Role of sodium in the growth of a ruminal selenomonad

Effects of dissolved CO2 levels on the growth ofMannheimia succiniciproducens and succinic acid production

A capnophilic rumen bacterium Mannheimia succiniciproducens produces succinic acid as a major fermentation end product under CO(2)-rich anaerobic condition. Since succinic acid is produced by carboxylation of C3 compounds during the fermentation, intracellular CO(2) availability is important for efficient succinic acid formation. Here, we investigated the metabolic responses of M. succiniciproducens to the different dissolved CO(2) concentrations (0-260 mM). Cell growth was severely suppressed when the dissolved CO(2) concentration was below 8.74 mM. On the other hand, cell growth and succinic acid production increased proportionally as the dissolved CO(2) concentration increased from 8.74 to 141 mM. The yields of biomass and succinic acid on glucose obtained at the dissolved CO(2) concentration of 141 mM were 1.49 and 1.52 times higher, respectively, than those obtained at the dissolved CO(2) concentration of 8.74 mM. It was also found that the additional CO(2) source provided in the form of NaHCO(3), MgCO(3), or CaCO(3) had positive effects on cell growth and succinic acid production. However, growth inhibition was observed when excessive bicarbonate salts were added. By the comparison of the activities of key enzymes, it was found that PEP carboxylation by PEP carboxykinase (PckA) is the most important for succinic acid production as well as the growth of M. succiniciproducens by providing additional ATP.

A cost effective fermentative production of succinic acid from cane molasses and corn steep liquor by Escherichia coli

AIM

Development and optimization of an efficient and inexpensive medium for succinic acid production by Escherichia coli under anaerobic conditions.

METHODS AND RESULTS

Initially, 0.8 gl(-1) of succinic acid was produced in 60 h in 300-ml medium. On optimization, glucose and peptone were replaced by cane molasses and corn steep liquor. Three hundred ml of this medium was inoculated with 4% (v/v) of seed inoculum, incubated at 39 degrees C for 72 h, resulted in 7.1 gl(-1) of succinic acid in 36 h. Scale up in a 10-l fermentor under conditions of controlled pH and continuous CO2 supply in this medium resulted in 17 gl(-1) of succinic acid in 30 h.

CONCLUSIONS

A ninefold increase in succinic acid production was obtained in 500-ml anaerobic bottles with optimized medium having cane molasses and corn steep liquor as against initial medium containing glucose and peptone. However, a subsequent scale up in a 10-l fermentor resulted in a 2.5-fold increase in succinic acid production as against optimized medium used in 500-ml anaerobic bottles.

SIGNIFICANCE AND IMPACT OF THE STUDY

Succinic acid production was enhanced in medium consisting of inexpensive carbon and nitrogen sources in a shorter span of time.

Succinic acid production from Bacteroides fragilis: process optimization and scale up in a bioreactor

We report the effect of different physiological and nutritional parameters on succinic acid production from Bacteroides fragilis. This strain initially produced 0.70gL(-1) of succinic acid in 60h. However, when process optimization was employed, 5.4gL(-1) of succinic acid was produced in medium consisting of glucose (1.5%); tryptone (2.5%); Na(2)CO(3) (1.5%), at pH 7.0, when inoculated with 4% inoculum and incubated at 37 degrees C, 100rpm for 48h. A marked enhancement in succinic acid production was observed when the optimized conditions were employed in a 10L bioreactor. A total of 12.5gL(-1) of succinic acid was produced in 30h. This is approximately 12-fold increase in succinic acid production when compared to the initial un-optimized medium production. This enhancement in succinic acid production may be due to the control of CO(2) supply and the impeller speed. This is also resulted in the reduction of the production time. The present study provides useful information to the industrialists seeking environmentally benign technology for the production of bulk biomolecules through manipulation of various chemical parameters.

