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

Introduction to Microbiology
Major Cell Types
Bacteria

As we mentioned during our preliminary study of prokaryotes, bacteria are relatively small organisms, typically enclosed within a rigid Cell wall. In many bacterial species, the outer surface of The Cell wall is covered by a resilient, viscous layer known as a capsule or slime layer. Bacteria are unicellular organisms; morphologically, they can be divided into three main groups (Fig. 1.6). Most bacteria are incapable of absorbing light energy, are capable of autonomous movement, and reproduce by dividing into two daughter Cells, although numerous exceptions to all these rules are known.

There is A large number of bacterial subgroups; some Major Types of bacteria and their characteristic features are listed in Table 1.2. The term "Gram reaction" refers to the response of bacteria to a relatively straightforward and rapid staining test. In this test, cells are first stained with the dye crystal violet, then treated with an iodine solution and rinsed with alcohol. Cells that retain the blue color of the dye after this Treatment are termed Gram-positive, whereas the loss of color indicates that the bacteria belong to the Gram-negative type. Many Characteristics of Bacteria correlate well with this color reaction, which reflects fundamental differences in their cell wall Structure.

In the industrial application of microorganisms, the question of whether oxygen supply to the nutrient medium is mandatory is of particular importance (Ch. 8, 12, and 14). In aerobic processes, oxygen is supplied to nourish the microorganisms, typically in the form of air. Such processes include the industrially important microbiological Methods for The production of vinegar, certain Antibiotics, and animal feed supplements. One of the main challenges in developing such processes is related to the limited solubility of oxygen in aqueous media typical of these systems (Ch. 8). In anaerobic processes, such as the production of certain alcohols or the treatment of organic wastes, microorganisms function in the absence of oxygen.

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FIG. 1.6. Three bacterial shapes.

Equally important in industrial Applications and in the control of bacterial contamination is the ability of bacteria to form so-called endospores under unfavorable conditions. The latter represent a "dormant" cell state that allows them to withstand the effects of elevated temperatures, radiation, and toxic chemicals without harm. When spores reach an environment suitable for their metabolic activity, they transform into normally functioning cells. In contrast to the spore form, this normal, biologically active state of cells is often referred to as the vegetative form. As the data in Table 1.2 indicate, There are two main groups of spore-forming bacteria. Aerobic bacteria of the genus Bacillus are extremely widespread in nature and easily adapt to any conditions. For the vegetative forms of certain Clostridium species, which develop normally under anaerobic conditions, oxygen is lethal, yet the spores of these bacteria are resistant to oxygen. Other bacteria, whose vegetative forms rapidly perish at 45°C, form spores that can withstand boiling in Water for several hours. It follows that if we wish to kill microorganisms by heating (thermal sterilization), higher temperatures are required to destroy spore-forming bacteria—typically boiling under pressure in an autoclave at temperatures above 120°C.

Table 1.2. Some major types of bacteria and their distinctive features

We will not discuss blue-green Algae (cyanobacteria) here, as they are of little industrial significance. It should be noted, however, that cyanobacteria, which actively participate in The Nitrogen Cycle, are important in the overall nutrient cycling of aquatic ecosystems (Ch. 14).