Nitrogen fixation associated with grasses and cereals: Recent progress and perspectives for the future

Over the last 20 years many new species of N₂-fixing bacteria have been discovered in association with grasses, cereals and other non-nodulating crops. Virtually all of these bacteria are microaerophylic, fixing N₂ only in the presence of low partial pressures of oxygen. Until a few years ago much attention was focussed on members the genusAzospirillum and it was assumed that N₂ fixation was restricted to the rhizosphere or rhizoplane of the host plants. Through the use of N balance and¹⁵N techniques it has been shown that in the case of lowland rice, several tropical pasture grasses and especially sugar cane, the contributions of biological N₂ fixation (BNF) are of agronomic significance.More detailed study of the N₂-fixing bacteria associated with sugar cane (Acetobacter diazotrophicus andHerbaspirillum spp.) has shown that they occur in high numbers not only in roots of this crop but also in the stems, leaves and trash but are rarely found in the soil. Some of these endophytic diazotrophs have now also been found in forage grasses, cereals, sweet potato and cassava, although evidence of significant BNF contributions is still lacking.The identification of these endophytic diazotrophs as the organisms probably responsible for the high contributions of N₂ fixation observed in sugar cane suggests that it may be possible to attain significant BNF contributions in some other gramineae and perhaps root crops.     

The Peculiar Structure of Acetobacter pasteurianus CIP103108 LPS Core Oligosaccharide

Acetobacter pasteurianus, a member of the Alphaproteobacteria, is an acetic acid-producing bacterium present on sugar-rich substrates such as such as fruits, flowers and vegetables and traditionally used in the production of fermented food. The preferred living habitat associated with acid conditions makes the structure of the bacterial cell wall interesting to study, due to expected uncommon features. We have used a combination of chemical, analytical and NMR spectroscopy approaches to define the complete structure of the core oligosaccharide from A. pasteurianus CIP103108 LPS. Interestingly, the core oligosaccharide displays a high concentration of negatively charged groups, structural features that might contribute to reinforcing the bacterial membrane.     

Mathematical modelling and its use for design of feeding strategies for L‐Sorbose fermentation

Batch sorbitol to sorbose bioconversion by Acetobacter suboxydans using initial sorbitol concentration (S₀ = 100 g/L) yielded a productivity of 10.11 g/L‐h and 98.6% conversion in 10 h time. The batch kinetics was then used to develop an unstructured mathematical model. Model parameters were identified using a nonlinear regression technique assisted by a computer program which minimized the deviation between the model predictions and actual batch experimental data. F test indicated 99% confidence on the prediction of model using optimized parameters. The batch model was eventually extrapolated to identify nutrient feeding strategies to maintain constant noninhibitory sorbitol supply and eliminate substrate limitation for fed‐batch fermentation in order to improve the sorbose productivity. The adequacy of the fed‐batch model was established by excellent agreement between experimental data and model simulation (except towards the end of fermentation).     

木醋杆菌发酵生产细菌纤维素的研究

利用木醋杆菌为实验菌种合成细菌纤维素,通过正交实验和单因素实验优化发酵培养基配方,确定了最佳制备条件,并利用扫描电镜、红外光谱对合成细菌纤维素的微结构进行了观察分析。     

利用椰子水生物合成CMC改性细菌纤维素

以椰子水为培养基,于培养基中添加羧甲基纤维素(CMC)后培养木醋杆菌可制备羧甲基纤维素改性细菌纤维素(CMC-BC)。当添加6g/LCMC时,CMC-BC产量达到最大(10.41g/L),是纯BC产量(4.73g/L)的2.2倍。采用FTIR表征了产物结构;通过SEM、XRD、TGA研究了产物性能;并测试了产物的特性黏度与含水率。结果显示,利用椰子水所制备的CMC-BC缩短了培养时间(3d)。适量添加CMC〔ρ(CMC)=2～18g/L〕时,CMC-BC的聚合度增大,且具有较好热稳定性及较高含水率。CMC-BC还表现出较好的溶解性能。     

