Enzyme Activities Affecting End Product Distribution by Lactobacillus plantarum in Response to Changes in pH and O(2)

Production of glycerol by Lactobacillus plantarum NRRL B-4496 and formation of hexamine during fermentation of pea protein enriched flour

Suspensions of pea protein enriched flour (PP) inoculated with Lactobacillus plantarum NRRL B-4496 and uninoculated PP suspensions were incubated in vials covered with airtight caps. Organic compound compositions of fermented and unfermented PP suspensions (F-PP and U-PP, respectively) were analyzed using solid phase microextraction (SPME) coupled with gas chromatography - mass-spectrometry (GCMS). Acetic acid was detected in all samples; pH dropped from pH 6.5 to pH 4.1 in L. plantarum F-PP and to pH 5.3 in uninoculated F-PP. Abundance of acetic acid and minuscule presence of lactic acid in L. plantarum F-PP suggested that fermentation proceeded preferentially via the pyruvate formate lyase (PFL) pathway. Nonetheless, glycerol appeared to be the most abundant compound in L. plantarum F-PP samples; colorimetric analysis indicated that its average concentration in these samples was 1.05 g/L. A metabolic switch from the PFL pathway to glycerol production might occur due to acidity tolerance limitations of L. plantarum, glycerol production being associated with the release of phosphate, which can act as a buffer. Fermentation of PP by L. plantarum also led to formation of hexamine, which is a known food preservation agent. Presence of naturally formed hexamine and glycerol in food products may render using chemical additives needless.

Sensorically and antimicrobially active metabolite production of Lactobacillus strains on Jerusalem artichoke juice

BACKGROUND

In the tubers of Jerusalem artichoke (Helianthus tuberosus L.) the main carbohydrate is the well-known prebiotic inulin, which is a good growth substrate for gut microorganisms. Jerusalem artichoke tuber is traditionally consumed boiled or pickled rather than in fermented form. Lactic acid bacteria are traditionally used in the production of fermented foods; nevertheless their behavior and metabolite production are considerably influenced by the substrate. The purpose of this study was to investigate the growth and production of the most important sensorically and antimicrobially active metabolites of different Lactobacillus strains on Jerusalem artichoke juice.

RESULTS

All investigated strains grew well (in the range 10(9) cfu mL(-1) ) in the media. The organic acids (lactic acid, 110-337 mmol L(-1) ; acetic acid, 0-180 mmol L(-1) ; and succinic acid, 0-79 mmol L(-1) ), hydrogen peroxide (0.25-1.77 mg L(-1) ), mannitol (0.06-3.24 g L(-1) ), acetoin and diacetyl production of strains varies not only according to the species but also from strain to strain, which will be demonstrated and discussed in the paper.

CONCLUSION

Our results showed that lactobacilli can be used for the fermentation of Jerusalem artichoke, which in this form could be used, alone or mixed with other raw food material, as a new synbiotic functional food.

Application of C Nuclear Magnetic Resonance To Elucidate the Unexpected Biosynthesis of Erythritol by Leuconostoc oenos

Natural-abundance C nuclear magnetic resonance (C-NMR) revealed the production of erythritol and glycerol by nongrowing cells of Leuconostoc oenos metabolizing glucose. The ratio of erythritol to glycerol was strongly influenced by the aeration conditions of the medium. The elucidation of the metabolic pathway responsible for erythritol production was achieved by C-NMR and H-NMR spectroscopy using specifically C-labelled d-glucose. The H-NMR spectrum of the cell supernatant resulting from the metabolism of [2-C]glucose showed that only 75% of the glucose supplied was metabolized heterofermentatively and that the remaining 25% was channelled to the production of erythritol. The synthesis of this polyol resulted from the reduction of the C-4 moiety of the intermediate fructose 6-phosphate. Oxygen has an inhibitory effect on the production of erythritol by L. oenos. Preaeration of a suspension of nongrowing cells of L. oenos resulted in 30% less erythritol and in 70% more glycerol formed during the anaerobic metabolism of glucose. The anaerobic production of erythritol from glucose was also found in growing cultures of L. oenos, although to a smaller extent.

