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Microbial Cell Structure
and Function
Summary
Chapter 2 is an excellent introductory overview of microscopic techniques and the structure and function of both prokaryotic and eukaryotic cells. For courses designed for nonscience majors, this chapter provides general details on each topic that, if supplemented with material from related chapters later in the text, may be sufficient background for most students. However, it is recommended that Chapter 2 be used to set the stage for more detailed coverage later in the course.
2.1–2.4 | Microscopy
The variety of microscopic methods available for observing microorganisms must be introduced early, as much of the presentation of structure–function relationships depends upon the excellent micrographs that appear throughout the book. Although details of microscopy are more easily introduced in the laboratory portion of the course, the material included here is pertinent to effective lecture presentation.
· Discuss the basic principles and components of the compound light microscope,
including the relationships between resolution and magnification, and numerical aperture (Figure 2.1). Note that although bright-field microscopy is fine for visualizing pigmented cells (Figure 2.2), it is not an efficient tool for viewing unstained cells with no natural pigmentation, such as nonphototrophic bacteria.
· This deficiency will lead to a discussion of various methods employed to increase contrast. Discuss the various simple dyes used to stain cells, most of which are positively charged, basic dyes capable of binding to negatively charged cell surfaces (e.g., methylene blue and crystal violet; Figure 2.3). Continue the discussion of differential stains, the most widely used of which is the Gram stain (Figure 2.4).
· Students should understand that while staining procedures increase the contrast of cells against the background to make them more visible, they also kill cells and often distort their appearance. Discuss phase-contrast microscopy and dark-field microscopy (Figure 2.5), two tools that allow one to look at living cells without the need for staining.
· Fluorescence microscopy is widely used in clinical diagnostic microbiology and environmental microbiology (Figure 2.6). Most students who enter the biotechnology industry or medical profession will work with fluorescent molecules (such as those used for fluorescence antibody staining methods). The variety and sensitivity of these molecules has
increased dramatically over the past decade. This has allowed the development of a wide variety of nonradioactive alternatives to biological assays that are now routinely used
in research.
· Students should be interested in the micrographs from three-dimensional imaging of cells. Depending upon the level of the course, you may choose to discuss the principles of differential interference contrast microscopy (Figure 2.7) and confocal scanning laser microscopy (Figure 2.8). Lastly, show and discuss the micrographs obtained from electron microscopy (Figures 2.9 and 2.10). Note the differences between scanning electron
microscopy (SEM), which provides an image of the external features of a specimen, and transmission electron microscopy (TEM), in which thin sections of the specimen show its detailed internal structure.
2.5 | Cell Morphology
Using Figure 2.11, point out the three major morphologies of prokaryotic cells (coccus, rod, and spirillum). Inform your students that, in some species, the cells remain attached following cell division, giving rise to different arrangements that are often genus-specific. For example, coccus cells may exist as short chains (Streptococcus) or grapelike clusters (Staphylococcus). Less common cell morphologies also exist, such as spirochetes, appendaged (budding) bacteria, and filamentous bacteria (Figure 2.11). Stress to students that these morphologies are only representative of those found in nature. Other unusual shapes have also been described in rare cases (for example, square and star-shaped cells!).
Before the molecular era, morphological and physiological properties were used to classify bacterial species. However, we now know that these criteria are poor predictors of evolutionary relationships. For example, certain species of Archaea may appear identical in size and shape to species of Bacteria under the microscope, but these organisms are of different phylogenetic domains and thus are not closely related to one another on an evolutionary basis. The cell morphology of a particular species is primarily a result of selective pressures in a given habitat that favored a particular cell shape for enhanced reproductive success.
2.6 | Cell Size and the Significance of Being Small
The presentation in the text on the significance of being small is an important concept for students to internalize as they progress in their study of microbiology. Table 2.1 shows the wide size range variability of prokaryotic cells, which range from a diameter of about 0.2 µm to over 700 µm. Use the two examples of unusually large prokaryotes discussed in this section to illustrate the current upper limit of prokaryotic cell size: (1) the surgeonfish gut symbiont Epulopiscium fishelsonsi (>600 µm in length; Figure 2.12a), and (2) the sulfur chemolitho-troph Thiomargarita namibiensis (750 µm; Figure 2.12b). The evolutionary “rationale” for the existence of unusually large-celled prokaryotes is a mystery when one considers that the metabolic rate of a cell varies inversely with the square of its size. Ask your students for ideas and/or hypotheses that might explain the selective advantage of large cell size in these two prokaryotes.
