Journal of Fermentation and Bioengineering最新文献

Dark fermentation in the marine green alga, Chlorococcum littorale, was investigated with emphasis on ethanol production. Under dark anaerobic conditions, 27% of cellular starch was consumed within 24 h at 25°C, the cellular starch decomposition being accelerated at higher temperatures. Ethanol, acetate, hydrogen and carbon dioxide were obtained as fermentation products. The maximum productivity of ethanol was 450 μmol/g-dry wt. at 30°C. The fermentation pathway for cellular starch was proposed from the yields of the end-products and the determined enzyme activities. Ethanol was formed from pyruvate by pyruvate decarboxylase and alcohol dehydrogenase. the change in fermentation pattern that varied with cell concentration in the reaction vials suggested that the hydrogen partial pressure affected the consumption mode of reducing equivalents under dark fermentation. Ethanol productivity was improved by adding methyl viologen, while hydrogen production decreased.

Xylitol, a five carbon sugar alcohol, occurs widely in nature but it is also a normal intermediate in human metabolism. As an alternative sweetener, it is recommended for diabetics and for the prevention of dental caries. Xylitol is currently produced chemically on a large scale. Microbial production is lately becoming more attractive since the downstream processing is expected to be cheaper. Among microorganisms, yeasts are the best xylitol producers, particularly those belonging to the genus Candida. The key enzymes for xylitol production in yeasts are d-xylose reductase which, using either NADH or NADPH, reduces d-xylose to xylitol, and predominantly, NAD-linked xylitol dehydrogenase which reoxidizes xylitol to d-xylulose. Xylitol accumulation in yeasts is sensitive to environmental conditions such as nutrition, temperature, pH, inoculum, substrate and aeration, with the last two being critical for yeast growth and fermentation. Hemicellulosic hydrolysates derived from hardwood and particularly from agricultural residues, such as sugar cane bagasse, corn cobs, wheat and rice straw, are used as feedstock for xylitol production. Due to the presence of inhibitory components, some of the hydrolysates have to be treated prior to microbial utilization. The most investigated types of processes have been batch ones, although fed-batch and immobilized systems have been characterized by the highest yields and productivities. Apart from the naturally occurring yeasts, recombinant strains of Saccharomyces cerevisiae in free and immobilized form were also investigated for xylitol production.

The chiA gene encoding chitinase A was cloned into Escherichia coli from Xanthomonas sp. strain AK and its nucleotide sequence was determined. The structural gene consists of 1788 bp encoding 596 amino acids with a predicted molecular weight of 62,122. The deduced ChiA is a modular enzyme composed of an N-terminal signal peptide and four domains in the following order: a chitin-binding domain, two fibronectin type III domains, and a family 18 catalytic domain. ChiA purified from the recombinant E. coli had temperature and pH optima at 35°C and 4.5, respectively. The K_m and V_max values for colloidal chitin were estimated to be 1.8 mg/ml and 8.7 μmol/min/mg, respectively.

Previously, we had developed a simple gene transfection technique for animal cells using cationic lipid vesicles; a commercially available synthetic cationic surfactant, dimethyldioctadecyl ammonium bromide (DDAB) was used for making lipid vesicles. In the present study, the lipid vesicles for receptor mediated gene transfer were modified with a ligand such as insulin and galactose residues to realize enhanced transfection efficiency and/or cell-specific gene transfection. The insulin-modified lipid vesicle solution mixed with the plasmid DNA (pCMVβ) was added to COS-7, NIH3T3, Hela or HepG2 cells; the transfection efficiency was increased 3–4-fold in all the cell lines tested. Furthermore, a mixture of the galactose-modified lipid vesicles and plasmid pCMVβ was added to HepG2 or HuH-6 cells expressing asialoglycoprotein receptors, and the transfection efficiency was increased 3–4-fold in these cell lines.

The effects of electric current on the fermentation characteristics of yeast were investigated. When 10 mA direct current (DC) or 100 mA alternating current (AC) was applied to the culture broth, significant increases in cell growth and alcohol production rates occurred. The contents of higher alcohols, esters and organic acids in the culture broths to which AC and DC were applied, were different from those in the control culture (no current application). Several compounds such as acetaldehyde and acetic acid, were formed from ethanol as a result of electrode reaction.