Identification of prebiotic fructooligosaccharide metabolism in Lactobacillus plantarum WCFS1 through microarrays

Short-chain fructooligosaccharides (scFOS) and other prebiotics are used to selectively stimulate the growth and activity of lactobacilli and bifidobacteria in the colon. However, there is little information on the mechanisms whereby prebiotics exert their specific effects upon such microorganisms. To study the genomic basis of scFOS metabolism in Lactobacillus plantarum WCFS1, two-color microarrays were used to screen for differentially expressed genes when grown on scFOS compared to glucose (control). A significant up-regulation (8- to 60-fold) was observed with a set of only five genes located in a single locus and predicted to encode a sucrose phosphoenolpyruvate transport system (PTS), a beta-fructofuranosidase, a fructokinase, an alpha-glucosidase, and a sucrose operon repressor. Several other genes were slightly overexpressed, including pyruvate dehydrogenase. For the latter, no detectable activity in L. plantarum under various growth conditions has been previously reported. A mannose-PTS likely to encode glucose uptake was 50-fold down-regulated as well as, to a lower extent, other PTSs. Chemical analysis of the different moieties of scFOS that were depleted in the growth medium revealed that the trisaccharide 1-kestose present in scFOS was preferentially utilized, in comparison with the tetrasaccharide nystose and the pentasaccharide fructofuranosylnystose. The main end products of scFOS fermentation were lactate and acetate. This is the first example in lactobacilli of the association of a sucrose PTS and a beta-fructofuranosidase that could be used for scFOS degradation.

Unravelling the multiple effects of lactic acid stress on Lactobacillus plantarum by transcription profiling

The organic acid lactate is the predominant fermentation product of Lactobacillus plantarum. The undissociated form of this organic acid is a strong growth inhibitor for the organism. Different theories have been postulated to explain the inhibitory effects of lactic acid: (i) toxicity arising from the dissipation of the membrane potential, (ii) acidification of the cytosol, or (iii) intracellular anion accumulation. In general, organic acid stresses are complex to study, since their toxicity is highly dependent on their degree of dissociation and thus on the pH. In this study, transcription profiles of L. plantarum grown in steady-state cultures that varied in lactate/lactic acid concentration, pH, osmolarity and absolute and relative growth rate, were compared by microarray analysis. By doing so, the differential expression of multiple groups of genes could specifically be attributed to the different aspects of lactic acid stress. A highly coherent group of lactic acid-responsive, cell surface protein-encoding genes was identified, to which no function has previously been assigned. Moreover, a group of genes that showed increased expression in response to the combination of lactic acid and a lower growth rate is expected to be involved in the formation of the alternative fermentation end-products malate, acetate and ethanol. One of these pathways is the phosphoketolase by-pass that is typical for bifidobacteria.

Characterization and functional analysis of the poxB gene, which encodes pyruvate oxidase in Lactobacillus plantarum

The pyruvate oxidase gene (poxB) from Lactobacillus plantarum Lp80 was cloned and characterized. Northern blot and primer extension analyses revealed that transcription of poxB is monocistronic and under the control of a vegetative promoter. poxB mRNA expression was strongly induced by aeration and was repressed by glucose. Moreover, Northern blotting performed at different stages of growth showed that poxB expression is maximal in the early stationary phase when glucose is exhausted. Primer extension and in vivo footprint analyses revealed that glucose repression of poxB is mediated by CcpA binding to the cre site identified in the promoter region. The functional role of the PoxB enzyme was studied by using gene overexpression and knockout in order to evaluate its implications for acetate production. Constitutive overproduction of PoxB in L. plantarum revealed the predominant role of pyruvate oxidase in the control of acetate production under aerobic conditions. The DeltapoxB mutant strain exhibited a moderate (20 to 25%) decrease in acetate production when it was grown on glucose as the carbon source, and residual pyruvate oxidase activity that was between 20 and 85% of the wild-type activity was observed with glucose limitation (0.2% glucose). In contrast, when the organism was grown on maltose, the poxB mutation resulted in a large (60 to 80%) decrease in acetate production. In agreement with the latter observation, the level of residual pyruvate oxidase activity with maltose limitation (0.2% maltose) was less than 10% of the wild-type level of activity.

