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Hydrogen peroxide production is a well-known trait of many bacterial species associated with the human body. In the presence of oxygen, the probiotic lactic acid bacterium Lactobacillus johnsonii NCC 533 excretes up to 1 mM H 2 O 2 , inducing growth stagnation and cell death. Disruption of genes commonly assumed to be involved in H 2 O 2 production (e.g., pyruvate oxidase, NADH oxidase, and lactate oxidase) did not affect this. Here we describe the purification of a novel NADH-dependent flavin reductase encoded by two highly similar genes ( LJ _ 0548 and LJ _ 0549 ) that are conserved in lactobacilli belonging to the Lactobacillus acidophilus group. The genes are predicted to encode two 20-kDa proteins containing flavin mononucleotide (FMN) reductase conserved domains. Reductase activity requires FMN, flavin adenine dinucleotide (FAD), or riboflavin and is specific for NADH and not NADPH. The K m for FMN is 30 ± 8 μM, in accordance with its proposed in vivo role in H 2 O 2 production. Deletion of the encoding genes in L. johnsonii led to a 40-fold reduction of hydrogen peroxide formation. H 2 O 2 production in this mutant could only be restored by in trans complementation of both genes. Our work identifies a novel, conserved NADH-dependent flavin reductase that is prominently involved in H 2 O 2 production in L. johnsonii .

The genome-wide transcriptional response of Saccharomyces cerevisiae to a temperature shift from 28 °C to 41 °C – a temperature at which cell viability was not impaired, but growth rate was – was determined over a period of 6 h under well-controlled conditions. Care was taken not to introduce other perturbations. The data shows that the cells respond by rapidly changing their gene expression. Two phases were observed in this response: A short term transitional response during the first hour in which cell growth ceased, followed by a transcriptionally stable phase that was growth permissive. The initial response was broadest, involving almost half of all the ORFs being induced or repressed to a statistically significant level (here 1.5 fold). Functional analysis showed that genes involved in energy transduction, including trehalose metabolism, and molecular-chaperone encoding genes were induced most prominently. The latter set of genes presumably functions to counteract the effect of the temperature increase on protein denaturation. Furthermore, genes encoding parts of the translation- and transcription systems were repressed temporarily, in line with the observed lag phase in growth. After the initial adaptation phase a new stable state of gene expression was attained, compatible with cell growth. This new transcriptome was characterised by a continued induction of stress response genes, except for those involved in trehalose biosynthesis and a normalisation of the expression of genes coding for the transcription and translation machinery. Finally, our data, together with information available from the literature, show that the adaptation of specific cellular processes to severe temperature stress can occur at different hierarchical control levels. While trehalose metabolism seems to be adjusted mainly at the transcriptional level, the metabolic or post-transcriptional level seems to be more significant for the adaptation of parts of glycolysis and respiration.

Enzyme reprofiling in bacteria during adaptation from one environmental condition to another may be regulated by both transcription and translation. However, little is known about the contribution of translational regulation. Recently, we have developed a pulse labeling method using the methionine analog azidohomoalanine to determine the relative amounts of proteins synthesized by Escherichia coli in a brief time frame upon a change in environmental conditions. Here we present an extension of our analytical strategy, which entails measuring changes in total protein levels on the same time scale as new protein synthesis. This allows identification of stable and labile proteins and demonstrates that altered levels of most newly synthesized proteins are the result of a change in translation rate rather than degradation rate. With this extended strategy, average relative translation rates for 10 min immediately after a switch from aerobiosis to anaerobiosis were determined. The majority of proteins with increased synthesis rates upon an anaerobic switch are involved in glycolysis and pathways aimed at preventing glycolysis grinding to a halt by a cellular redox imbalance. Our method can be used to compare relative translation rates with relative mRNA levels at the same time. Discrepancies between these parameters may reveal genes whose expression is regulated by translation rather than by transcription. This may help unravel molecular mechanism underlying changes in translation rates, e. g. mediated by small regulatory RNAs. Molecular & Cellular Proteomics 9:2508-2516, 2010

Realistic quantitative models require data from many laboratories. Therefore, standardization of experimental systems and assay conditions is crucial. Moreover, standards should be representative of the in vivo conditions. However, most often, enzyme-kinetic parameters are measured under assay conditions that yield the maximum activity of each enzyme. In practice, this means that the kinetic parameters of different enzymes are measured in different buffers, at different pH values, with different ionic strengths, etc. In a joint effort of the Dutch Vertical Genomics Consortium, the European Yeast Systems Biology Network and the Standards for Reporting Enzymology Data Commission, we have developed a single assay medium for determining enzyme-kinetic parameters in yeast. The medium is as close as possible to the in vivo situation for the yeast Saccharomyces cerevisiae, and at the same time is experimentally feasible. The in vivo conditions were estimated for S. cerevisiae strain CEN.PK113-7D grown in aerobic glucose-limited chemostat cultures at an extracellular pH of 5.0 and a specific growth rate of 0.1 h(-1). The cytosolic pH and concentrations of calcium, sodium, potassium, phosphorus, sulfur and magnesium were determined. On the basis of these data and literature data, we propose a defined in vivo-like medium containing 300 mM potassium, 50 mM phosphate, 245 mM glutamate, 20 mM sodium, 2 mM free magnesium and 0.5 mM calcium, at a pH of 6.8. The V(max) values of the glycolytic and fermentative enzymes of S. cerevisiae were measured in the new medium. For some enzymes, the results deviated conspicuously from those of assays done under enzyme-specific, optimal conditions.

