphomannomutases within the haloacid dehalogenase superfamily. In contrast to known bacterial alpha-PGMs, this 28-kDa enzyme is highly specific for alpha-glucose 1-phosphate and glucose 6-phosphate and showed no activity for mannose phosphate. To elucidate the function of pgmH, the metabolism of glucose and galactose was characterized in mutants overproducing or with a deficiency of alpha-PGM activity. Overproduction of alpha-PGM led to increased glycolytic flux and growth rate on galactose. Despite several attempts, we failed to obtain a deletion mutant of pgmH. The essentiality of this gene was proven by using a conditional knock-out strain in which a native copy of the gene was provided in trans under the control of the nisin promoter. Growth of this strain was severely impaired when alpha-PGM activity was below the control level. We show that the novel L. lactis alpha-PGM is the only enzyme that mediates the interconversion of alpha-glucose 1-phosphate to glucose 6-phosphate and is essential for growth.

Parameter estimation in modulated, unbranched reaction chains within biochemical systems

Modern biology is increasingly developing techniques for measuring time series of global gene expression and of many simultaneous proteins or metabolites. These data contain valuable information on the dynamics of cells, which has to be extracted with computational means. Given a suitable mathematical model, this extraction is in principle a straightforward regression task, but the complexity and nonlinearity of the differential equations that describe biological systems cause severe difficulties when the systems are of realistic size. We propose a method of stepwise regression that can be applied effectively to linear portions of pathways. The method may be combined with other estimation methods and either directly yields reasonable parameter estimates or at least provides appropriate start values for subsequent nonlinear search algorithms. We illustrate the method with the analysis of in vivo NMR data describing the dynamics of glycolytic metabolites in Lactococcus lactis.

Sorbitol synthesis by an engineered Lactobacillus casei strain expressing a sorbitol-6-phosphate dehydrogenase gene within the lactose operon

Sorbitol is claimed to have important health-promoting effects and Lactobacillus casei is a lactic acid bacterium relevant as probiotic and used as a cheese starter culture. A sorbitol-producing L. casei strain might therefore be of considerable interest in the food industry. A recombinant strain of L. casei was constructed by the integration of a d-sorbitol-6-phosphate dehydrogenase-encoding gene (gutF) in the chromosomal lactose operon (strain BL232). gutF expression in this strain followed the same regulation as that of the lac genes, that is, it was repressed by glucose and induced by lactose. (13)C-nuclear magnetic resonance analysis of supernatants of BL232 resting cells demonstrated that, when pre-grown on lactose, cells were able to synthesize sorbitol from glucose. Inactivation of the l-lactate dehydrogenase gene in BL232 led to an increase in sorbitol production, suggesting that the engineered route provides an alternative pathway for NAD(+) regeneration.

Unity in organisation and regulation of catabolic operons in Lactobacillus plantarum, Lactococcus lactis and Listeria monocytogenes

Global regulatory circuits together with more specific local regulators play a notable role when cells are adapting to environmental changes. Lactococcus lactis is a lactic acid bacterium abundant in nature fermenting most mono- and disaccharides. Comparative genomics analysis of the operons encoding the proteins and enzymes crucial for catabolism of lactose, maltose and threhalose revealed an obvious unity in operon organisation . The local regulator of each operon was located in a divergent transcriptional direction to the rest of the operon including the transport protein-encoding genes. Furthermore, in all three operons a catabolite responsive element (CRE) site was detected inbetween the gene encoding the local regulator and one of the genes encoding a sugar transport protein. It is evident that regardless of type of transport system and catabolic enzymes acting upon lactose, maltose and trehalose, respectively, Lc. lactis shows unity in both operon organisation and regulation of these catabolic operons. This knowledge was further extended to other catabolic operons in Lc. lactis and the two related bacteria Lactobacillus plantarum and Listeria monocytogenes. Thirty-nine catabolic operons responsible for degradation of sugars and sugar alcohols in Lc. lactis, Lb. plantarum and L. monocytogenes were investigated and the majority of those possessed the same organisation as the lactose, maltose and trehalose operons of Lc. lactis. Though, the frequency of CRE sites and their location varied among the bacteria. Both Lc. lactis and Lb. plantarum showed CRE sites in direct proximity to genes coding for proteins responsible for sugar uptake. However, in L. monocytogenes CRE sites were not frequently found and not in the vicinity of genes encoding transport proteins, suggesting a more local mode of regulation of the catabolic operons found and/or the use of inducer control in this bacterium.