Serine metabolism in Lactobacillus plantarum

This study investigated the metabolism of (L-) serine by Lactobacillus plantarum B3089 isolated from cheese. Serine was deaminated by growing cells to ammonia with the corresponding formation of acetate and formate. Serine was also deaminated by non-growing cells to ammonia but with the formation of acetate only (no production of formate). Phosphoserine and threonine were not catabolised. It is proposed that serine was deaminated by serine dehydratase (deaminase) to ammonia and pyruvate. Pyruvate was further catabolised predominantly to acetate, carbon dioxide and formate in growing cells, catalysed by pyruvate-formate lyase and pyruvate oxidase; some of the pyruvate was converted to acetoin. In non-growing cells, however, pyruvate-formate lyase was inactive and pyruvate oxidase degraded the pyruvate to acetate and carbon dioxide. Serine dehydratase activity could not be detected in cell-free extracts, presumably because of enzyme instability. The growth of L. plantarum was neither enhanced nor stimulated by serine under the current conditions. Whereas there was little difference in serine utilisation between pH 7.0 and pH 5.8, serine utilisation was decreased by 30% at pH 5.0. NaCl of up to 4% (w/v) concentration had little effect on serine utilisation. Serine had no impact on lactose metabolism. Lactose was fermented mainly to lactate (73%) with the remainder converted to an unidentified polysaccharide (27%).

13C nuclear magnetic resonance analysis of glucose and citrate end products in an ldhL-ldhD double-knockout strain of Lactobacillus plantarum

We have examined the metabolic consequences of knocking out the two ldh genes in Lactobacillus plantarum using 13C nuclear magnetic resonance. Unlike its wild-type isogenic progenitor, which produced lactate as the major metabolite under all conditions tested, ldh null strain TF103 mainly produced acetoin. A variety of secondary end products were also found, including organic acids (acetate, succinate, pyruvate, and lactate), ethanol, 2,3-butanediol, and mannitol.

Lactate and acetate production in Listeria innocua

Listeria innocua NCTC 11289 was grown aerobically in continuous culture in defined media at 30 degrees C. Both acetate and lactate were produced, the proportion of acetate decreased with increasing dilution rate. Enzymatic analysis showed lactate dehydrogenase was activated 10-fold by fructose-1, 6-bisphosphate. The presence of phosphate acetyltransferase and acetate kinase but not pyruvate oxidase was detected, suggesting the sequential action of phosphate acetyltransferase and acetate kinase to produce acetate from acetyl CoA via acetylphosphate.

Storage of poultry meat under modified atmospheres or vacuum packs: possible role of microbial metabolites as indicator of spoilage

The effect of carbon dioxide (100%), nitrogen (100%), carbon dioxide/oxygen (20%:80%) or vacuum pack at 3 and 10 degrees C was studied on the microbial flora, in skinless poultry breast fillets or thigh meat. Lactic acid bacteria and Brochothrix thermosphacta were the predominant organisms in samples stored in vacuum packs, carbon dioxide and nitrogen. Pseudomonads grew only in oxygen/carbon dioxide packaging systems. The concentration of lactate diminished in both thigh and breast meat during storage at 3 and 10 degrees C. This decrease was more pronounced in thigh meat stored under 20%:80% carbon dioxide/oxygen. Acetate increased to varying degrees in all samples regardless of the storage conditions.