A comparative physiological and transcriptional study is presented on wild-type Saccharomyces cerevisiae and mutants with altered levels of catabolic regulators: hxk2Delta lacking hexokinase2, HAP4 / overproducing hap4p and hxk2 Delta HAP4 upward arrow. Relative to the wild-type, HAP4 / showed the same growth rate with some increased yield on glucose, and hxk2Delta grew 28% slower but with a dramatically improved yield. Hxk2 Delta HAP4 / grew 14% slower but showed fully oxidative growth. A higher yield correlated with increased respiration. For both hxk2 Delta strains, glucose repression was suppressed (upregulation of high-affinity sugar transporters, invertase and oxidative phosphorylation). T-profiler analysis showed that genes under control of the hap2/3/4/5-binding motif were significantly altered in expression in all strains. HAP4 overexpression, directly or in hxk2 knockouts, led to repression of the genes containing the Zap1p motif including ZAP1 itself, indicating altered zinc metabolism. Whereas HAP4 overexpression resulted in a shift towards oxidative metabolism only, deletion of HXK2 resulted in a strain that, in addition to being oxidative, almost completely lacked the ability to sense glucose. As the double mutant had an energy efficiency close to the maximum even with excess glucose and was derepressed to a larger extent and over a broader range, the functioning of the two regulators is in general considered to be additive.

Proteins that are covalently linked to the skeletal polysaccharides of the cell wall of Candida albicans play a major role in the colonization of the vaginal mucosal surface, which may result in vaginitis. Here we report on the variability of the cell-wall proteome of C. albicans as a function of the ambient O(2) concentration and iron availability. For these studies, cells were cultured at 37 degrees C in vagina-simulative medium and aerated with a gas mixture consisting of 6 % (v/v) CO(2), 0.01-7 % (v/v) O(2) and N(2), reflecting the gas composition in the vaginal environment. Under these conditions, the cells grew exclusively in the non-hyphal form, with the relative growth rate being halved at approximately 0.02 % (v/v) O(2). Using tandem MS and immunoblot analysis, we identified 15 covalently linked glycosylphosphatidylinositol (GPI) proteins in isolated walls (Als1, Als3, Cht2, Crh11, Ecm33, Hwp1, Pga4, Pga10, Phr2, Rbt5, Rhd3, Sod4, Ssr1, Ywp1, Utr2) and 4 covalently linked non-GPI proteins (MP65, Pir1, Sim1/Sun42, Tos1). Five of them (Als3, Hwp1, Sim1, Tos1, Utr2) were absent in cells grown in rich medium. Immunoblot analysis revealed that restricted O(2) availability resulted in higher levels of the non-GPI protein Pir1, a putative beta-1,3-glucan cross-linking protein, and of the GPI-proteins Hwp1, an adhesion protein, and Pga10 and Rbt5, which are involved in iron acquisition. Addition of the iron chelator ferrozine at saturating levels of O(2) resulted in higher cell wall levels of Hwp1 and Rbt5, suggesting that the responses to hypoxic conditions and iron restriction are related.

Due to the spread of resistance, antibiotic exposure receives increasing attention. Ecological consequences for the different niches of individual microbiomes are, however, largely ignored. Here, we report the effects of widely used antibiotics (clindamycin, ciprofloxacin, amoxicillin, and minocycline) with different modes of action on the ecology of both the gut and the oral microbiomes in 66 healthy adults from the United Kingdom and Sweden in a two-center randomized placebo-controlled clinical trial. Feces and saliva were collected at baseline, immediately after exposure, and 1, 2, 4, and 12 months after administration of antibiotics or placebo. Sequences of 16S rRNA gene amplicons from all samples and metagenomic shotgun sequences from selected baseline and post-antibiotic-treatment sample pairs were analyzed. Additionally, metagenomic predictions based on 16S rRNA gene amplicon data were performed using PICRUSt. The salivary microbiome was found to be significantly more robust, whereas the antibiotics negatively affected the fecal microbiome: in particular, health-associated butyrate-producing species became strongly underrepresented. Additionally, exposure to different antibiotics enriched genes associated with antibiotic resistance. In conclusion, healthy individuals, exposed to a single antibiotic treatment, undergo considerable microbial shifts and enrichment in antibiotic resistance in their feces, while their salivary microbiome composition remains unexpectedly stable. The health-related consequences for the gut microbiome should increase the awareness of the individual risks involved with antibiotic use, especially in a (diseased) population with an already dysregulated microbiome. On the other hand, understanding the mechanisms behind the resilience of the oral microbiome toward ecological collapse might prove useful in combating microbial dysbiosis elsewhere in the body., IMPORTANCE Many health care professionals use antibiotic prophylaxis strategies to prevent infection after surgery. This practice is under debate since it enhances the spread of antibiotic resistance. Another important reason to avoid nonessential use of antibiotics, the impact on our microbiome, has hardly received attention. In this study, we assessed the impact of antibiotics on the human microbial ecology at two niches. We followed the oral and gut microbiomes in 66 individuals from before, immediately after, and up to 12 months after exposure to different antibiotic classes. The salivary microbiome recovered quickly and was surprisingly robust toward antibiotic-induced disturbance. The fecal microbiome was severely affected by most antibiotics: for months, health-associated butyrate-producing species became strongly underrepresented. Additionally, there was an enrichment of genes associated with antibiotic resistance. Clearly, even a single antibiotic treatment in healthy individuals contributes to the risk of resistance development and leads to long-lasting detrimental shifts in the gut microbiome.

This paper reports on physiological and molecular responses of Saccharomyces cerevisiae to heat stress conditions. We observed that within a very narrow range of culture temperatures, a shift from exponential growth to growth arrest and ultimately to cell death occurred. A detailed analysis was carried out of the accumulation of trehalose and the activation of the protein kinase C1 (PKC1) (cell integrity) pathway in both glucose- and ethanol-grown cells upon temperature upshifts within this narrow range of growth temperatures. It was observed that the PKC1 pathway was hardly activated in a tps1 mutant that is unable to accumulate any trehalose. Furthermore, it was observed that an increase of the extracellular osmolarity during a continuous heat stress prevented the activation of the pathway. The results of these analyses support our hypothesis that under heat stress conditions the activation of the PKC1 pathway is triggered by an increase in intracellular osmolarity, due to the accumulation of trehalose, rather than by the increase in temperature as such.

