Fundamentals of Biochemical Engineering Part 1 - Bailey J., Ollis D. 1989

Introduction to Microbiology
Major Cell Types
Molds

Molds are higher Fungi characterized by a vegetative Structure known as the mycelium. As shown in Fig. 1.8, the mycelium is a highly branched network of microscopic tubes containing a mobile, multinucleated Cytoplasm. It may consist of several types of related Cells. The long, thin filaments of mycelial cells are called hyphae. In some cases, the mycelium can form a very dense mass. Given that molds require an adequate oxygen supply for normal metabolic activity, this density can pose significant challenges in their cultivation, as the mycelium can create substantial resistance to mass transfer. This problem, along with the unusual hydrodynamic properties of mycelial Suspensions, will be discussed in more detail in Chapters 4 and 8.

Like Yeasts, molds lack chlorophyll and are generally non-motile. Typically, molds reproduce via spores through either sexual or asexual pathways. Spore characteristics play a major role in the Taxonomy of fungi.

From an industrial perspective, the most important molds are Aspergillus and Penicillium (Fig. 1.9). The primary metabolic products of these microorganisms include Antibiotics (substances that kill certain microorganisms or inhibit their growth), organic acids, and biological catalysts.

Under normal conditions, one strain of Aspergillus niger produces oxalic acid (HO₂CCO₂H); however, when the nutrient medium is depleted of phosphates and certain Metal Ions such as copper, iron, and magnesium, citric acid [HOOCCH₂C(OH)(COOH)CH₂COOH] becomes the predominant product. This characteristic forms The basis of the industrial biochemical process for citric acid production. Thus, A. niger serves as an interesting example of how approaches to the design and optimization of biochemical and non-biological processes can differ. In biological systems, a relatively minor adjustment in medium composition can sometimes yield a dramatically higher selectivity.

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FIG. 1.8. Structure of mold mycelium. Conditions in the core of a dense mycelium may differ significantly from those in its peripheral regions.

This example, much like the penicillin case study below, demonstrates how vital it is for a biochemical engineer to understand Cell Structure, METABOLISM, and function—a point that will be reinforced repeatedly throughout the text. Without accounting for fundamental cell properties and intracellular processes, all the expertise a process engineer applies to the development, design, and analysis of industrial biochemical systems can prove entirely useless, as the key Biological features of the system under study would be overlooked.

A second fundamental distinction between microbiological and non-biological processes can be illustrated by penicillin production. Major breakthroughs in penicillin manufacturing were achieved by isolating high-yielding mutants of the parent Penicillium strain through ultraviolet irradiation of its spores (Fig. 1.10). Mutations can be induced by various agents and frequently increase the yield of a desired metabolite by several orders of magnitude. Recombinant DNA technology, discussed in Chapter 6, offers an alternative, targeted, and strictly controlled method for generating genetically modified variants of certain organisms. This approach has made it possible, for instance, to produce specific animal Proteins using simple Bacteria. These Examples highlight the crucial role that genetics plays in biochemical engineering. Today, genetics commands major attention in academic curricula, research institutions, and industrial biotechnology centers alike. The practical importance of developing genetically engineered organisms (or, conversely, preserving an Organism's original genetic makeup) underscores the necessity of close collaboration between engineers, biologists, and biochemists when planning and critically evaluating industrial biochemical processes. The history of penicillin production exemplifies The Development of novel techniques, including deep-culture Fermentation, solvent extraction of large quantities of labile compounds, high-throughput sterilization of massive volumes of air, and the isolation of mutant microorganisms yielding high titers of penicillin.

FIG. 1.9. Hyphae of Aspergillus and Penicillium, two industrially important molds.

FIG. 1.10. Increase in penicillin yield over a 30-year research period. The development of specialized mutant mold strains led to an exponential rise in yield over the past 25 years. A similar trend characterizes The production of the antibiotic streptomycin. [Reprinted with permission from: Demain, A. L., Overproduction of Microbial Metabolites due to Alteration of Regulation, in Advances in Biochemical Engineering, Vol. 1, Ghose, T. K., and Fiechter, A., Eds., p. 129, Springer-Verlag, New York, 1971.]

Before concluding this section on bacteria and fungi, we should briefly mention actinomycetes, a group of microorganisms that share characteristics of both fungi and bacteria. These microbes are widely used in the industrial production of vital antibiotics. Although formally classified as bacteria, actinomycetes resemble fungi in their ability to form long, highly branched hyphae. The manufacturing processes for antibiotics utilizing actinomycetes and molds also share many similarities. What allies actinomycetes with bacteria is their susceptibility to infection by the same Bacteriophages and viral diseases. Viruses will be briefly discussed later in Chapter 6.