Effect of sugars and malate on ruminal microorganisms

The objective of this study was to examine the effects of a commercial feed supplement that contains sugars and malate on lactate fermentation by Selenomonas ruminantium grown in batch culture. Experiments also were conducted to examine the effects of this feed supplement on the mixed ruminal microorganism fermentation of ground corn and soluble starch in the presence and absence of 5 mg/kg of monensin. When S. ruminantium strains HD4 and H18 were incubated in basal medium that contained DL-lactate, some DL-lactate was utilized by both strains after 24 h. In the presence of 1 g/L of sugars plus malate commercial feed supplement, both strains used most of the carbohydrate associated with the feed supplement between 6 and 8 h, and lactate was the main end product. In ground corn fermentations by mixed ruminal microorganisms, 2.25 and 3.25 g/L of sugars plus malate commercial feed supplement increased concentrations of acetate, propionate, and total volatile fatty acids, while 3.25 g/L increased lactate and decreased final pH and butyrate. Fermentation of soluble starch in the presence of both concentrations of sugars plus malate commercial feed supplement increased concentrations of acetate, propionate, and total volatile fatty acids and decreased the acetate:propionate ratio. In the presence of 5 mg/kg of monensin, sugars plus malate treatment increased concentrations of propionate and total volatile fatty acids in ground corn and soluble starch fermentations. Collectively, these results suggest that the sugars plus malate commercial feed supplement stimulates the ruminal fermentation.

In situ disappearance of malate from alfalfa and bermudagrass hay

The objectives of this study were to determine the rate of malate and dry matter disappearance from different forages in the rumen. Four nonlactating, ruminally cannulated Holstein cows were fed a hay-based diet. Samples of early and late harvested alfalfa, Coastal bermudagrass, and Tifton 85 bermudagrass hays were ground, placed in nylon in situ bags, and ruminally incubated for 0, 0.5, 1, 2, 4, 8, 12, 24, and 48 h. After incubation, samples were rinsed, freeze-dried, extracted, and analyzed for malate content by HPLC with an organic acid column. When forages were incubated in the rumen, malate concentrations were less than 0.55 mg/g of dry matter at 0.5 h and remained low for the 48-h incubation period. These results suggest that malate was solublized and utilized within 30 min after reaching the rumen. Dry matter digestibility of both forages increased with time and was different across forages. Both alfalfa samples were digested to a greater extent between 0.5 and 24 h than either type of bermudagrass, but after 48 h the early maturity Tifton 85 digestibility was similar to alfalfa. Even though it is more common to feed unground forages to ruminants, these in situ results suggest that once malate is available in the rumen it will disappear quickly.

Malate content of forage varieties commonly fed to cattle

The objective of this study was to determine the concentration of malate in forage varieties at different stages of maturity. Five alfalfa varieties (Alfagraze, Apollo Supreme Cimarron, Crockett, and Magnum III) and three bermudagrass varieties (Coastal, Tifton-78, and Tifton-85) were collected at different stages of maturity. Samples were collected from replicate plots (n = 3) of each alfalfa variety at 9, 18, 28, 35, and 42 d of maturity; bermudagrass hay samples were composited from six bales of each variety from two cuttings staged to be harvested at 27 and 41 d of maturity. Malate was extracted from the samples and quantitated by high performance liquid chromatography using an organic acid column. As maturity increased, the concentration of malate declined in both plant species. Concentrations of malate were numerically higher in two alfalfa varieties (Crockett and Magnum III) at 35 and 42 d of maturity than in all other alfalfa varieties. Concentrations of malate in bermudagrass at 41 d of maturity were lower than concentrations of malate in all alfalfa varieties at 42 d of maturity. Malate declined as maturity increased in the Coastal and Tifton-78 varieties. Because malate stimulates the utilization of lactate by the predominant ruminal bacterium Selenomonas ruminantium, some of the benefits associated with alfalfa in the diets of dairy cattle may be due to the malate in this forage.

Why do many ruminal bacteria die and lyse so quickly?

