Study on High-Cell-Density Continuous Culture of Microorganism

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In order to achieve high-cell-density, fed-batch culture has conventionally been applied as the only existing technique. This technique has, however, disadvantages in terms of productivity because it takes longer time, the culture volume increases, down time exists, etc. On the other hand, in continuous culture, the productivity increases strictly in proportion to the increase in the cell-density, because the productivity of a continuous culture= (dilution rate) X (cell-density) X (content of the metabolite).In this project, both basic methodology and experimental techniques have been investigated in order to achieve high-cell-density in continuous culture, using a microbial strain that produces biodegradable plastics (poly-3-hydroxybutyrate, PHB). The strain used, Alcaligenes latus, is known as the one that accumulates PHB intracellularly in growth-associated manner so that this strain is suitable as a test strain for one-stage high-cell-density continuous culture. After a number of trials of continuous cultures under various conditions, a continuous culture keeps steady state for many days by a method that all the nutrients except nitrogen source is supplied by chemostat-mode whereas 28% ammonia water is supplied as the nitrogen source by pH auxostat-mode. By supplying the sucrose(150g/l)and minerals at the constant feed rate and supplying 28% ammonia water by pH auxostat-mode, the exit cell-density was ca.70g dry cell/l and dilution rate was 0.18/h. The PHB productivity was 6.0 (g dry cell) / (l・h) , which was twice as the conventional high-cell-density fed-batch culture.