Test Bank for Brock Biology of Microorganisms16th Edition by Michael T. Madigan, David A. Stahl.|| All Chapters || 1-34 Chapters , Newest 2024

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CHAPTER
The Microbial World nn nn




1
Summary
Chapter 1 introduces the study of microbiology for students, most of whom will
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have had little or no exposure to the subject. Consequently, you have a good
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opportunity to provide an overview of microbiology that demonstrates the
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important roles microorganisms (or microbes) play in human activities and in the
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ecology of the entire biosphere. The ability of microorganisms to exist
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independently in nature as free-living cells within larger microbial communities
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(Figure 1.1) confers enormous adaptive advantages over cells of macroorganisms,
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which are incapable of an independent existence.
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1.1 | Microorganisms, Tiny Titans of the Earth
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Describe to students ways in which microbiology serves as both a basic and an applied
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biological science. From a basic perspective, the study of microorganisms in laboratory
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culture has been the primary means by which the fundamental genetic and biochemical
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properties of living cells have been revealed. From an applied perspective, microorganisms
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directly affect the quality of human life in both detrimental and beneficial ways. Although
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microorganisms are the causative agents of some of the most important human, animal, and
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plant diseases, they are also used for the industrial production of antibiotics,
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pharmaceuticals, and foods. Microbes are also increasingly being used for beneficial
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purposes as diverse as bioremediation of polluted sites, gene therapies for genetic diseases,
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and the production of biofuels. Microbiology is therefore a science of far-reaching scope,
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with applications that affect the quality of human life in a variety of ways (Figure 1.2).
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You should also emphasize to students the importance of microorganisms in the
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emergence and maintenance of higher forms of life. From the production of molecular
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oxygen (by cyanobacteria) to the biogeochemical cycling of key elements, such as carbon,
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nitrogen, and sulfur, microorganisms play a major role in sustaining all life on the planet.
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Point out in your course introduction that for all the reasons summarized in this section,
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microbiology is the foundation of all biological sciences.
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1.2 | Structure and Activities of Microbial Cells
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Because microorganisms generally exist as free-living cells, it is important to discuss the
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characteristics of cells in general. Emphasize that all cells exhibit a nonrandom organization with a
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semipermeable membrane boundary that encompasses an internal system that is not in equilibrium
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with its environment. Point out that prokaryotic cells (i.e., all Bacteria and Archaea) do not contain
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membrane-bound, internal organelles as traditionally described for eukaryotic cells (the Eukarya;
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Figure 1.4). In particular, the organization of prokaryotic DNA as a single chromosome in an
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Copyright © 2021 Pearson Education, Inc.
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Madigan

arrangement called a nucleoid, an aggregated mass of genetic material within the cytoplasm, is in
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stark contrast to the compartmentalized, multichromosomal configuration typically found in
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eukaryotes. However, despite the structural and morphological similarities of Bacteria and Archaea,
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make sure your students are aware early on that these groups of microorganisms have quite distinct
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evolutionary lineages and are, therefore, not closely related on a genetic level. This concept is
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discussed in more detail in Section 1.5.
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The ability of cells to maintain a thermodynamic energy flow far from equilibrium
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defines what we refer to as a living system. All living systems display some form of
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enzyme-driven metabolism in which both energy-yielding (catabolic) and energy-
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consuming (anabolic) biochemical reactions are catalyzed concurrently. These chemical
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transformations allow for biosynthesis of new cell structures and, ultimately, cell division
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(microbial growth). Figure 1.5 shows the characteristics that define cellular life, some of
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which are universal (e.g., metabolism and evolution) and some of which occur only in
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some cells (e.g., differentiation and motility).
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1.3 | Cell Size and Morphology
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The presentation in the text of the significance of being small is an important concept for students
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to internalize as they begin their study of microbiology. Table 1.1 shows the wide size range
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variability of bacterial cells, which range from a diameter of about 0.2 µm to over 700 µm. Use the
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two examples of unusually large bacteria discussed in this section to illustrate the current upper
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limit of bacterial cell size: (1) the surgeonfish gut symbiont Epulopiscium fishelsoni (>600 µm in
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length; Figure 1.6a), and (2) the sulfur chemolithotroph Thiomargarita namibiensis (750 µm;
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Figure 1.6b). The evolutionary “rationale” for the existence of unusually large-celled bacteria is a
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mystery when one considers that the metabolic rate of a cell varies inversely with the square of its
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size. Ask your students for ideas and/or hypotheses that might explain the selective advantage of
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large cell size in these two prokaryotes.
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The fact that bacteria can live independently as single cells (unlike an individual cell of a
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multicellular organism) suggests that they must possess some capabilities that provide a selective
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advantage over their multicellular counterparts that ensure their survival on the planet. Small cells
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have more surface area to volume (i.e., a higher surface-to-volume ratio), and this alone confers
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many of the evolutionary advantages of being small, including the following:
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● Rapid nutrient and waste transport into and out of the cell allows for faster metabolic rates and
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growth rates. nn nn