Pleiotropic effects of lactate dehydrogenase inactivation in Lactobacillus casei

In lactic acid bacteria, conversion of pyruvic to lactic acid through the activity of lactate dehydrogenase (Ldh) constitutes the final step of the homofermentative pathway. Lactobacillus casei has two characterized genes encoding Ldh activities. The ldhL gene codes for an L-Ldh, which specifically catalyzes the formation of L-lactate, whereas the hicD gene codes for a D-hydroxyisocaproate dehydrogenase (HicDH), which catalyzes the conversion of pyruvate into D-lactate. In L. casei cells fermenting glucose, a mixture of L-/D-lactate with a 97:3% ratio was formed. Inactivation of hicD led to undetectable D-lactate levels after glucose fermentation, while L-lactate levels remained constant. Inactivation of ldhL did not abolish the production of L-lactate, but the lactate final concentration decreased about 25% compared to the wild type, suggesting the presence of at least a second L-Ldh. Moreover, part of the pyruvate flux was rerouted and half of the lactate produced was in the D-isomer form. ldhL inactivation in L. casei showed additional interesting effects. First, the glycolytic flux from pyruvate to lactate was redirected and other fermentation products, including acetate, acetoin, pyruvate, ethanol, diacetyl, mannitol and CO(2), were produced. Second, a lack of carbon catabolite repression of lactose metabolism and N-acetyl-glucosaminidase activity was observed. This second effect could be partly avoided by growing the cells under aeration, since NADH oxidases could account for NAD+ regeneration.

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.

High-level acetaldehyde production in Lactococcus lactis by metabolic engineering

Efficient conversion of glucose to acetaldehyde is achieved by nisin-controlled overexpression of Zymomonas mobilis pyruvate decarboxylase (pdc) and Lactococcus lactis NADH oxidase (nox) in L. lactis. In resting cells, almost 50% of the glucose consumed could be redirected towards acetaldehyde by combined overexpression of pdc and nox under anaerobic conditions.

On-line monitoring of CO2 production inLactococcus lactis during physiological pH decrease using membrane inlet mass spectrometry with dynamic pH calibration

Monitoring CO2 production in systems, where pH is changing with time is hampered by the chemical behavior and pH-dependent volatility of this compound. In this article, we present the first method where the concentration and production rate of dissolved CO2 can be monitored directly, continuously, and quantitatively under conditions where pH changes rapidly ( approximately 2 units in 15 min). The method corrects membrane inlet mass spectrometry (MIMS) measurements of CO2 for pH dependency using on-line pH analysis and an experimentally established calibration model. It is valid within the pH range of 3.5 to 7, despite pH-dependent calibration constants that vary in a non-linear fashion with more than a factor of 3 in this interval. The method made it possible to determine the carbon dioxide production during Lactococcus lactis fermentations, where pH drops up to 3 units during the fermentation. The accuracy was approximately 5%. We used the method to investigate the effect of initial extracellular pH on carbon dioxide production during anarobic glucose fermentation by non-growing Lactocoocus lactis and demonstrated that the carbon dioxide production rate increases considerably, when the initial pH was increased from 6 to 6.8.

Natural-abundance isotope ratio mass spectrometry as a means of evaluating carbon redistribution during glucose-citrate cofermentation by Lactococcus lactis

The cometabolism of citrate and glucose by growing Lactococcus lactis ssp. lactis bv. diacetylactis was studied using a natural-abundance stable isotope technique. By a judicious choice of substrates differing slightly in their 13C/12C ratios, the simultaneous metabolism of citrate and glucose to a range of compounds was analysed. These end-products include lactate, acetate, formate, diacetyl and acetoin. All these products have pyruvate as a common intermediate. With the objective of estimating the degree to which glucose and citrate metabolism through pyruvate may be differentially regulated, the delta13C values of the products accumulated over a wide range of concentrations of citrate and glucose were compared. It was found that, whereas the relative accumulation of different products responds to both the substrate concentration and the ratio between the substrates, the delta13C values of the products primarily reflect the availability of the two substrates over the entire range examined. It can be concluded that in actively growing L. lactis the maintenance of pyruvate homeostasis takes precedence over the redox status of the cells as a regulatory factor.

