Scientists and engineers have realized the industrial and environmental significance of biofilm accumulation and activity. The ability to predict and control biofilm formation has led to less fouling and corrosion in industrial systems and a better understanding of biofilm importance in natural aquatic systems. Understanding the fundamental processes contributing to biofilm formation is beneficial to anyone involved with natural or industrial systems where biofilms may play a significant role in determining variables such as bulk water quality, toxic compound biodegradation, or product quality.
We have reviewed the general forces through which cells interact with substrata in their first nonspecific contact. The complex, fast-emerging biology of specific cell adhesion and the structure of the extracellular matrix were reviewed in substantial detail, and the most updated conceptual model of biological cell adhesion was assembled from past efforts and new literature data. The chemistries of the various possible substrata for cell adhesion have been reviewed extensively in the past, and here only a brief summary was presented, with particular emphasis on the materials for traditional and porous microcarriers. The fascinating molecular and cellular implications of cell adhesion were reviewed in detail to establish that cell adhesion and the extracellular matrix provide more than structural support for the cells and their assemblies, and that in fact they constitute fundamental regulators of cell function, metabolism, and differentiation. We reviewed the fluid-mechanical mechanisms of cell damage in microcarrier systems and provided experimental evidence for the importance of the cell-adhesion quality in the ability of cells to withstand fluid forces in bioreactors. We provided evidence that the interplay of cell adhesion and fluid forces is likely to produce cell responses more complex than that of simple life and death, and we suggested that such responses are awaiting investigation and exploration for new applications and culturing possibilities. We also reviewed the experimental evidence on the importance of cell adhesion in cell and microcarrier aggregation and discussed the implications of such aggregation on the culturing environment and the operation of bioreactors. Finally, we discussed the possible implications of cell adhesion as it relates to the developing field of tissue engineering, using the example of bone marrow culture, which involves a large variety of cells and constitutes one of the most complex cell culture systems.
The fundamental knowledge available about the microorganism, substrates, and process represent the basis on which a rational approach for the design and formulation of media for microbial processes can be attempted. In this respect, it is necessary to analyze critically the main objective to be optimized (yield, productivity, quality of final product, etc.). The first step of medium development is concerned with the decision to be taken about the adequate components to be used, followed by the calculation of their concentrations. After medium preparation and experimentation, the application of the most appropriate statistical optimization methodology will make it possible to attain the optimal medium. Medium design and formulation for plant cell processes are still carried out on an empirical basis owing to the lack of fundamental knowledge. However, some general guidelines can be given for growth and production media considering the experimental evidence available about the functions and influence of the medium components for promoting growth and product formation. Adequate manipulation of the carbon, nitrogen, and phosphorus sources and phytohormones and the inclusion of precursors and elicitors in the medium represent the best strategy for improving medium development for plant cell processes. The development of cost-effective medium supplies for mammalian cell culture production of proteins is a key element for a commercially successful process. Medium development is a task that requires experience, time, and resources in its solutions, which may be several. An optimal formulation is elusive because there will always be room for improvement, but through systematic, comprehensive work, practical combinations of nutrients, factors, and feeding schedules can be developed in a reasonable time. A properly developed formulation can bring the burden of the medium below 10% of the total cost of a mammalian cell process.
The range of possible yield of a reactor in which a process is due to take place is limited by basic principles derived from the conservation of mass (stoichiometry) and energy (thermodynamics). Those limits remain valid as well in the case of the biological and biochemical processes, which are the subject of this book. The design of the reactor for a given output within the range mentioned must be based on knowledge of the rate at which the process takes place (kinetics).
Sterilization is the process of rendering materials to a condition which is totally devoid of living microorganisms. It is an absolute condition and, in practice, it is an ideal which is approached in terms of achieving an acceptable probability of the absence of viable organisms. In the context of bioreactor design, sterilization has three applications:In reality, this means that having ensured that all viable organisms are absent it is necessary to prevent entry of potentially infecting microorganisms during the entire period of plant operation.
To initiate a culture of animal cells, one removes the cells from the animal and often then immobilizes the cells on an appropriate surface [1,2]. If the subsequent culture environment is suitable, the cells will grow to cover the surface and can be used to inoculate a new, larger surface. The initial growth surface is generally the interior bottom wall of a simple vessel, such as a T-flask or petri plate [1,2]. For large-scale operations, which require billions of cells or more,extensive surface area is often provided through highly engineered reactors. The design of such reactors is the topic of this chapter.
The aggregation of cells into clumps or flocs has been exploited for decades in such applications as biological wastewater treatment, beer brewing, antibiotic fermentation, and enhanced sedimentation to aid in cell recovery or retention. More recent research has included the use of cell aggregation and sedimentation to selectively separate subpopulations of cells. Potential biotechnological applications include overcoming contamination, maintaining plasmid-bearing cells in continuous fermentors, and selectively removing nonviable hybridoma cells from perfusion cultures.
This chapter overviews matrices and activation processes employed in cell studies. Three factors are involved in the choice of an appropriate matrix for any solid-phase biochemical procedure, whether it be affinity chromatography, enzyme immobilization, or the immobilization of whole cells. Those three factors are the mechanical, chemical, and biological properties of the matrix. In cell adhesion and immobilization a somewhat broader group of matrices has been studied with reasonable diligence. Matrices for all aspects of solid-phase biochemistry are most easily considered when they are divided into various categories with similar properties. Naturally occurring matrices can further be divided into two subclasses, those of organic origin, often consisting of various polysaccharides, and those that are inorganic, consisting of various minerals that have been processed to yield the proper shape and size. No single activation method, just as no single matrix, will give a complete solution to all matrix problems.
