Effects of hydrodynamic and interfacial forces on plant cell suspension systems.

Plant cells are perceived to be sensitive to the hydrodynamic environment in conventional bioreactors. Heightened sensitivity, relative to most bacterial cultures, is frequently attributed to larger plant cell sizes, extensive vacuolization and aggregation patterns. Early studies of shear sensitivity focused on cell lysis and/or loss of viability. More recently, an extensive array of sub-lethal responses has been identified. A fuller understanding of these sub-lytic effects may assist in the optimization of large-scale cultivation conditions. This paper reviews recent work on the hydrodynamic shear sensitivity of plant cell systems, under cultivation conditions and in purpose-built shearing devices. The relevance of different approaches to the characterization of the intensity of a given hydrodynamic environment is discussed. Indicators of cell response to hydrodynamic stress are evaluated. The potential significance of cellular defense mechanisms, observed in response to mechanical stimulants, is identified.

Plant cell suspension cultures: some engineering considerations.

Higher plants are the source of a vast array of biochemicals which are used as drugs, pesticides, flavourings and fragrances. For some of these compounds, plant cell culture can provide a potential production alternative to traditional cultivation methods or chemical synthesis routes. Many systems have been patented and the last 20 years have seen considerable industrial and academic interest in the development of large scale cultures to produce pharmaceutically active, high value substances. However, the industrial application of plant cell suspension cultures has, to date, been limited. Commercialisation has essentially been impeded by economic feasibility, arising from both biological and engineering considerations. This paper reviews the commercial development of the technology to date and focuses on the impact of specific engineering-related factors, in particular, the shear sensitivity of plant cell suspension cultures. Evidence of sensitivity to hydrodynamic shear in bioreactors has generally been attributed to the physical characteristics of the suspended cells. Recent studies indicate that shear sensitivity may not be as important, in some cases, as initially anticipated.

Fluid shear effects on suspension cultures of Morinda citrifolia.

The shear susceptibility of cell suspension cultures of the plant cell Morinda citrifolia was investigated by subjecting the cells to the well-defined shear field generated in turbulent flow through a capillary. Suspensions were circulated using a peristaltic pump and average shear stresses between 25 and 350 N m(-2) were generated in the capillary test section. Control experiments were performed to assess the possible contribution of the peristaltic pump to the observed cell damage. There was clear evidence of pump-induced damage at the more severe test conditions and all viability measurements were corrected accordingly. Both shake flask suspension cultures (aged between 9 and 15 days) and repeated batch fermentation cultures, grown in a stirred tank reactor (STR) under a variety of controlled agitation conditions, were tested in the capillary shear loop. The cell damage incurred was evaluated in terms of suspension viability, as determined by a dye exclusion technique. Viability loss was found to conform closely to a first-order model in which the rate constant was observed to increase with the imposed shear stress. Furthermore, a linear relationship was identified between the specific death constant and the cumulative energy dissipated. Post-shear morphological measurements showed that the chain length distribution is shifted toward markedly lower values. In comparison with shake flask cultures, repeated batch fermentation cultures exhibited a marked increase in sensitivity to capillary shear. Based upon the determined morphological characteristics, this result is primarily attributable to the increased chain lengths characteristic of the repeated batch cultures.