Studies using 15N have indicated that as much as 50% of the microbial mass turns over before N passes to the lower gut, and this N recycling significantly decreases the availability of microbial protein. Protozoa digest bacteria and smaller protozoa, but bacterial protein can turn over even if protozoa are not present. Fibrobacter succinogenes cultures lyse even when they are growing, and the lysis rate is independent of growth rate. When extracellular sugar is depleted, F. succinogenes secretes an extracellular proteinase that inactivates the autolysins. This method of autolytic regulation decreases the turnover of stationary cells. Bacteriophage and anaeroplasma can cause lysogeny, but, as yet, there is little proof that these processes are important determinants of bacterial turnover in vivo. Dietary manipulations (e.g., salt feeding and particle size reduction) that increase liquid and solid dilution rates can increase bacterial flow by decreasing bacterial residence time and turnover. Some dead ruminal bacteria are able to maintain their cellular integrity, and the ratio of dead to live cells in ruminal fluid may be as great as 10:1. Bacterial survival appears to be at least partially explained by the method of sugar transport. When bacteria rely solely on mechanisms of ion-coupled sugar symport, an energized membrane is necessary for the reinitiation of growth. If group translocation (phosphotransferase system) is the mechanisms of transport, uptake can be driven by phosphoenolpyruvate, and an energized membrane and the storage of intracellular reserve materials are not an absolute criteria for survival. In some cases, N deprivation accelerates death. When Prevotella ruminicola was limited for N under conditions of excess energy, methylglyoxal production caused a rapid decrease in viability. The impact of bacterial death in the rumen is not clear-cut. If the rate of fermentation is zero-order with respect to cell concentration (substrate-limited), cell death would have little impact on digestion.

Hexose phosphorylation by the ruminal bacterium Selenomonas ruminantium

Three strains of Selenomonas ruminantium (D, GA192, and H18) were surveyed for phosphorylation of D-glucose and 2-deoxyglucose by phosphoenolpyruvate and ATP. Cells of all three strains that had been treated with toluene had high rates of hexose phosphorylation with either phosphoryl donor; this activity was constitutive in strain D. Glucose phosphorylation that was dependent on phosphoenolpyruvate was maximal at pH 7.2, remained fairly high at pH 6.5, but decreased (> or = 65%) at pH 5.0 for all strains. Cell extracts were used to evaluate the involvement of soluble kinases in 2-deoxyglucose phosphorylation. Both glucose and 2-deoxyglucose were phosphorylated by ATP, but phosphorylation of either hexose was negligible with phosphoenolpyruvate in each bacterium. Because phosphoenolpyruvate could not serve as a phosphoryl donor, the activity dependent on phosphoenolpyruvate in cells treated with toluene might have been due to a phosphotransferase system associated with the membrane. Unlabeled 2-deoxyglucose was a strong inhibitor (> or = 59%) of [14C]glucose phosphorylation with ATP by cell extracts of all S. ruminantium strains, and unlabeled glucose was a strong inhibitor (> or = 78%) of [14C]2-deoxyglucose phosphorylation with ATP. Based on Lineweaver-Burk kinetics, 2-deoxyglucose was a competitive inhibitor of initial rates of kinase activity in strains D, GA192, and H18. These results collectively suggest that glucose phosphorylation with phosphoenolpyruvate and 2-deoxyglucose phosphorylation with ATP are common traits in these strains of S. ruminantium.

Ecology, metabolism, and genetics of ruminal selenomonads

Selenomonas ruminantium is one of the more prominent and functionally diverse bacteria present in the rumen and can survive under a wide range of nutritional fluctuations. Selenomonas is not a degrader of complex polysaccharides associated with dietary plant cell wall components, but is important in the utilization of soluble carbohydrates released from initial hydrolysis of these polymers by other ruminal bacteria. Selenomonads have multiple carbon flow routes for carbohydrate catabolism and ATP generation, and subspecies differ in their ability to use lactate. Some soluble carbohydrates (glucose, sucrose) appear to be transported via the phosphoenolpyruvate phosphotransferase system, while arabinose and xylose are transported by proton symport. High cell yields and the presence of electron transport components in Selenomonas strains has been documented repeatedly and this may partially account for the energy partitioning observed between energy consumed for growth and maintenance functions. Most strains can utilize ammonia, protein, and/or amino acids as a nitrogen source. Some strains can hydrolyze urea and/or reduce nitrate and use the ammonia for the biosynthesis of amino acids. Experimental evidence suggests that ammonia assimilatory enzymes in some strains may possess unique properties with respect to other presumably similar bacteria. Little is known about the genetics of ruminal selenomonads. Plasmid DNA has been isolated from some strains, but it is unknown what physiological functions may be encoded on these extrachromosomal elements. Due to the predominance of S. ruminantium in the rumen, it is an ideal candidate for genetic manipulation. Once the genetics of this bacterium are better understood, it may be possible to amplify its role in the rumen.