Pathway and regulation of erythritol formation in Leuconostoc oenos

It was recently observed that Leuconostoc oenos GM, a wine lactic acid bacterium, produced erythritol anaerobically from glucose but not from fructose or ribose and that this production was almost absent in the presence of O2. In this study, the pathway of formation of erythritol from glucose in L. oenos was shown to involve the isomerization of glucose 6-phosphate to fructose 6-phosphate by a phosphoglucose isomerase, the cleavage of fructose 6-phosphate by a phosphoketolase, the reduction of erythrose 4-phosphate by an erythritol 4-phosphate dehydrogenase and, finally, the hydrolysis of erythritol 4-phosphate to erythritol by a phosphatase. Fructose 6-phosphate phosphoketolase was copurified with xylulose 5-phosphate phosphoketolase, and the activity of the latter was competitively inhibited by fructose 6-phosphate, with a Ki of 26 mM, corresponding to the Km of fructose 6-phosphate phosphoketolase (22 mM). These results suggest that the two phosphoketolase activities are borne by a single enzyme. Extracts of L. oenos were also found to contain NAD(P)H oxidase, which must be largely responsible for the reoxidation of NADPH and NADH in cells incubated in the presence of O2. In cells incubated with glucose, the concentrations of glucose 6-phosphate and of fructose 6-phosphate were higher in the absence of O2 than in its presence, explaining the stimulation by anaerobiosis of erythritol production. The increase in the hexose 6-phosphate concentration is presumably the result of a functional inhibition of glucose 6-phosphate dehydrogenase because of a reduction in the availability of NADP.

Sugar-glycerol cofermentations in lactobacilli: the fate of lactate

The simultaneous fermentation of glycerol and sugar by lactobacillus brevis B22 and Lactobacillus buchneri B190 increases both the growth rate and total growth. The reduction of glycerol to 1,3-propanediol by the lactobacilli was found to influence the metabolism of the sugar cofermented by channelling some of the intermediate metabolites (e.g., pyruvate) towards NADH-producing (rather than NADH-consuming) reactions. Ultimately, the absolute requirement for NADH to prevent the accumulation of 3-hydroxypropionaldehyde leads to a novel lactate-glycerol cofermentation. As a result, additional ATP can be made not only by (i) converting pyruvate to acetate via acetyl phosphate rather than to the ethanol usually found and (ii) oxidizing part of the intermediate pyruvate to acetate instead of the usual reduction to lactate but also by (iii) reoxidation of accumulated lactate to acetate via pyruvate. The conversion of lactate to pyruvate is probably catalyzed by NAD-independent lactate dehydrogenases that are found only in the cultures oxidizing lactate and producing 1,3-propanediol, suggesting a correlation between the expression of these enzymes and a raised intracellular NAD/NADH ratio. The enzymes metabolizing glycerol (glycerol dehydratase and 1,3-propanediol dehydrogenase) were expressed in concert without necessary induction by added glycerol, although their expression may also be influenced by the intracellular NAD/NADH ratio set by the different carbohydrates fermented.

Bioenergetic consequences of catabolic shifts by Lactobacillus plantarum in response to shifts in environmental oxygen and pH in chemostat cultures

Proton motive force (PMF), intracellular end product concentrations, and ATP levels were determined when a steady-state Lactobacillus plantarum 8014 anaerobic chemostat culture was shifted to an aerobic condition or was shifted from pH 5.5 to 7.5. The PMF and intracellular ATP levels increased immediately after the culture was shifted from anaerobic to aerobic conditions. The concentrations of intracellular lactate and acetate, which exported protons that contributed to the proton gradient, changed in the same fashion. The H+/lactate stoichiometry, n, varied from 0.8 to 1.2, and the H+/acetate n value changed from 0.8 to 1.6 at 2 h after the shift to aerobic conditions. The n value for acetate excretion remained elevated at aerobic steady state. When the anaerobic culture was shifted from pH 5.5 to 7.5, intracellular ATP increased 20% immediately even though the PMF decreased 50% as a result of the depletion of the transmembrane proton gradient. The H+/lactate n value changed from 0.7 to 1.8, and n for H+/acetate increased from 0.9 to 1.9 at pH 7.5 steady state. In addition, the H+/acetate stoichiometry was always higher than the n value for H+/lactate; both were higher in alkaline than aerobic conditions, demonstrating that L. plantarum 8014 coexcreted more protons with end products to maintain intracellular pH homeostasis and generate proton gradients under aerobic and alkaline conditions. During the transient to pH 7.5, the n value for H+/acetate approached 3, which would spare one ATP.