The chitinolytic activity of Penicillium janthinellum P9 was studied in shaken cultures and in a 3-l bench-top bioreactor by varying culture conditions such as initial medium pH, growth temperature, stirrer speed and aeration site. In shaken flasks, the highest levels of enzyme activity (468 and 483 U·l⁻¹) were obtained at a growth temperature of 24°C and at an initial medium pH of 4.0, respectively. In the bioreactor, both agitation and aeration significantly influenced the enzyme production: the highest level of enzyme activity (497 U·l⁻¹) was obtained at an impeller speed of 500 rpm and an aeration rate of 1.5 vvm. Culturing P. janthinellum P9 under optimised conditions led to an increase in the enzyme activity of ca. 65% (686 U·l⁻¹ as compared to the 415 U·l⁻¹ obtained under the initial culture conditions).

The effect of leucine addition on the production yield of lysine by Corynebacterium glutamicum AJ-3462 was studied and a method of increasing a fermentation time while maintaining high lysine producing activity was developed. When leucine was not added during the lysine production phase, a high molar production yield of lysine from glucose of 35.6% was obtained. However, lysine production did not continue for a long time and almost stopped at around 90 h. When leucine was added to the medium in the fermentor at 24 h intervals, lysine producing activity was maintained for a long time; however, the production yield of lysine from glucose was reduced to 23.5%. When leucine was added at 30 h intervals, the production yield of lysine from glucose was improved to 32.9%. However, leucine addition did not recover lysine producing activity after 162 h. From these results, in order to extend the fermentation time while maintaining lysine productivity and production yield at high values, a leucine addition strategy was developed as follows. The initial leucine concentration was set as 0.1 g/l. After depletion of leucine, it was added intermittently at 30 h intervals. The interval for intermittent addition was changed to 24 h when a decrease in cellular activity was recognized by on-line monitoring of the lysine production rate based on a metabolic reaction model. As a result, the fermentation time was extended to 174 h, and lysine productivity and total molar production yield of lysine from glucose became 0.104 g/l/h and 29.2%, respectively.

Fed-batch cultures were performed to improve the productivity of erythritol. When only glucose was added to the culture broth, the specific erythritol production rate was markedly decreased from 0.042 g/g/h to 0.023 g/g/h. On addition of both glucose and corn steep liquor, the rate was kept almost constant at 0.042 g/g/h throughout the fed-batch fermentation period. The repeated fed-batch culture method showed a maximum erythritol productivity of 1.86 g/l/h and a 45% of total erythritol conversion yield, corresponding to a 23% and 15% increase compared with batch fermentation.

We investigated the influence of a magnetic field on photosynthesis in, and the growth of Spirulina platensis (S. platensis), under magnetic fields with magnetic flux densities varying from 0.5 gauss (geomagnetic field) to 700 gauss. The specific growth rate of S. platensis was the highest at 100 gauss, being 1.5-fold that at 0.5 gauss, while the growth was obviously inhibited at 700 gauss. The existence of a magnetic field had no effect on the growth when S. platensis was cultured in the dark on a medium containing 0.3% glucose. The oxygen evolution rate during photosynthesis increased with increase in magnetic flux density, and the intracellular concentrations of sugar and phycocyanin, a light-harvesting pigment present in the thylakoid membrane in which reactions of the photosynthetic electron transfer system occur, reached maximal values at 100 gauss. At magnetic flux densities of a over 100 gauss, the phycocyanin content decreased with increase in magnetic flux density. The content of glyceroglycolipid, which exists exclusively in the thylakoid membrane, decreased with increase in magnetic flux density, especially so at 700 gauss, when it became 44% that at 0.5 gauss. From the aforementioned results, it is evident that magnetic fields accelerate the growth of S. platensis associated with activation of light excitation in the photosynthetic electron transfer system and increase in phycocyanin contents, and that these effects are maximal at magnetic flux densities of around 100 gauss. Nevertheless, when a magnetic flux density of over 400 gauss was applied, growth inhibition was observed with decrease in the phycocyanin content, and production of chemical energy necessary for sugar synthesis.