The fact that bacteria can live independently as single cells (unlike an individual cell of a multicellular organism) suggests that they must possess some capabilities that provide a
selective advantage over their multicellular counterparts that ensure their survival on the planet. Small cells have more surface area to volume (i.e., a higher surface-to-volume ratio), and this alone confers many of the evolutionary advantages of being small, including the
following:
· Rapid nutrient and waste transport into and out of the cell allows for faster metabolic rates and growth rates.
· Rapid growth rates result in the rapid production of large populations of cells. These populations, in turn, can greatly affect the physiochemical conditions of an ecosystem within a short time period.
· Transport rates are a function of the surface area of the cell membrane relative to cell
volume. Use Figure 2.13 to mathematically demonstrate to students that the surface area of a sphere is a function of the square of the radius, whereas the volume of a sphere is a function of the cube of the radius. This means that the surface-to-volume ratio of a spherical cell can be expressed as 3/r, where r equals the radius of the cell. Therefore, a coccus cell having a smaller radius has more surface area per volume, and thus more efficient transport capabilities, than a coccus cell having a larger radius.
· Rates of evolutionary change are higher in smaller, faster growing haploid cells than in larger, slower growing diploid cells. This allows for greater adaptive potential through
rapid selection for advantageous mutations and counterselection against deleterious
mutations.
The theoretical lower limit of size for a living cell is likely near 0.2 mm in diameter. This limit is dictated by the amount of volume required to contain cellular components that are crucial for maintaining life, such as (1) the presence of essential genes on the chromosome; (2) having a sufficient number of ribosomes; and (3) containing a minimal number of metabolic, structural, and transport proteins within the cell. Challenge students to list these and other molecular components a cell would have to contain to maintain life. Remind students that some cells are parasitic in nature. Inform them that, much like viruses, such microorganisms often have streamlined genomes that lack important genes and may make them dependent upon their hosts for growth. Can such organisms truly be considered living? This might make a good outside project for group debate, requiring students to view the cell as a three-dimensional physical structure constrained in space and to research a problem that is
currently being debated.
2.7 | Membrane Structure
The structure of the cytoplasmic membrane, a phospholipid bilayer, should be discussed in considerable detail because it plays a critical role in establishing and maintaining the cell’s internal environment. Students must understand that the cytoplasmic membrane is the selectively permeable boundary between the cytoplasm of the cell and the cell’s immediate environment. If the integrity of the membrane becomes compromised, then essential cellular components can leak out of the cytoplasm and into the environment, thereby destroying the cell. Convey to students that the cytoplasmic membrane generally does not confer a specific shape and provide rigid support to the cell (these are roles of the cell wall, to be discussed
later), but rather the membrane has a fluid nature that allows for a degree of lateral movement of phospholipids and proteins (Figures 2.14 and 2.15). Proteins embedded in the membrane
consist of both hydrophobic regions that are situated within the lipid portion of the phospholipid bilayer and hydrophilic regions that are oriented toward either the external environment or the aqueous cytoplasm of the cell.
In contrast to eukaryotic cells, which contain rigid sterol molecules to strengthen and stabilize membranes (especially those of animal cells, which lack cell walls), most prokaryotic membranes instead contain planar molecules called hopanoids that serve a similar function. Exceptions to this generalization include methanotrophic bacteria, which contain large amounts of sterols in internal membranes, and the mycoplasmas, a group of parasitic bacteria that lack cell walls.
While members of the Bacteria and Eukarya contain ester linkages that bond the fatty
acids to glycerol in their membranes (Figure 2.16a and b), Archaea contain ether linkages
between the glycerol and lipid portions of their membranes. In addition, archaeal membrane lipids are not composed of fatty acids but instead consist of repeating five-carbon isoprene units that combine to form 20-carbon phytanyl side chains (Figures 2.16c and 2.17a and b). Together, the glycerol and phytanyl form a glycerol diether. In some Archaea, glycerol diethers are joined at their hydrophobic ends to create a lipid monolayer of diglycerol tetraethers (Figure 2.17b and e). This structural conformation provides superior thermostability of the membrane, and indeed lipid monolayers are most commonly found in hyperthermophilic
archaeal species. Finally, members of the Crenarchaeota often contain crenarchaeol, a unique monolayer membrane lipid having four cyclopentyl rings and one cyclohexyl ring (Figure 2.17c). Despite the molecular differences between archaeal membranes and bacterial/eukaryotic membranes, their basic structural properties are the same in that each
possesses hydrophobic interior hydrocarbon chains attached to polar (hydrophilic)
glycerophosphate molecules.