In glucose-limited aerobic chemostat cultures of a wild-type Saccharomyces cerevisiae and a derived hxk2 null strain, metabolic fluxes were identical. However, the concentrations of intracellular metabolites, especially fructose 1,6-bisphosphate, and hexose-phosphorylating activities differed. Interestingly, the hxk2 null strain showed a higher maximal growth rate and higher Crabtree threshold dilution rate, revealing a higher oxidative capacity for this strain. After a pulse of glucose, aerobic glucose-limited cultures of wild-type S. cerevisiae displayed an overshoot in the intracellular concentrations of glucose 6-phosphate, fructose 6-phosphate, and fructose 1,6-bisphosphate before a new steady state was established, in contrast to the hxk2 null strain which reached a new steady state without overshoot of these metabolites. At low dilution rates the overshoot of intracellular metabolites in the wild-type strain coincided with the immediate production of ethanol after the glucose pulse. In contrast, in the hxk2 null strain the production of ethanol started gradually. However, in spite of the initial differences in ethanol production and dynamic behaviour of the intracellular metabolites, the steady-state fluxes after transition from glucose limitation to glucose excess were not significantly different in the wild-type strain and the hxk2 null strain at any dilution rate.

A study has been initiated to integrate molecular and physiological responses of Saccharomyces cerevisiae to heat stress conditions. We focus our research on a quantification of the energetics of the stress response. A series of continuous heat stresses was applied to exponentially growing cells of the strain X2180-1A at 28°C, by increasing the growth temperature to 37, 39, 40, 41, 42, or 43°C. Here, the results on cell growth and viability, as well as on anabolic and catabolic rates are presented. We observed a surprisingly ‘thin line’ for the cells between growing, surviving, and dying, with regard to growth temperature. The heat stress showed a dual effect on catabolism: immediately after the temperature increase a strong peak was seen, after which a new, steady level was reached. In addition, the yield on glucose decreased with increasing temperature. Our results indicate that life at elevated temperatures is energetically unfavourable and a non-lethal heat stress invokes a redistribution of catabolic and anabolic fluxes.

For adaptation between anaerobic, micro-aerobic and aerobic conditions Escherichia coli's metabolism and in particular its electron transport chain (ETC) is highly regulated. Although it is known that the global transcriptional regulators FNR and ArcA are involved in oxygen response it is unclear how they interplay in the regulation of ETC enzymes under micro-aerobic chemostat conditions. Also, there are diverse results which and how quinones (oxidised/reduced, ubiquinone/other quinones) are controlling the ArcBA two-component system. In the following a mathematical model of the E. coli ETC linked to basic modules for substrate uptake, fermentation product excretion and biomass formation is introduced. The kinetic modelling focusses on regulatory principles of the ETC for varying oxygen conditions in glucose-limited continuous cultures. The model is based on the balance of electron donation (glucose) and acceptance (oxygen or other acceptors). Also, it is able to account for different chemostat conditions due to changed substrate concentrations and dilution rates. The parameter identification process is divided into an estimation and a validation step based on previously published and new experimental data. The model shows that experimentally observed, qualitatively different behaviour of the ubiquinone redox state and the ArcA activity profile in the micro-aerobic range for different experimental conditions can emerge from a single network structure. The network structure features a strong feed-forward effect from the FNR regulatory system to the ArcBA regulatory system via a common control of the dehydrogenases of the ETC. The model supports the hypothesis that ubiquinone but not ubiquinol plays a key role in determining the activity of ArcBA in a glucose-limited chemostat at micro-aerobic conditions.

In this article, we aim to find an explanation for the surprisingly thin line, with regard to temperature, between cell growth, growth arrest and ultimately loss of cell viability. To this end, we used an integrative approach including both experimental and modelling work. We measured the shortand long-term effects of increases in growth temperature from 28°C to 37, 39, 41, 42 or 43°C on the central metabolism of Saccharomyces cerevisiae. Based on the experimental data, we developed a kinetic mathematical model that describes the metabolic and energetic changes in growing bakers' yeast when exposed to a specific temperature upshift. The model includes the temperature dependence of core energy-conserving pathways, trehalose synthesis, protein synthesis and proteolysis. Because our model focuses on protein synthesis and degradation, the net result of which is important in determining the cell's capacity to grow, the model includes growth, i.e. glucose is consumed and biomass and adenosine nucleotide cofactors are produced. The model reproduces both the observed initial metabolic response and the subsequent relaxation into a new steady-state, compatible with the new ambient temperature. In addition, it shows that the energy consumption for proteome reprofiling may be a major determinant of heat-induced growth arrest and subsequent recovery or cell death. © Copyright 2014 Federation of European Biochemical Societies. All rights reserved.

Reduction of aerobic fermentation on sugars by altering the fermentative/oxidative balance is of significant interest for optimization of industrial production of Saccharomyces cerevisiae . Glucose control of oxidative metabolism in baker's yeast is partly mediated through transcriptional regulation of the Hap4p subunit of the Hap2/3/4/5p transcriptional activator complex. To alleviate glucose repression of oxidative metabolism, we constructed a yeast strain with constitutively elevated levels of Hap4p. Genetic analysis of expression levels of glucose-repressed genes and analysis of respiratory capacity showed that Hap4p overexpression (partly) relieves glucose repression of respiration. Analysis of the physiological properties of the Hap4p overproducer in batch cultures in fermentors (aerobic, glucose excess) has shown that the metabolism of this strain is more oxidative than in the wild-type strain, resulting in a significant reduced ethanol production and improvement of growth rate and a 40% gain in biomass yield. Our results show that modification of one or more transcriptional regulators can be a powerful and a widely applicable tool for redirection of metabolic fluxes in microorganisms.