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.

Engineering Lactococcus lactis for production of mannitol: high yields from food-grade strains deficient in lactate dehydrogenase and the mannitol transport system

Mannitol is a sugar polyol claimed to have health-promoting properties. A mannitol-producing strain of Lactococcus lactis was obtained by disruption of two genes of the phosphoenolpyruvate (PEP)-mannitol phosphotransferase system (PTS(Mtl)). Genes mtlA and mtlF were independently deleted by double-crossover recombination in strain L. lactis FI9630 (a food-grade lactate dehydrogenase-deficient strain derived from MG1363), yielding two mutant (Delta ldh Delta mtlA and Delta ldh Delta mtlF) strains. The new strains, FI10091 and FI10089, respectively, do not possess any selection marker and are suitable for use in the food industry. The metabolism of glucose in nongrowing cell suspensions of the mutant strains was characterized by in vivo (13)C-nuclear magnetic resonance. The intermediate metabolite, mannitol-1-phosphate, accumulated intracellularly to high levels (up to 76 mM). Mannitol was a major end product, one-third of glucose being converted to this hexitol. The double mutants, in contrast to the parent strain, were unable to utilize mannitol even after glucose depletion, showing that mannitol was taken up exclusively by PEP-PTS(Mtl). Disruption of this system completely blocked mannitol transport in L. lactis, as intended. In addition to mannitol, approximately equimolar amounts of ethanol, 2,3-butanediol, and lactate were produced. A mixed-acid fermentation (formate, ethanol, and acetate) was also observed during growth under controlled conditions of pH and temperature, but mannitol production was low. The reasons for the alteration in the pattern of end products under nongrowing and growing conditions are discussed, and strategies to improve mannitol production during growth are proposed.

IS981-mediated adaptive evolution recovers lactate production by ldhB transcription activation in a lactate dehydrogenase-deficient strain of Lactococcus lactis

Lactococcus lactis NZ9010 in which the las operon-encoded ldh gene was replaced with an erythromycin resistance gene cassette displayed a stable phenotype when grown under aerobic conditions, and its main end products of fermentation under these conditions were acetate and acetoin. However, under anaerobic conditions, the growth of these cells was strongly retarded while the main end products of fermentation were acetate and ethanol. Upon prolonged subculturing of this strain under anaerobic conditions, both the growth rate and the ability to produce lactate were recovered after a variable number of generations. This recovery was shown to be due to the transcriptional activation of a silent ldhB gene coding for an Ldh protein (LdhB) with kinetic parameters different from those of the native las operon-encoded Ldh protein. Nevertheless, cells producing LdhB produced mainly lactate as the end product of fermentation. The mechanism underlying the ldhB gene activation was primarily studied in a single-colony isolate of the recovered culture, designated L. lactis NZ9015. Integration of IS981 in the upstream region of ldhB was responsible for transcription activation of the ldhB gene by generating an IS981-derived -35 promoter region at the correct spacing with a natively present -10 region. Subsequently, analysis of 10 independently isolated lactate-producing derivatives of L. lactis NZ9010 confirmed that the ldhB gene is transcribed in all of them. Moreover, characterization of the upstream region of the ldhB gene in these derivatives indicated that site-specific and directional IS981 insertion represents the predominant mechanism of the observed recovery of the ability to produce lactate.

Metabolic pathway for propionate utilization by phosphorus-accumulating organisms in activated sludge: 13C labeling and in vivo nuclear magnetic resonance