Although molecular adaptations of membranes to high and low temperatures are discussed in some detail in Chapter 5, this may be a good opportunity to introduce the topic of saturated versus unsaturated hydrocarbon chains and discuss how they relate to membrane fluidity
under high and low temperature extremes (e.g., why vegetable shortening is a solid at room temperature, and vegetable oil is a liquid under the same conditions).
2.8 | Membrane Function
The major functions of the cytoplasmic membrane are summarized in Figure 2.18 and include its role as (1) a permeability barrier, (2) a protein anchor, and (3) a means of energy conservation. With respect to acting as a permeability barrier, impress upon students that even
extremely small ions do not freely pass through the hydrophobic interior of the membrane due to their charges (Table 2.2). While water molecules do diffuse through membranes (due to their small size and only weak polarity) in a process called osmosis, the movement of water across membranes is greatly accelerated by water transport proteins called aquaporins. These transport proteins have been identified in the membranes of organisms from all domains of life but are perhaps best studied in the bacterium Escherichia coli.
Introduce students to the concept that a membrane can function much like a battery in that it can store potential energy. By separating protons to the outside of the membrane from
hydroxyl ions on the inside, the membrane becomes “energized” (i.e., polarized), and this
energized state is referred to as the proton motive force (PMF). The dissipation of this force results in the conversion of potential energy to kinetic energy. When protons stored outside
of the membrane return to the inside of the cell through an ATPase enzyme complex, ADP and Pi are converted to ATP, the cell’s energy currency. This concept will be discussed in
detail in Chapter 3.
Discuss with your students the necessity for membrane-bound transport proteins by comparing the rate of simple diffusion of a solute across a membrane to the greatly accelerated rate of carrier-mediated transport of a solute across a membrane (Figure 2.19). Transport proteins allow for the accumulation inside a cell of a solute that may be in very low concentration in the environment. Point out that each carrier-mediated transport protein shows high specificity for a given solute.
2.9 | Nutrient Transport
Some students may find the variety of nutrient transport mechanisms difficult to comprehend initially, so discuss these mechanisms in detail using Figures 2.20–2.23 to illustrate the
concepts and provide examples of each type of transport event. When describing the three classes of membrane transport systems—simple transport, group translocation, and the ABC (ATP-binding cassette) system—highlight the following points to your students:
· Some transport mechanisms require only a membrane-spanning component (e.g., the simple transporters shown in Figure 2.21).
· Some require a series of proteins that cooperate in a phosphorylation/dephosphorylation cascade to carry out the transport event (e.g., the group translocation phosphotransferase system; Figure 2.22).
· Some require a membrane-spanning transporter, a substrate-binding protein, and an
ATP-hydrolyzing protein (e.g., the monosaccharide ABC transporters; Figure 2.23).
In addition to small molecule transport, larger molecules, such as proteins, need to
be inserted into membranes or transported outside the cell (e.g., toxins, amylases, and
cellulases). This movement of materials is accomplished by translocases, the most well-characterized being the SecYEG system that is found in many prokaryotes and the Type III Secretion System employed for the export of toxins by several pathogenic bacteria.
2.10 | Peptidoglycan
The bacterial cell wall warrants extensive coverage in the classroom because research on its structure and function can be traced back to the early history of microbiology. It began with Ferdinand Cohn’s early observation of the differential reaction of various bacterial cells to the Gram stain. This stain distinguished two types of bacteria based on the composition of the cell wall: gram-positive and gram-negative. Research proceeded with the discovery that both lysozyme and penicillin induced cell lysis and with the realization that some of the bacterial cell wall constituents (diaminopimelic acid and N-acetylmuramic acid) were unique. These discoveries were exciting. They increased our understanding of prokaryotic cells and helped to obtain better chemotherapy with which to combat bacterial diseases. The mechanisms of peptidoglycan biosynthesis, cell division (covered in Chapter 5), osmotic lysis, and the activity of penicillin are important topics of discussion because they provide striking examples
of the interrelationship of basic knowledge and practical applications of great significance.