Pentose utilization and transport by the ruminal bacterium Prevotella ruminicola

Plant cell wall polysaccharides are primarily composed of hexose or hexose derivatives, but a significant fraction is hemicellulose which contains pentose sugars. Prevotella ruminicola B14, a predominant ruminal bacterium, simultaneously metabolized pentoses and glucose or maltose, but the organism preferentially fermented pentoses over cellobiose and preferred xylose to sucrose. Xylose and arabinose transport at either low (2 microM) or high (1 mM) substrate concentrations were observed only in the presence of sodium and if oxygen was excluded during the harvest and assay procedures. An artificial electrical potential (delta psi) or chemical gradient of sodium (delta pNa) drove transport in anaerobically prepared membrane vesicles. Because (i) transport was electrogenic, (ii) a delta pNa drove uptake, and (iii) the number of sodium binding sites was approximately 1, it appeared that P. ruminicola possessed pentose/sodium support mechanisms for the transport of arabinose and xylose at low substrate concentrations. Pentose uptake exhibited a low affinity for xylose or arabinose (> 300 microM), and transport of xylose exhibited bi-phasic kinetics which suggested that a second sodium-dependent xylose transport system was present. Little study has been made on solute transport by Prevotella (Bacteroides) species and this work represents the first use of isolated membrane vesicles from these organisms.

Effect of direct-fed microbials on rumen microbial fermentation

Nonbacterial, direct-fed microbials added to ruminant diets generally consist of Aspergillus oryzae fermentation extract, or Saccharomyces cerevisiae cultures, or both. Results from in vivo research have been variable regarding effects of direct-fed microbials on ruminant feedstuff utilization and performance. Some research has shown increased weight gains, milk production, and total tract digestibility of feed components, but others have shown little influence of direct-fed microbials on these parameters. In vitro research with mixed ruminal microorganisms likewise has been inconsistent regarding the effects of direct-fed microbials. Several researchers observed that direct-fed microbials increased cellulolytic bacterial numbers in the rumen and stimulated the production of some fermentation end products. This suggests that direct-fed microbials may be providing growth factors for the ruminal microbes. However, other researchers have reported no effect of direct-fed microbials on in vitro fiber digestion. Recent research demonstrated that growth of the predominant ruminal bacterium Selenomonas ruminantium in lactate medium as well as lactate uptake by whole cells of Sel. ruminantium were markedly increased by an A. oryzae fermentation extract and an S. cerevisiae culture. In addition, both products increased the production of acetate, propionate, succinate, total VFA, and cell yield (grams of cells per mole of lactate). Therefore, it appears that these direct-fed microbials provide soluble factors that stimulate lactate utilization by Sel. ruminantium. Evidence is presented indicating that the malate content of the A. oryzae fermentation extract and S. cerevisiae culture may be involved in this stimulation.

Effect of pH and Monensin on Glucose Transport by Fibrobacter succinogenes , a Cellulolytic Ruminal Bacterium

Fibrobacter succinogenes S85, a cellulolytic ruminal bacterium, required sodium for growth and glucose uptake. Cells which were deenergized with iodoacetate (500 μM) could not take up [ 14 C]glucose. However, deenergized cells which were treated with valinomycin, loaded with potassium, and diluted into sodium or sodium plus potassium to create an artificial electrical gradient (ΔΨ) plus a chemical gradient of sodium (ΔpNa) or ΔpNa alone transported glucose at a rapid rate. Cells which were loaded with potassium plus sodium and diluted into sodium (ΔΨ with sodium, but no ΔpNa) also took up glucose at a rapid rate. Potassium-loaded cells that were diluted into buffers which did not contain sodium (ΔΨ without sodium) could not take up glucose. An artificial ZΔpH which was created by acetate diffusion could not drive glucose transport even if sodium was present. The maximum rate and affinity of glucose transport (pH 6.7) were 62.5 nmol/mg of protein per min and 0.51 mM, respectively. S85 was unable to grow at a pH of less than 5.5, and there was little glucose transport at this pH. When the extracellular pH was decreased, the glucose carrier was inhibited, intracellular pH declined, the cells were no longer able to metabolize glucose, and ΔΨ declined. Monensin (1 μM) or lasalocid (5 μM) decreased intracellular ATP and dissipated both the ΔΨ and ΔpNa. Since there was no driving force for transport, glucose transport was inhibited. These results indicated that F. succinogenes used a pH-sensitive sodium symport mechanism to take up glucose and that either a ΔΨ or a ΔpNa was required for glucose transport.