In vivo 13C and 31P nuclear magnetic resonance techniques were used to study propionate metabolism by activated sludge in enhanced biological phosphorus removal systems. The fate of label supplied in [3-13C]propionate was monitored in living cells subjected to anaerobic/aerobic cycles. During the anaerobic phase, propionate was converted to polyhydroxyalkanoates (PHA) with the following monomer composition: hydroxyvalerate, 74.2%; hydroxymethylvalerate, 16.9%; hydroxymethylbutyrate, 8.6%; and hydroxybutyrate, 0.3%. The isotopic enrichment in the different carbon atoms of hydroxyvalerate (HV) produced during the first anaerobic stage was determined: HV5, 59%; HV4, 5.0%; HV3, 1.1%; HV2, 3.5%; and HV1, 2.8%. A large proportion of the supplied label ended up on carbon C-5 of HV, directly derived from the pool of propionyl-coenzyme A (CoA), which is primarily labeled on C-3; useful information on the nature of operating metabolic pathways was provided by the extent of labeling on C-1, C-2, and C-4. The labeling pattern on C-1 and C-2 was explained by the conversion of propionyl-CoA to acetyl-CoA via succinyl-CoA and the left branch of the tricarboxylic acid cycle, which involves scrambling of label between the inner carbons of succinate. This constitutes solid evidence for the operation of succinate dehydrogenase under anaerobic conditions. The labeling in HV4 is explained by backflux from succinate to propionyl-CoA. The involvement of glycogen in the metabolism of propionate was also demonstrated; moreover, it was shown that the acetyl moiety to the synthesis of PHA was derived preferentially from glycogen. According to the proposed metabolic scheme, the decarboxylation of pyruvate is coupled to the production of hydrogen, and the missing reducing equivalents should be derived from a source other than glycogen metabolism.

Effect of different NADH oxidase levels on glucose metabolism by Lactococcus lactis: kinetics of intracellular metabolite pools determined by in vivo nuclear magnetic resonance

Three isogenic strains of Lactococcus lactis with different levels of H(2)O-forming NADH oxidase activity were used to study the effect of oxygen on glucose metabolism: the parent strain L. lactis MG1363, a NOX(-) strain harboring a deletion of the gene coding for H(2)O-forming NADH oxidase, and a NOX(+) strain with the NADH oxidase activity enhanced by about 100-fold. A comprehensive description of the metabolic events was obtained by using (13)C nuclear magnetic resonance in vivo. The most noticeable results of this study are as follows: (i) under aerobic conditions the level of fructose 1,6-bisphosphate [Fru(1,6)P(2)] was lower than the level under anaerobic conditions, and the rate of Fru(1,6)P(2) depletion was very high; (ii) the levels of 3-phosphoglycerate and phosphoenolpyruvate were considerably enhanced under aerobic conditions and significantly lower in the NOX(-) strain; and (iii) the glycolytic flux decreased in the presence of saturating levels of oxygen, but it was not altered in response to changes in the NADH oxidase activity. In particular, the observation that the glycolytic flux was not enhanced in the NOX(+) strain indicated that glycolytic flux was not primarily determined by the level of NADH in the cell. The patterns of end products were identical for the NOX(-) and parent strains; in the NOX(+) strain the carbon flux was diverted to the production of alpha-acetolactate-derived compounds, and at a low pH this strain produced diacetyl at concentrations up to 1.6 mM. The data were integrated with the goal of identifying the main regulatory aspects of glucose metabolism in the presence of oxygen.

Catabolism of mannitol in Lactococcus lactis MG1363 and a mutant defective in lactate dehydrogenase

Mannitol metabolism in Lactococcus lactis MG1363 and in a derivative strain deficient in lactate dehydrogenase (LDH(d)) was characterized. Both strains had the ability to grow on mannitol as an energy source, although this polyol was a poorer substrate for growth than glucose. When compared to glucose, the metabolism of mannitol caused an NADH burden due to formation of an additional NADH molecule at the reaction catalysed by mannitol-1-phosphate dehydrogenase (Mtl1PDH). This resulted in a prominent accumulation of mannitol 1-phosphate (Mtl1P) both in growing and resting cells, suggesting the existence of a severe bottleneck at Mtl1PDH. Growth on mannitol induced the activity of Mtl1PDH in both the LDH(d) and MG1363 strains. The lower accumulation of Mtl1P in mannitol-grown cells when compared to glucose-grown LDH(d) cells, as monitored by in vivo (13)C-NMR, reflects this induction. A clear shift towards the production of ethanol was observed on mannitol, indicating pressure to regenerate NAD(+) when this substrate was used. A strategy to obtain a mannitol-overproducing strain is proposed.

Carbohydrate cycling in micro-organisms: what can (13)C-NMR tell us?