The capacity of Escherichia coli to adapt its catabolism to prevailing redox conditions resides mainly in three catabolic branch points involving (i) pyruvate formate-lyase (PFL) and the pyruvate dehydrogenase complex (PDHc), (ii) the exclusively fermentative enzymes and those of the Krebs cycle, and (iii) the alternative terminal cytochrome bd and cytochrome bo oxidases. A quantitative analysis of the relative catabolic fluxes through these pathways is presented for steady-state glucose-limited chemostat cultures with controlled oxygen availability ranging from full aerobiosis to complete anaerobiosis. Remarkably, PFL contributed significantly to the catabolic flux under microaerobic conditions and was found to be active simultaneously with PDHc and cytochrome bd oxidase-dependent respiration. The synthesis of PFL and cytochrome bd oxidase was found to be maximal in the lower microaerobic range but not in a ΔArcA mutant, and we conclude that the Arc system is more active with respect to regulation of these two positively regulated operons during microaerobiosis than during anaerobiosis.

Among the signal transfer systems in bacteria two types predominate: two-component regulatory systems and quorum sensing systems. Both types of system can mediate signal transfer across the bacterial cell envelope; however, the signalling molecule typically is not taken up into the cells in the former type of system, whereas it usually is in the latter. The Two-component systems include the recently described (eukaryotic) phosphorelay systems; quorum sensing systems can be based upon autoinducers of the N-acylated homoserine lactones, and on autoinducers of a peptidic nature. A single bacterial cell contains many signalling modules that primarily operate in parallel. This may give rise to neural-network behaviour. Recently, however, for both types of basic signal transfer modules, it has been demonstrated that they also can be organised in series (i.e. in a hierarchical order). Besides their hierarchical position in the signal transduction network of the cell, the spatial distribution of individual signalling modules may also be an important factor in their efficiency in signal transfer. Many challenges lie hidden in future work to understand these signal transfer processes in more detail. These are discussed here, with emphasis on the mutual interactions between different signal transfer processes. Successful contributions to this work will require rigorous mathematical modelling of the performance of signal transduction components, and -networks, as well as studies on light-sensing signal transduction systems, because of the unsurpassed time resolution obtainable in those latter systems, the opportunity to apply repeated reproducible stimuli, etc. The increased understanding of bacterial behaviour that already has resulted--and may further result--from these studies, can be used to fine-tune the beneficial activities of bacteria and/or more efficiently inhibit their deleterious ones.

Streptomyces coelicolor was grown in variously limited chemostat cultures and the specific rate of extracellular actinorhodin production (q(actinorhodin)) was measured. The highest q(actinorhodin) values were observed in glucose- or ammonia-limited cultures, whereas almost no actinorhodin was produced in sulfate-, phosphate-, potassium-, or magnesium-limited cultures. The effect of the dilution rate on actinorhodin production was studied in glucose-limited cultures. It was found that q(actinorhodin) was highest at D = 0.06h(-1), which was well below the maximal D value tested (0.14 h(-1)). This explains why, in batch cultures, actinorhodin production starts at the onset of the stationary phase. It was also found that the use of nitrilotriacetate instead of citrate as a chelating agent had a negative effect on actinorhodin production. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 577-582, 1997.

Oxygen relieves the CO2 and acetate dependency of Lactobacillus johnsonii NCC 533. The probiotic Lactobacillus johnsonii NCC 533 is relatively sensitive to oxidative stress; the presence of oxygen causes a lower biomass yield due to early growth stagnation. We show however that oxygen can also be beneficial to this organism as it relieves the requirement for acetate and CO2 during growth. Both on agar- and liquid-media, anaerobic growth of L. johnsonii NCC 533 requires CO2 supplementation of the gas phase. Switching off the CO2 supply induces growth arrest and cell death. The presence of molecular oxygen overcomes the CO2 dependency. Analogously, L. johnsonii NCC 533 strictly requires media with acetate to sustain anaerobic growth, although supplementation at a level that is 100-fold lower (120 microM) than the concentration in regular growth medium for lactobacilli already suffices for normal growth. Analogous to the CO2 requirement, oxygen supply relieves this acetate-dependency for growth. The L. johnsonii NCC 533 genome indicates that this organism lacks genes coding for pyruvate formate lyase (PFL) and pyruvate dehydrogenase (PDH), both CO2 and acetyl-CoA producing systems. Therefore, C1- and C2- compound production is predicted to largely depend on pyruvate oxidase activity (POX). This proposed role of POX in C2/C1-generation is corroborated by the observation that in a POX deficient mutant of L. johnsonii NCC 533, oxygen is not able to overcome acetate dependency nor does it relieve the CO2 dependency.

The cyanobacterium Microcystis PCC7806 was found to possess an NAD-dependent lactate dehydrogenase (EC 1.1.1.27) which catalyzes the reduction of pyruvate to l-lactate. The enzyme required fructose 1,6-bisphosphate for activity and displayed positive cooperativity towards pyruvate. Lactate was not formed during fermentation by cell suspensions, possibly due to low intracellular concentrations of fructose 1,6-bisphosphate and/or pyruvate.

Enterococcus faecalis was grown in chemostat culture on various energy sources at dilution rates ranging from 0.05 h-1 to 0.5 h-1, under both aerobic and anaerobic conditions. NADH/NAD ratios and total nicotinamide adenine dinucleotide pool size (NAD(H)) were determined. It was found that the NADH/NAD ratio was controlled by the steady state product concentrations rather than by the degree of reduction of the energy source. Highest ratios were observed when NADH was reoxidized via ethanol formation, whereas in aerobic cultures, in which predominantly acetate was produced and oxidation of NADH occurred via the NADH oxidase, ratios were lowest. Addition of ethanol to the medium resulted in an increase of the NADH/NAD ratio, both aerobically and anaerobically. The total amount of NAD(H) was found to be influenced by the culture conditions. Under anaerobic conditions, the NADH oxidation (NAD reduction) rate appeared to correlate with the total amount of nicotinamide nucleotides. In contrast, no effect of the culture conditions on the total amount of NAD(H) was observed in aerobically grown cells.

