Growth energetics ofClostridium sporogenesNCIB 8053: modulation by CO2

Fermentation of oxidized hexose derivatives by Clostridium acetobutylicum

Background

Clostridium acetobutylicum fermentations are promising for production of commodity chemicals from heterogeneous biomass due to the wide range of substrates the organism can metabolize. Much work has been done to elucidate the pathways for utilization of aldoses, but little is known about metabolism of more oxidized substrates. Two oxidized hexose derivatives, gluconate and galacturonate, are present in low cost feedstocks, and their metabolism will contribute to overall metabolic output of these substrates.

Results

A complete metabolic network for glucose, gluconate, and galacturonate utilization was generated using online databases, previous studies, genomic context, and experimental data. Gluconate appears to be metabolized via the Entner-Doudoroff pathway, and is likely dehydrated to 2-keto-3-deoxy-gluconate before phosphorylation to 2-keto-3-deoxy-6-P-gluconate. Galacturonate appears to be processed via the Ashwell pathway, converging on a common metabolite for gluconate and galacturonate metabolism, 2-keto-3-deoxygluconate. As expected, increasingly oxidized substrates resulted in increasingly oxidized products with galacturonate fermentations being nearly homoacetic. Calculations of expected ATP and reducing equivalent yields and experimental data suggested galacturonate fermentations were reductant limited. Galacturonate fermentation was incomplete, which was not due solely to product inhibition or the inability to utilize low concentrations of galacturonate. Removal of H₂ and CO₂ by agitation resulted in faster growth, higher cell densities, formation of relatively more oxidized products, and higher product yields for cultures grown on glucose or gluconate. In contrast, cells grown on galacturonate showed reduced growth rates upon agitation, which was likely due to loss in reductant in the form of H₂. The growth advantage seen on agitated glucose or gluconate cultures could not be solely attributed to improved ATP economics, thereby indicating other factors are also important.

Conclusions

The metabolic network presented in this work should facilitate similar reconstructions in other organisms, and provides a further understanding of the pathways involved in metabolism of oxidized feedstocks and carbohydrate mixtures. The nearly homoacetic fermentation during growth on galacturonate indicates further optimization of this and related organisms could provide a route to an effective biologically derived acetic acid production platform. Furthermore, the pathways could be targeted to decrease production of undesirable products during fermentations of heterogeneous biomass.

The influence of dissolved CO(2) concentration on the death kinetics of Saccharomyces cerevisiae

AIMS

The effects of temperature and concentration of dissolved CO(2) on the inactivation of Saccharomyces cerevisiae were investigated using a plug-flow system.

METHODS AND RESULTS

Several combinations of pressure (4, 6, 8, 10 mega-Pa (MPa)) and temperature (30, 34, 36, 38 degrees C) were used. The D-values obtained were 0.14 min at 8 MPa and 38 degrees C, and 0.15 min at 10 MPa and 36 degrees C. The log D-values were related linearly to the treatment temperature and to the dissolved CO(2) concentration. The thermal resistance constant (zCO(2)(T)) was 9.5 degrees C in the media, including significant levels of CO(2), and the CO(2) resistance constant was z(temp.)(gamma)=7.2 gamma.

CONCLUSION

This work has shown that inactivation followed first-order death kinetics, and the effects of temperature and CO(2) concentration were consistent through the critical temperature and pressure of CO(2). Therefore, it is feasible to estimate D-values at any temperature and any CO(2) concentration.

SIGNIFICANCE AND IMPACT OF THE STUDY

Non-thermal inactivation of micro-organisms in acidic beverages could be realized by the present technique.

Influence of oxygenation and carbon dioxide on the growth ofRhizobium meliloti in a fermenter

A decrease in dissolved O2 from 90% to 50% saturation in a fermenter adversely affected both blomass production ofRhizobium meliloti ATCC 9930 and viable cell number, although oxygen was never limiting. Lower amounts of dissolved oxygen, or accidental decreases in dissolved oxygen concentration, also caused appreciable acidification of the culture broth, which was the result of CO2 accumulation in the medium. Adding CO2 to the aeration gas mixture affected both biomass production and mean generation time in proportion to the CO2 concentration; the effect on viable cell number was less pronounced.R. meliloti may be considered as a microorganism moderately sensitive to CO2. GoodR. meliloti growth requires, among other things, not only sufficient aeration (oxygenation) but also good ventilation of the CO2 evolved during the fermentation.