● Rapid growth rates result in the rapid production of large populations of cells. These
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populations, in turn, can greatly affect the physiochemical conditions of an ecosystem within a
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short time period.
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● Transport rates are a function of the surface area of the cytoplasmic membrane relative to cell nn nn nn nn nn nn nn nn nn nn nn nn nn nn nn




volume. Use Figure 1.7 to mathematically demonstrate to students that the surface area of a
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sphere is a function of the square of the radius, whereas the volume of a sphere is a function of the cube
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of the radius. This means that the surface-to-volume ratio of a spherical cell can be expressed as
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3/r, where r equals the radius of the cell. Therefore, a coccus cell having a smaller radius has more
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surface area per volume and, thus, more efficient transport capabilities, than a coccus cell having a
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larger radius. nn nn




● Rates of evolutionary change are higher in smaller, faster growing haploid cells than in larger,
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2 INSTRUCTOR'S MANUAL FOR BROCK BIOLOGY OF MICROORGANISMS, 16e
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slower growing diploid cells. This allows for greater adaptive potential through rapid selection for
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advantageous mutations and counterselection against deleterious mutations.
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The theoretical lower limit of size for a living cell is likely near 0.2 μm in diameter. This limit is
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dictated by the amount of volume required to contain cellular components that are crucial for
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maintaining life, such as (1) the presence of essential genes on the chromosome; (2) having a
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sufficient number of ribosomes; and (3) containing a minimal number of metabolic,
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structural, and transport proteins within the cell. Challenge students to list these and other
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molecular components that a cell would have to contain to maintain life. Remind students
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that some cells are parasitic in nature. Inform them that, much like viruses, such
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microorganisms often have streamlined genomes that lack important genes and may make
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them dependent upon their hosts for growth. Can such organisms truly be considered living?
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This might make a good outside project for group debate, requiring students to view the cell
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as a three-dimensional physical structure constrained in space and to research a problem
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that is currently being debated.
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Using Figure 1.8, point out the three major morphologies of prokaryotic cells (coccus, rod, and
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spirillum). Inform your students that, in some species, the cells remain attached following cell
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division, giving rise to different arrangements that are often genus-specific. For example, coccus
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cells may exist as long chains (Streptococcus) or grapelike clusters (Staphylococcus). Less
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common cell morphologies also exist, such as spirochetes, appendaged (budding) bacteria, and
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filamentous bacteria (Figure 1.8). Stress to students that these morphologies are only representative
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of those found in nature. Other unusual shapes have also been described in rare cases (e.g., square-
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and star-shaped cells!).
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Before the molecular era, morphological and physiological properties were used to classify
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bacterial species. However, we now know that these criteria are poor predictors of evolutionary
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relationships. For example, certain species of Archaea may appear identical in size and shape to
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species of Bacteria under the microscope, but these organisms are of different phylogenetic
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domains and thus are not closely related to one another on an evolutionary basis. The cell
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morphology of a particular species is primarily a result of selective pressures in a given habitat that
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favored a particular cell shape for enhanced reproductive success.
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1.4 | An Introduction to Microbial Life
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This section of the chapter provides an overview of the key factors that distinguish cells of the
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different domains of life, and how these are further distinguished from acellular microbes—the
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viruses. Figure 1.9 beautifully compares the size relationships of a variety of microbial forms and
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components, from the uncommonly large bacterium Epulopiscium fishelsoni at a length of 600 μm
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(see also Figure 1.6) to individual proteins at the limit of resolution of an electron microscope
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(0.2 nm). Mention the following key points:
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● The greatest diversity of Bacteria and Archaea has never been cultured in the laboratory, and
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such species are known only by their DNA sequences found in environmental samples.
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● Although they both have a prokaryotic cell structure, Bacteria and Archaea are not closely nn nn nn nn nn nn nn nn nn nn nn nn nn




related. On a molecular level, Archaea have much in common with the eukaryotes.
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● The metabolic and physiological diversity of microorganisms, especially the Bacteria and
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Archaea, is astonishing, with species capable of aerobic respiration, anaerobic respiration, nn nn nn nn nn nn nn nn nn nn




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