Cultured meat (i.e., meat produced in vitro using tissue engineering techniques) is being developed as a potentially healthier and more efficient alternative to conventional meat. Life cycle assessment (LCA) research method was used for assessing environmental impacts of large-scale cultured meat production. Cyanobacteria hydrolysate was assumed to be used as the nutrient and energy source for muscle cell growth. The results showed that production of 1000 kg cultured meat requires 26-33 GJ energy, 367-521 m(3) water, 190-230 m(2) land, and emits 1900-2240 kg CO(2)-eq GHG emissions. In comparison to conventionally produced European meat, cultured meat involves approximately 7-45% lower energy use (only poultry has lower energy use), 78-96% lower GHG emissions, 99% lower land use, and 82-96% lower water use depending on the product compared. Despite high uncertainty, it is concluded that the overall environmental impacts of cultured meat production are substantially lower than those of conventionally produced meat.

Size distributions and glucose and pH profiles of aggregates of the d -(-)-lactic acid-producing organism Bacillus laevolacticus were measured. The organisms were grown in continuous culture with a medium glucose concentration of either 280 or 110 mM. A maximal aggregate diameter of 2.2 mm, with a Sauter mean of 1.46 mm, was determined for the former culture condition, whereas aggregates from a culture with 110 mM glucose input had a maximal diameter of 1.9 mm (Sauter mean of 1.07 mm). A pH gradient of approximately 2 U was observed for large aggregates (above 1.5 mm). In smaller aggregates (0.75 mm), the pH value in the interior part was approximately 0.4 U lower than that in the culture fluid. It could be concluded that, in cultures with the high glucose input, lactic acid accumulated within the aggregates to such an extent that metabolism in the central region of the larger aggregates could not proceed further. In these cultures, approximately 90% of the total biomass was active. In aggregates from cultures with a low glucose input, glucose only partly penetrated the larger-sized aggregates, and the activity of this culture was reduced to approximately 70% of the biomass. These aggregates were found to decrease in size after prolonged periods of cultivation. It is suggested that this is caused by glucose depletion in the interior of the aggregates. It is concluded that the availability of glucose is an important factor in determining the size of aggregates of B. laevolacticus.

The stability of several pBR322-derived recombinant plasmids, carrying thethr operon fromEscherichia coli, was investigated in sulfur-limited chemostat cultures ofE. coli GT123. A marked increase in the segregational stability of one of these plasmids was observed. It is concluded that the increased stability was due to the spontaneous insertion of Tn1000 from the chromosome of the host into the plasmid.

Potassium-limited cultures of Candida utilis grown with ammonium chloride as the sole nitrogen source attained a higher dry weight than similar cultures grown with sodium nitrate as the sole nitrogen source. This increase depended on the extracellular ammonia concentration and was not due to accumulation of storage polymers. We conclude that in this yeast ammonium ions can fulfill to some extent the physiological role of potassium ions and that the intracellular concentration of ammonium ions is predominantly determined by the ammonia concentration in the culture medium.

Realistic quantitative models require data from many laboratories. Therefore, standardization of experimental systems and assay conditions is crucial. Moreover, standards should be representative of the in vivo conditions. However, most often, enzyme-kinetic parameters are measured under assay conditions that yield the maximum activity of each enzyme. In practice, this means that the kinetic parameters of different enzymes are measured in different buffers, at different pH values, with different ionic strengths, etc. In a joint effort of the Dutch Vertical Genomics Consortium, the European Yeast Systems Biology Network and the Standards for Reporting Enzymology Data Commission, we have developed a single assay medium for determining enzyme-kinetic parameters in yeast. The medium is as close as possible to the in vivo situation for the yeast Saccharomyces cerevisiae, and at the same time is experimentally feasible. The in vivo conditions were estimated for S. cerevisiae strain CEN.PK113-7D grown in aerobic glucose-limited chemostat cultures at an extracellular pH of 5.0 and a specific growth rate of 0.1 h(-1). The cytosolic pH and concentrations of calcium, sodium, potassium, phosphorus, sulfur and magnesium were determined. On the basis of these data and literature data, we propose a defined in vivo-like medium containing 300 mM potassium, 50 mM phosphate, 245 mM glutamate, 20 mM sodium, 2 mM free magnesium and 0.5 mM calcium, at a pH of 6.8. The V(max) values of the glycolytic and fermentative enzymes of S. cerevisiae were measured in the new medium. For some enzymes, the results deviated conspicuously from those of assays done under enzyme-specific, optimal conditions.

The field of Systems Biology is a rapidly evolving area of research. It follows on from the previous experimental and theoretical ‘omics’ revolution in biology. Now that we have through the use of these tools many ‘indices’ of biological systems available the next step is to actually start composing the systems that these indices specify. In this paper we will discuss the developments in the field of Systems Biology as they pertain to predictive food microbiology and give an example of state of the art current approaches. The data discussed in the case study deal with the resistance of the yeast Saccharomyces cerevisiae towards environmental temperature changes through adaptation of its metabolism, protein signalling and gene-expression.The results are integrated and its implications for the definition of new experiments discussed; the iteration between experiment driven model definition and model driven experimentation being characteristic for contemporary Systems Biology approaches. The stress condition discussed represents in no way a practical situation in food microbiology but what it teaches may well be applied in such cases. We will indicate how the latter may be achieved.

