High Cell Density Growth of Micro-organisms

What is high cell density growth?The scope of this chapter It is difficult to define exactly what is meant by high cell density growth because it is a relative term.Thus when working with protozoa a cell density of 2.2 x 107 and 54 g dry weight 1-' has been considered as growth at high cell density, whereas with bacteria and fungi this would be only a moderate growth density (Chang and Furusaki, 1991; Kiy and Tiedtke, 1992).The highest biomass levels ever recorded in a bioreactor were those described by Chung, Chang and Kim (1987).In these experiments a Nocardia strain was grown in a dual hollow-fibre bioreactor, reaching densities of 550 g dry weight 1-'.No record of the cell number was given, but 1012 Escherichia coli cells ml-' of accessible reactor space has been reported, again for a hollow-fibre based system (Inloes etal., 1983).The common factor in most of these reports is the comparison of a system with batch culture in a shake flask, or a stirred tank reactor.Therefore I will use this approach when discussing the alternatives available for increasing the culture cell density of micro-organisms.As far as possible, all types of micro-organisms will be covered, although inevitably the information available relates mainly to a relatively narrow range of bacteria and fungi.It is rare that figures are given for both the dry weight and cell numbers present in a particular reactor, yet the two are not always proportional.Therefore the culture conditions are sometimes given in more detail than may seem necessary to allow readers to get as good an impression as possible about the physiological state of the microbes in the systems described.The demand for high cell density growth is mainly to improve the volumetric productivity of a process, although there is growing evidence that it can also be a strategy for stabilizing recombinant organisms, slowing down the loss of plasmid-encoded genes (Huang, Perelti and Bryers, 1993).Interest in high cell density culture is also growing because of a need to obtain increased biomass levels of microbes that are reputed to be difficult to cultivate, for biochemical Abbreviations: ATP, adenosine triphosphate; GRAS, generally regarded as safe.