The extension of (13)C-nuclear magnetic resonance (NMR) techniques to study cellular metabolism over recent years has provided valuable data supporting the occurrence, diversity and extent of carbon cycling in the carbohydrate metabolism of micro-organisms. The occurrence of such cycles, resulting from the simultaneous operation of different and sometimes opposite individual steps, is inherently related to the network organisation of cellular metabolism. These cycles are tentatively classified here as 'reversibility', 'metabolic' and 'substrate' cycles on the basis of their balance in carbon and cofactors. Current hypotheses concerning the physiological relevance of carbohydrate cycles are discussed in light of the (13)C-NMR data. They most likely represent system-level mechanisms for coherent and timely partitioning of carbon resources to fit with the various biosynthetic, energetic or redox needs of cells and/or additional strategies in the adaptive capacity of micro-organisms to face variation in environmental conditions.

Nutraceutical production with food-grade microorganisms

Over the past few years a number of new food ingredients labelled as being nutraceuticals have been launched on the food and pharmaceutical market. These include components that have a proven beneficial effect on human health, such as low-calorie sugars and B vitamins. Lactic acid bacteria, in particular Lactococcus lactis, have been demonstrated to be ideal cell factories for the production of these important nutraceuticals. Developments in the genetic engineering of food-grade microoganisms means that the production of certain nutraceuticals can be enhanced or newly induced through overexpression and/or disruption of relevant metabolic genes.

Methods for detection and visualization of intracellular polymers stored by polyphosphate-accumulating microorganisms

Polyphosphate-accumulating microorganisms (PAOs) are important in enhanced biological phosphorus (P) removal. Considerable effort has been devoted to understanding the biochemical nature of enhanced biological phosphorus removal (EBPR) and it has been shown that intracellular polymer storage plays an important role in PAO's metabolism. The storage capacity of PAOs gives them a competitive advantage over other microorganisms present that are not able to accumulate internal reserves. Intracellular polymers stored by PAOs include polyphosphate (poly-P), polyhydroxyalkanoates (PHAs) and glycogen. Staining procedures for qualitative visualization of polymers by optical microscopy and combinations of these procedures with molecular tools for in situ identification are described here. The strengths and weaknesses of widely used polymer quantification methods that require destruction of samples, are also discussed. Finally, the potential of in vivo nuclear magnetic resonance (NMR) spectroscopy for on-line measurement of intracellular reserves is reported.

Is the glycolytic flux in Lactococcus lactis primarily controlled by the redox charge? Kinetics of NAD(+) and NADH pools determined in vivo by 13C NMR

The involvement of nicotinamide adenine nucleotides (NAD(+), NADH) in the regulation of glycolysis in Lactococcus lactis was investigated by using (13)C and (31)P NMR to monitor in vivo the kinetics of the pools of NAD(+), NADH, ATP, inorganic phosphate (P(i)), glycolytic intermediates, and end products derived from a pulse of glucose. Nicotinic acid specifically labeled on carbon 5 was synthesized and used in the growth medium as a precursor of pyridine nucleotides to allow for in vivo detection of (13)C-labeled NAD(+) and NADH. The capacity of L. lactis MG1363 to regenerate NAD(+) was manipulated either by turning on NADH oxidase activity or by knocking out the gene encoding lactate dehydrogenase (LDH). An LDH(-) deficient strain was constructed by double crossover. Upon supply of glucose, NAD(+) was constant and maximal (approximately 5 mm) in the parent strain (MG1363) but decreased abruptly in the LDH(-) strain both under aerobic and anaerobic conditions. NADH in MG1363 was always below the detection limit as long as glucose was available. The rate of glucose consumption under anaerobic conditions was 7-fold lower in the LDH(-) strain and NADH reached high levels (2.5 mm), reflecting severe limitation in regenerating NAD(+). However, under aerobic conditions the glycolytic flux was nearly as high as in MG1363 despite the accumulation of NADH up to 1.5 mm. Glyceraldehyde-3-phosphate dehydrogenase was able to support a high flux even in the presence of NADH concentrations much higher than those of the parent strain. We interpret the data as showing that the glycolytic flux in wild type L. lactis is not primarily controlled at the level of glyceraldehyde-3-phosphate dehydrogenase by NADH. The ATP/ADP/P(i) content could play an important role.

Spontaneous formation of a mannitol-producing variant of Leuconostoc pseudomesenteroides grown in the presence of fructose

We report the spontaneous formation of a stable mannitol-producing variant of Leuconostoc pseudomesenteroides. The mannitol-producing variant showed mannitol dehydrogenase activity which was absent in the parental strain. It was also able to use fructose and glucose simultaneously, whereas the parental strain showed diauxic growth with these sugars. A possible explanation of these observations is discussed.