A major challenge in systems biology lies in the integration of processes occurring at different levels, such as transcription, translation, and metabolism, to understand the functioning of a living cell in its environment. We studied the high temperature induced glycolytic flux increase in Saccharomyces cerevisiae and investigated the regulatory mechanisms underlying this increase. We used glucose-limited chemostat cultures to separate regulatory effects of temperature from effects on growth rate. Growth at increased temperature (38 °C versus 30 °C) resulted in a strongly increased glycolytic flux, accompanied by a switch from respiration to a partially fermentative metabolism. We observed an increased flux through all enzymes, ranging from 5- to 10-fold. We quantified the contributions of direct temperature effects on enzyme activities, the gene expression cascade and shifts in the metabolic network, to the increased flux through each enzyme. To do this we adapted flux regulation analysis. We show that the direct effect of temperature on enzyme kinetics can be included as a separate term. Together with hierarchical regulation and metabolic regulation, this term explains the total flux change between two steady states. Surprisingly, the effect of the cultivation temperature on enzyme catalytic capacity, both directly through the Arrhenius effect and indirectly through adapted gene expression, is only a moderate contribution to the increased glycolytic flux for most enzymes. The changes in flux are therefore largely caused by changes in the interaction of the enzymes with substrates, products, and effectors. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.

In this contribution we describe the occurrence of futile cycles in growing bacteria. These cycles are thought to be active when organisms contain two uptake systems for a particular nutrient (one with a high, the other with a low affinity for its substrate). The high-affinity system is responsible for uptake of the nutrient, some of which is subsequently lost to the medium again via leakage through the low-affinity-system. A special futile cycle is caused under some growth conditions by the extremely rapid diffusion of ammonia through bacterial membranes. When the ammonium ion is taken up via active transport, the couple NH 3 NH + 4 will act as an uncoupler. This is aggravated by the chemical similarity of the potassium and the ammonium ion, which leads to ammonium ion transport via the Kdp potassium transport system when the potassium concentration in the medium is low. Other examples of futile cycles, such as those caused by the production of fatty acids by fermentation, are briefly discussed.

Two-component regulation systems (TCRSs) are the dominant type of signal transduction system in prokaryotes that are used to inform the cellular trancriptional machinery (and additional targets for regulation, like the motility apparatus) about actual changes in the extracellular physico-chemical conditions. We now review their molecular structure and enzymatic characteristics, their mutual interactions and its implications, and their role in cellular physiology. Specific emphasis is placed on the ArcB/A system, a representative of the phosphorelay type of TCRS, and a key player in the adjustment of the cellular make-up of enterobacteria in response to alterations in the oxygen availability. Also some applied aspects of the TCRSs are discussed, i.e. their role as a target to develop new anti-bacterials and their application in biotechnology (or: ‘synthetic biology’).

14-3-3 proteins form a family of highly conserved eukaryotic proteins involved in a wide variety of cellular processes, including signalling, apoptosis, cell-cycle control and transcriptional regulation. More than 150 binding partners have been found for these proteins. The yeast Saccharomyces cerevisiae has two genes encoding 14-3-3 proteins, BMH1 and BMH2. A bmh1 bmh2 double mutant is unviable in most laboratory strains. Previously, we constructed a temperature-sensitive bmh2 mutant and showed that mutations in RTG3 and SIN4, both encoding transcriptional regulators, can suppress the temperature-sensitive phenotype of this mutant, suggesting an inhibitory role of the 14-3-3 proteins in Rtg3-dependent transcription [van Heusden and Steensma (2001) Yeast 18, 1479–1491]. In the present paper, we report a genome-wide transcription analysis of a temperature-sensitive bmh2 mutant. Steady-state mRNA levels of 60 open reading frames were increased more than 2.0-fold in the bmh2 mutant, whereas those of 78 open reading frames were decreased more than 2.0-fold. In agreement with our genetic experiments, six genes known to be regulated by Rtg3 showed elevated mRNA levels in the mutant. In addition, several genes with other cellular functions, including those involved in gluconeogenesis, ergosterol biosynthesis and stress response, had altered mRNA levels in the mutant. Our data show that the yeast 14-3-3 proteins negatively regulate Rtg3-dependent transcription, stimulate the transcription of genes involved in ergosterol metabolism and in stress response and are involved in transcription regulation of multiple other genes.

The tendency of Saccharomyces cerevisiae to favor alcoholic fermentation over respiration is a complication in aerobic, biomass-directed applications of this yeast. Overproduction of Hap4p, a positive transcriptional regulator of genes involved in respiratory metabolism, has been reported to positively affect the balance between respiration and fermentation in aerobic glucose-grown batch cultures. In this study, the effects of HAP4 overexpression have been quantified in the prototrophic S. cerevisiae strain CEN.PK 113-7D under a variety of growth conditions. In aerobic glucose-limited chemostat cultures, overexpression of HAP4 increased the specific growth rate at which aerobic fermentation set in by about 10% relative to the isogenic wild-type. Upon relief of glucose-limited conditions, the HAP4-overexpressing strain produced slightly less ethanol than the wild-type strain. The effect of Hap4p overproduction was most drastic in aerobic, glucose-grown chemostat cultures in which ammonium was limiting. In such cultures, the biomass yield on glucose was double that of the wild-type.