珍珠梅果醋的工艺研究

以云南珍珠梅为原料,沪酿1．41醋酸菌为菌种,经液态深层发酵酿制珍珠梅果醋。分析了温度、接种量、初始酸度、初始酒精度对醋酸发酵的影响,结果表明,珍珠梅果醋酿制的最佳工艺参数为温度32℃,接种量11%(v/v),初始酸度1．8g/100mL(以醋酸计),初始酒精度5．0%(v/v),发酵率达到84．8%。建立了相关方程Y=3．524-3．16X_1~2 1．500X_2~2-0．267X_3 0．567X_4,复相关系数R~2=0．941,醋酸菌在发酵第7d达到对数生长期;酿制出来的珍珠梅果醋经澄清化处理颜色金黄、澄清透亮、醋味浓郁,同时具有珍珠梅的特殊风味。     

大豆乳清细菌纤维素在冰淇淋中的应用

研究使用AcetobacterxylinumC5菌种 ,利用大豆乳清发酵得到的细菌纤维素作为稳定剂 ,应用到冰淇淋的加工当中。试验证明 ,细菌纤维素可以替代黄原胶、卡拉胶等稳定剂添加到冰淇淋中 ,它能够改善口感 ,同时呈现爽口的香甜味。细菌纤维素冰淇淋的抗融性和融化特性都比较理想 ,融化率 2 1 7%,膨胀率 67%。同时产品具有一定的膳食保健功能     

Effects of polyacrylamide‐co‐acrylic acid on cellulose production by Acetobacter xylinum

Polyacrylamide‐co‐acrylic acid (PA) added to shake flask cultures of Acetobacter xylinum at concentrations up to 3 g dm^?3 resulted in increased production of bacterial cellulose. For PA concentrations of 0–3 g dm^?3, 7‐day cellulose production rose monotonically from 2.7 ± 0.8 to 6.5 ± 0.5 g dm^?3 at a shaker speed of 175 rpm, and from 1.7 ± 0.01 to 3.7 ± 0.5 g dm^?3 at shaker speed of 375 rpm. Addition of PA also changed the morphology of the biomass from amorphous/stringy forms to spheroidal particles with diameters ≤2 mm. Similarly, bioreactor cultures grown in the absence of PA formed long fibrous masses which deposited on the internals, while those grown in the presence of 1–2 g dm^?3 PA formed small discrete particles with diameters ≤0.1 mm. Tests performed with 1 and 2 g dm^?3 PA, and stirrer speeds from 500 to 900 rpm, appeared to give the highest cellulose concentration of 5.3 ± 0.7 g dm^?3 in 64–68.5 h in the presence of 2 g dm^?3 PA at 700 rpm, although this value was statistically indistinguishable from that obtained at 1 g dm^?3 PA and 900 rpm. A qualitative model is proposed to describe the mechanisms by which PA affects biomass morphology, resulting in its advantageous formation as small, dispersed, spheroidal pellets. Quantitative analysis of the results gave inverse correlations between both the fraction of fructose carbon going to cellulose synthesis and the specific fructose consumption rate, and the maximum cellulose concentration and the fraction of fructose carbon going to by‐product formation. Since cellulose yield was almost universally improved by higher polyacrylamide concentration, it appears likely that increased viscosity reduces fructose uptake rate by limiting mass transfer. Copyright © 2003 Society of Chemical Industry     

胡柚果醋生产菌种的筛选与鉴定

目的:筛选1株适合胡柚果醋生产的醋酸菌。方法:从自然发酵胡柚果醋中筛选产醋酸菌种,研究其酒精耐受性、酒精消耗速度、产醋酸速度;分别发酵生产胡柚果醋,对其进行感官分析,以Vitek-32型微生物分析系统对最适菌种进行鉴定。结果:筛选出3株性能较好的醋酸菌株,分别命名为HY05、HY19与HY36,其中HY05菌株性能最优,能够耐受14%的酒精浓度,起始酒精浓度为10%时酒精转化最快,产酸速度快,所生产的果醋色泽淡黄,质地澄清透亮,风味芳香怡人;根据形态观察与Vitek-32系统的分析结果,将其鉴定为醋杆菌属的巴氏醋杆菌(Acetobacter pasteurianus),命名为A.pasteurianus HY05。结论:A.pasteurianus HY05适宜胡柚果醋的生产。