Arnaud J P, Lacroix C, Foussereau C, Choplin L
Centre de recherche STELA, Université Laval, Ste Foy, Québec, Canada.
J Biotechnol. 1993 May;29(1-2):157-75. doi: 10.1016/0168-1656(93)90048-r.
Cells are frequently submitted to shear stresses during industrial processes. Shear stress can be either beneficial or detrimental to the cells depending on the organism and on the level of intensity. The present work was designed to study the effect of shear stress on cell activity of a widely used lactic acid bacterium, Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). A constant shear stress bioreactor, based on Couette device, was developed and used to control shear stress from 0 to 72 Pa during a 4-h cultivation of a supplemented whey permeate medium with L. bulgaricus at 42 degrees C. In order to reach high shear stresses and to perform experiments within the laminar flow range where hydrodynamic conditions are accurately defined, the medium was thickened with carboxymethylcellulose (CMC). pH, cell counts, optical density, lactose and lactic acid concentrations were monitored during culture, and cell activity was evaluated after culture and after a freezing treatment at -80 degrees C, using a standardized activity test based on optical density measurement. Cell metabolism was significantly improved by intermediate shear stress levels (36 and 54 Pa) during culture. Furthermore, biomass concentration, evaluated by optical density, was clearly higher at 36 Pa. Cell lengthening was observed, which was mainly related to the presence of CMC and partly to shear stress level, especially at 36 Pa. Hydrodynamic conditions during culture could affect the membrane permeability of the cell and its resistance to freezing. Cells cultivated at 72 Pa were certainly weakened by shearing forces, and these cultures exhibited lag times twice as long after freezing as those grown at 36 Pa. Furthermore, after freezing, cultures grown at 36 Pa had shorter lag times than at 0 Pa (1.1 and 1.3 h, respectively) and higher specific growth rates (1.24 and 0.99 h-1, respectively).
在工业生产过程中，细胞经常会受到剪切应力的作用。根据生物体和应力强度水平的不同，剪切应力对细胞可能有益也可能有害。本研究旨在探究剪切应力对广泛使用的乳酸菌德氏乳杆菌保加利亚亚种（L. bulgaricus）细胞活性的影响。基于库埃特装置开发了一种恒剪切应力生物反应器，并用于在42℃下对添加了L. bulgaricus的乳清渗透液培养基进行4小时培养过程中，将剪切应力控制在0至72 Pa之间。为了达到高剪切应力并在准确界定流体动力学条件的层流范围内进行实验，使用羧甲基纤维素（CMC）对培养基进行增稠。在培养过程中监测pH值、细胞计数、光密度、乳糖和乳酸浓度，并在培养后以及在-80℃冷冻处理后，使用基于光密度测量的标准化活性测试评估细胞活性。培养过程中，中等剪切应力水平（36和54 Pa）可显著改善细胞代谢。此外，通过光密度评估的生物量浓度在36 Pa时明显更高。观察到细胞伸长，这主要与CMC的存在有关，部分与剪切应力水平有关，尤其是在36 Pa时。培养过程中的流体动力学条件可能会影响细胞膜的通透性及其抗冻性。在72 Pa下培养的细胞肯定受到了剪切力的削弱，这些培养物在冷冻后的延迟期是在36 Pa下生长的培养物的两倍。此外，冷冻后，在36 Pa下生长的培养物的延迟期比在0 Pa下生长的培养物短（分别为1.1和1.3小时），比生长速率更高（分别为1.24和0.99 h-1）。