In Escherichia coli, the two-component regulatory ArcAB system functions as a major control system for the regulation of expression of genes encoding enzymes involved in both aerobic and anaerobic catabolic pathways. Previously, we have described the physiological response of wild-type E. coli to changes in oxygen availability through the complete range from anaerobiosis to full aerobiosis (S. Alexeeva, B. de Kort, G. Sawers, K. J. Hellingwerf, and M. J. Teixeira de Mattos, J. Bacteriol. 182:4934-4940, 2000, and S. Alexeeva, K. J. Hellingwerf, and M. J. Teixeira de Mattos, J. Bacteriol. 184:1402-1406, 2002). Here, we address the question of the contribution of the ArcAB-dependent transcriptional regulation to this response. Wild-type E. coli and a mutant lacking the ArcA regulator were grown in glucose-limited chemostat cultures at controlled levels of oxygen availability ranging from full aerobiosis to complete anaerobiosis. A flux analysis of the distribution of catabolic fluxes over parallel pathways was carried out, and the intracellular redox state (as reflected by the NADH/NAD ratio) was monitored for all steady states. Deletion of ArcA neither significantly altered the in vivo activity of the pyruvate dehydrogenase complex and pyruvate formate lyase nor significantly affected catabolism under fully aerobic and fully anaerobic conditions. In contrast, profound effects of the absence of ArcA were seen under conditions of oxygen-restricted growth: increased respiration, an altered electron flux distribution over the cytochrome o - and d -terminal oxidases, and a significant change in the intracellular redox state were observed. Thus, the ArcA regulator was found to exert major control on flux distribution, and it is concluded that the ArcAB system should be considered a microaerobic redox regulator.

A study has been initiated to integrate molecular and physiological responses of Saccharomyces cerevisiae to heat stress conditions. We focus our research on a quantification of the energetics of the stress response. A series of continuous heat stresses was applied to exponentially growing cells of the strain X2180-1A at 28 degrees C, by increasing the growth temperature to 37, 39, 40, 41, 42, or 43 degrees C. Here, the results on cell growth and viability, as well as on anabolic and catabolic rates are presented. We observed a surprisingly 'thin line' for the cells between growing, surviving, and dying, with regard to growth temperature. The heat stress showed a dual effect on catabolism: immediately after the temperature increase a strong peak was seen, after which a new, steady level was reached. In addition, the yield on glucose decreased with increasing temperature. Our results indicate that life at elevated temperatures is energetically unfavourable and a non-lethal heat stress invokes a redistribution of catabolic and anabolic fluxes.

Despite a large number of studies on the role of oxygen in cellular processes, there is no consensus as to how oxygen availability to the cell should be defined, let alone how it should be quantified. Here, a quantitative definition for oxygen availability (perceived aerobiosis) is presented; the definition is based on a calibration with reference to the minimal oxygen supply rate needed for fully oxidative catabolism (i.e., complete conversion of the energy source to CO 2 and water for glucose-limited conditions). This quantitative method is used to show how steady-state electron fluxes through the alternative cytochrome oxidases of Escherichia coli are distributed as a function of the extent of aerobiosis of glucose-limited chemostat cultures. At low oxygen availability the electron flux is mainly via the high-affinity cytochrome bd oxidase, and, at higher oxygen availability, a similar phenomenon occurs but now via the low-affinity cytochrome bo oxidase. The main finding is that the catabolic activities of E. coli (and specifically its respiratory activity) are affected by the actual oxygen availability per unit of biomass rather than by the residual dissolved oxygen concentration of the culture.

Among the signal transfer systems in bacteria two types predominate: two-component regulatory systems and quorum sensing systems. Both types of system can mediate signal transfer across the bacterial cell envelope; however, the signalling molecule typically is not taken up into the cells in the former type of system, whereas it usually is in the latter. The Two-component systems include the recently described (eukaryotic) phosphorelay systems; quorum sensing systems can be based upon autoinducers of the N-acylated homoserine lactones, and on autoinducers of a peptidic nature. A single bacterial cell contains many signalling modules that primarily operate in parallel. This may give rise to neural-network behaviour. Recently, however, for both types of basic signal transfer modules, it has been demonstrated that they also can be organised in series (i.e. in a hierarchical order). Besides their hierarchical position in the signal transduction network of the cell, the spatial distribution of individual signalling modules may also be an important factor in their efficiency in signal transfer. Many challenges lie hidden in future work to understand these signal transfer processes in more detail. These are discussed here, with emphasis on the mutual interactions between different signal transfer processes. Successful contributions to this work will require rigorous mathematical modelling of the performance of signal transduction components, and -networks, as well as studies on light-sensing signal transduction systems, because of the unsurpassed time resolution obtainable in those latter systems, the opportunity to apply repeated reproducible stimuli, etc. The increased understanding of bacterial behaviour that already has resulted--and may further result--from these studies, can be used to fine-tune the beneficial activities of bacteria and/or more efficiently inhibit their deleterious ones.

The kinetics of glucose transport and the transcription of all 20 members of the HXT hexose transporter gene family were studied in relation to the steady state in situ carbon metabolism of Saccharomyces cerevisiae CEN.PK113-7D grown in chemostat cultures. Cells were cultivated at a dilution rate of 0.10 h-1 under various nutrient-limited conditions (anaerobically glucose- or nitrogen-limited or aerobically glucose-, galactose-, fructose-, ethanol-, or nitrogen-limited), or at dilution rates ranging between 0.05 and 0.38 h-1 in aerobic glucose-limited cultures. Transcription of HXT1-HXT7 was correlated with the extracellular glucose concentration in the cultures. Transcription of GAL2, encoding the galactose transporter, was only detected in galactose-limited cultures. SNF3 and RGT2, two members of the HXT family that encode glucose sensors, were transcribed at low levels. HXT8-HXT17 transcripts were detected at very low levels. A consistent relationship was observed between the expression of individual HXT genes and the glucose transport kinetics determined from zero-trans influx of 14C-glucose during 5 s. This relationship was in broad agreement with the transport kinetics of Hxt1-Hxt7 and Gal2 deduced in previous studies on single-HXT strains. At lower dilution rates the glucose transport capacity estimated from zero-trans influx experiments and the residual glucose concentration exceeded the measured in situ glucose consumption rate. At high dilution rates, however, the estimated glucose transport capacity was too low to account for the in situ glucose consumption rate.