pH regulation of urease levels in Streptococcus salivarius

Potential mechanisms for regulation of urease levels in Streptococcus salivarius were examined, including: induction by urea, nitrogen or carbon source repression, and effects of pH and CO2 (because CO2 enrichment enhanced urease detection on urea agar plates). Regulation by either pH or CO2 was confirmed by comparison of the urease accumulation pattern during anaerobic growth under CO2 with that under N2. Under CO2, there was an initial buffering plateau at pH 6.2 and a rate of Streptococcus salivarius urease accumulation three-fold that under N2, with a pH 7.6 plateau. With both gas phases there was also an increase in the rate of urease appearance coincident with the decrease in medium pH following the pH plateau. The effects of pH, CO2, and HCO3- on urease levels and on growth were separately assessed by culture in media containing 0, 25, 100 mmol/L KHCO3 buffered at different pH levels. There was an inverse relationship between the logarithm of the urease level after 24-hour growth and the pH during growth-the urease specific activity was 100-fold higher at pH 5.5, compared with pH 7.0 and above. HCO3-/CO2 (100 mmol/L) had little effect on urease levels, but was essential for growth at pH 5.5. There was no significant urease induction by urea, or repression by ammonia or glucose. There was also evidence of pH regulation of urease levels in some staphylococci, Klebsiella pneumonia, and Corynebacterium renale, but not in Actinomyces naeslundii and several other species. We conclude that the external pH is a major factor regulating urease levels in S. salivarius and possibly some other species-a mechanism equivalent to urease repression by OH-.

Effect of different carbohydrates on growth, polysaccharidase and glycosidase production by Bacteroides ovatus, in batch and continuous culture

Bacteroides ovatus was grown in batch culture on 12 different carbon sources (five polysaccharides, seven monosaccharides and disaccharides). Specific growth rates were determined for each substrate together with polysaccharidase and glycosidase activities. Growth rates on polymerized carbohydrates were as fast or faster than on corresponding simple sugars, demonstrating that the rate of polysaccharide depolymerization was not a factor limiting growth. Bacteroides ovatus synthesized a large range of polymer-degrading enzymes. These polysaccharidases and glycosidases were generally repressed during growth on simple sugars, but arabinose was required for optimal production of alpha-arabinofuranosidase. Polysaccharidase and glycosidase activities were measured in continuous cultures grown with either xylan or guar gum under putative carbon limitation. With the exception of beta-xylosidase, activities of the polymer-degrading enzymes were inversely related to growth rate. This correlated with polysaccharide utilization which was greatest at low dilution rates. These results show that Bact. ovatus is highly adapted for growth on polymerized carbohydrate in the human colon and confirm that the utilization of polysaccharides is partly regulated at the level of enzyme synthesis.

Effect of concentration of substrates and products on the growth of Klebsiella pneumoniae in chemostat cultures

Non-equilibrium thermodynamics (NET) can be used to describe microbial growth. In this description, the concentrations of products contribute to the driving forces of the metabolic processes (anabolism and catabolism). Thus, in contrast to the model of bacterial growth of Monod (Recherches sur la Croissance les Cultures Bactériennes (1942) Herman et Cie, Paris), it is predicted that the growth rate of a bacterial chemostat culture is, in principle, dependent on the concentration of the catabolic product (for instance HCO3-) during catabolite limitation and on the concentration of the anabolic product (for instance biomass) during anabolite limitation. In order to test this prediction, Klebsiella pneumoniae was grown in aerobic citrate-limited, glucose-limited or ammonia-limited chemostat cultures. Ammonia-limited cultures were considered to be essentially anabolite-limited, whereas citrate limitation was used as a representative for catabolite limitation. In ammonia-limited or in glucose-limited cultures it was found that the growth rate was independent of the biomass concentration present. In the NET description this means that the 'back' reaction (i.e., in the direction from biomass to substrates) is saturated with respect to biomass. On the other hand, in citrate-limited cultures, the steady-state concentration of citrate increased with the concentration of the catabolic product HCO3-. At relatively low concentrations of HCO3-, 'thermodynamic back-pressure' of growth (i.e., increase in product concentration was compensated by an increase in substrate concentration so that the driving force for growth remained almost constant) was demonstrated as predicted by the NET model. At concentrations above 40 mM, a kinetic (allosteric) effect of HCO3- was detected. This was concluded from a reduced growth yield on citrate, and from a significant decrease in the maximal growth rate and the maximal oxygen consumption rate after relief of the citrate limitation.