The effect of NADH on the activity of the purified pyruvate dehydrogenase complexes (PDHc) of Enterococcus (Ec.) faecalis, Lactococcus lactis, Azotobacter vinelandii and Escherichia coli was determined in vitro. It was found that the PDHc of E. coli and L. lactis was active only at relatively low NADH/NAD ratios, whereas the PDHc of Ec. faecalis was inhibited only at high NADH/NAD ratios. The PDHc of Azotobacter vinelandii showed an intermediate sensitivity. The organisms were grown in chemostat culture under conditions that led to different intracellular NADH/NAD ratios and the PDHc activities in vivo could be calculated from the specific rates of product formation. Under anaerobic growth conditions, only Ec. faecelis expressed PDHc activity in vivo. The activities in vivo of the complexes of the different organisms were in good agreement with their properties determined in vitro. The physiological consequences of these results are discussed.

When grown in carbon source-limited chemostat cultures with lactate or glucose as the carbon and energy source and xylose as an additional source of reducing equivalents. Pseudomonas putida NCTC 10936 oxidized xylose to xylonolactone and xylonate. No other products were formed from this pentose, nor was it incorporated into biomass. The presence of xylose in these cultures resulted in higher Yglucose and Ylactate values as compared to cultures without xylose indicating that biologically useful energy was conserved during the periplasmic oxidation of xylose. As the Y0 values for growth on glucose or on lactate alone were equal to the Y0 values for growth with xylose as co-substrate, it is concluded that for glucose- or lactate-limited growth energy conservation by PQQH2 oxidation is as efficient as by NADH2 oxidation.

Escherichia coli Frag1 was grown under various nutrient limitations in chemostat culture at a fixed temperature, dilution rate and pH both in the presence and the absence of a high concentration of ammonium ions by using either ammonium chloride or DL-alanine as the sole nitrogen source. The presence of high concentrations of ammonium ions in the extracellular fluids of potassium-limited cultures of E. coli Frag1 caused an increase of the specific rate of oxygen consumption of these cultures. In contrast, under phosphate-, sulphate- or magnesium-limited growth conditions no such increase could be observed. The presence of high concentrations of ammonium ions in potassium-limited cultures of E. coli Frag5, a mutant strain of E. coli Frag1 which lacks the high affinity potassium uptake system (Kdp), did not increase the specific rate of oxygen consumption. These results indicate that ammonium ions, very similar to potassium ions both in charge and size, are transported via the Kdp leading to a futile cycle of ammonium ions and ammonia molecules (plus protons) across the cytoplasmic membrane. Both the uptake of ammonium ions and the extrusion of protons would increase the energy requirement of the cells and therefore increase their specific rate of oxygen consumption. The involvement of a (methyl)ammonium transport system in this futile cycle could be excluded.

Magnesium-limited chemostat cultures of Klebsiella pneumoniae NCTC 418 with 20 μM CaCl2 in the medium showed a low rate of gluconate plus 2-ketogluconate production relative to potassium- or phosphate-limited cultures. However, when the medium concentration of CaCl2 was increased to 1 mM, the glucose dehydrogenase (GDH) activities also increased and became similar to those observed in potassium- or phosphate limited cultures. It is concluded that this is due to Mg2+ and Ca2+ ions being involved in the binding of pyrroloquinoline quinone (PQQ) to the GDH apoenzyme. There seems to be an absolute requirement of divalent cations for proper enzyme functioning and in this respect Ca2+ ions could replace Mg2+ ions. The high GDH activity which has been found in cells grown under Mg2−-limited conditions in the presence of higher concentrations of Ca2+ ions, is compatible with the earlier proposal that GDH functions as an auxiliary energy generating system involved in the maintenance of high transmembrane ion gradients., Centro de Investigación y Desarrollo en Fermentaciones Industriales

Cells of Klebsiella pneumoniae NCTC 418 grown at low culture pH values (4.5–5) in a glucose-limited chemostat culture contained elevated levels of glutamate synthase (EC 2.6.1.53). This can be taken as an indication that these cells show the physiology of nitrogen-limited cells, in spite of the fact that high concentrations (about 80 mM) of ammonium ions were present in the culture extracellular fluids. This phenomenon can be explained by the rapid diffusion of ammonia (NH3) through the cell membrane, leading to very low cytoplasmic ammonium (NH4+) and NH3 levels in cells that possess an almost neutral cytoplasmic pH value, but are growing at low culture pH values.

The mechanism of granule formation by Selenomonas ruminantium was investigated. A basic protein has been isolated from the lysate of S. ruminantium which triggers cluster formation (small aggregates of 20–100 cells) of suspended cells. Evidence is presented that these basic proteins were of ribosomal origin. It is suggested that ribosomes are released into the culture broth by lysis and that the associated basic proteins are subsequently dissociated by high monovalent cation concentrations. It was found that these positively charged basic proteins interact with the negatively charged lipopolysaccharide of the organism to form the clusters. Adding lysate to suspended cells, followed by lowering of the pH from 5.8 to 4.5 also induced clustering. At dilution rates exceeding the maximum growth rate clusters were retained in anaerobic gas-lift reactors and grew into granules (1–3 mm). It is postulated that granules evolve from clusters. Within the clusters, lysis and a low pH are induced due to diffusion limitations. As a consequence dividing cells are entrapped within the clusters, resulting in growth.

A study was undertaken of the microbial composition of aggregates from an acidifying anaerobic gas-lift reactor. For this purpose a simple 100 ml anaerobic gas lift reactor was developed. It was found that the predominant organism in the aggregates was Selenomonas ruminantium. Both, microscopical observations and a newly developed enumeration technique led to the conclusion that the mixed granules consisted mainly of this organism. Grown in pure culture, S. ruminantium was capable of forming aggregates. These aggregates resembled the mixed aggregates both macro- as well as microscopically. Furthermore the fermentation pattern of this pure aggregated culture was similar to that of a mixed aggregated culture.