General Structure of Prokaryotic and Eukaryotic Cells

Cells are the basic structural and functional unit of all living organisms. The name cell comes from the Latin word “cella,” a small room. Robert Hooke (see Chapter 1, Scope of Microbiology) selected this term in 1655, when he discovered cells in a piece of cork with his microscope, and compared the cork cells with small rooms. In 1838, Schleiden and Schwann proposed a formal cell theory, or cell doctrine, that marked the start of modern cell biology. The current cell theory states:

The major events leading to the knowledge of cell biology today are shown in Table 3.1.

Two different types of cells exist, prokaryotes and eukaryotes, which are structurally and functionally different from one another (Table 3.2). However, both cell types share certain properties, and these are as follows:

Plasma Membrane and Cell Wall

In order to survive, all living cells need to be protected from the continuous changes of their surrounding environment. This is achieved by the plasma membrane and in some types of cells by an additional structure, a cell wall.

Plasma Membrane

All living cells, both prokaryotes and eukaryotes, are surrounded by a plasma membrane, about 8 nm (nanometer) thick, creating an intracellular and extracellular environment/compartment. The intracellular and extracellular fluid compartments (ICF and ECF, respectively) are aqueous and the plasma membrane provides the barrier between them. The membrane is composed primarily of phospholipids and proteins. Phospholipids are polar molecules with a polar (hydrophilic) head and a nonpolar (hydrophobic) tail (see Chapter 2, Chemistry of Life). Because the environment on each side of the plasma membrane is aqueous, the hydrophobic tails face each other and the hydrophilic heads are exposed to water on both surfaces of the membrane. This results in the formation of a phospholipid bilayer, illustrated in Figure 3.1.

Embedded in the phospholipid bilayer are peripheral and integral (transmembrane) proteins and cholesterol. The peripheral proteins are partially embedded on one side of the membrane, whereas integral proteins extend from one side through the membrane to the other side. Although the specific lipid and protein components vary in different membranes, the basic structure and composition of the plasma membrane are the same, as are those membranes that surround cell organelles. Because the plasma membrane is not solid, its components are freely movable, presenting a constantly changing fluid-mosaic membrane structure (see Figure 3.1, A).

Whereas the phospholipids are the main lipid component of the cell membrane, cholesterol is another major cellular membrane lipid and the amount varies with the type of plasma membrane. For example, the plasma membrane of animal cells contains almost one cholesterol molecule per phospholipid molecule, plant cells contain much less, and prokaryotic cells have no cholesterol but instead contain sterol-like molecules called hopanoids. Cholesterol and hopanoids (pentacyclic sterol–like compounds) immobilize the first few hydrocarbon groups of the phospholipid molecules. This makes the lipid bilayer less elastic and decreases permeability to small water-soluble molecules.

Glycolipids, also a component of membranes, project into the extracellular space. Functionally, these may protect, insulate, and serve as receptor-binding sites. In addition, the outer membrane of gram-negative cell walls (see Chapter 6, Bacteria and Archaea) contains lipopolysaccharide bacterial endotoxins that are released on lysis of these cells.

The functions of membrane proteins differ depending on their composition and location in the plasma membrane. They can:

• Provide structural support

• Transport molecules across the membrane

• Act as enzyme regulators to control chemical reactions

• Act as specific receptors for hormones and other regulatory substances

• Act as surface antigens (see Chapter 20, The Immune System)

MEDICAL HIGHLIGHTS

Endotoxin Shock

Endotoxins located within the gram-negative cell wall are released when the cell lyses, either due to an immune response or due to the action of antibiotics. Endotoxin shock is generally associated with a systemic spread of gram-negative bacteria, in which the massive release of endotoxins acting as pyrogens (see the section on fever in Chapter 20, The Immune System) causes systemic inflammatory reactions that may have serious, life-threatening consequences.

Glycocalyx

Many cells have a matrix formed outside the plasma membrane. This extracellular fabric is composed of polymeric material called a glycocalyx, produced by some bacteria, epithelia, and other cells. In general the glycocalyx is a network of polysaccharides that project from the cellular surface and functions in/as:

• Cell-to-cell communication

• Interaction with other extracellular matrix

• Anchoring of cells to the extracellular matrix

• Protection from pathogens

• Modulation of the immune responses

• Attachment site for bacteria to inert surfaces (formation of biofilms)

In the case of bacteria the glycocalyx is a coating of macromolecules that protects the cell and sometimes helps the bacteria to adhere to its environment. The chemical composition, thickness, and organization of the glycocalyx differ among bacteria. One specialized function of the bacterial glycocalyx is the formation of capsules (Figure 3.2). Capsules protect pathogens, such as Streptococcus pneumoniae and Bacillus anthracis, from phagocytosis by white blood cells, thus adding to their pathogenicity.

Cell Wall

A cell wall is located immediately below the glycocalyx surrounding the plasma membranes. Bacteria, archaea, fungi, plants, and algae can all have cell walls of different chemical composition. The purpose of the cell wall is to maintain the shape of a cell and to protect the cell from any physical or chemical damage.

The bacterial cell wall is a unique rigid structure that maintains the shape of bacteria and protects them from hostile environments, including protection from the immune system of a host (see Chapter 20, The Immune System). The strength of the cell wall is due to the presence of peptidoglycan (mucopeptide or murein), a mixed polymer of hexose sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by short peptide fragments (Figure 3.3). The amount and composition of peptidoglycan vary among the major bacterial groups and provide the basis for Gram staining (see Chapter 4, Microbiological Laboratory Techniques), dividing bacteria into so-called gram-positive and gram-negative organisms.

However, some bacteria do not have a characteristic cell wall structure and some lack a cell wall altogether (i.e., Mycobacterium; see Chapter 6, Bacteria and Archaea). Differences in cell wall composition are an important consideration in the selection of antimicrobial drugs in treatment regimens of bacterial diseases. The cell wall structure is also important in choosing specific methods to control microbial growth in a variety of environments (see Chapter 19, Physical and Chemical Methods of Control).

Characteristics of cell walls in the different microorganisms are as follows:

Gram-positive bacteria have a thick peptidoglycan layer (20–80 nm) located external to the cell membrane (Figure 3.4, A). It also contains acidic polysaccharides such as teichoic acid and lipoteichoic acid, which aid in cell wall maintenance and contribute to an acidic cell surface. The small space between the plasma membrane and the cell wall is the periplasmic space, which can be minimal in some of the gram-positive bacteria.

Gram-negative bacteria have a thin (5- to 10-nm) peptidoglycan layer that is more complex because it has an outer membrane that provides a cover that is anchored to the lipoprotein molecules of the peptidoglycan layer (Figure 3.4, B). The outer membrane is similar in structure to the plasma membrane, but it contains lipopolysaccharides extending from its surface. These lipopolysaccharides can function as receptors or as antigens. In addition, the membrane contains porin proteins, which allow penetration only of small molecules. This serves as a defense mechanism against larger molecules such as antibiotics. The periplasmic space between the plasma membrane and the outer membrane may constitute up to 40% of the total cell volume. This space houses biochemical pathways for nutrient acquisition, peptidoglycan synthesis, electron transport, and detoxification of substances otherwise harmful to the cell.

• The cell wall of archaea does not contain peptidoglycan, but some may contain a similar chemical called pseudopeptidoglycan.

• Plant cell walls are generally made of polysaccharides, mostly cellulose, hemicellulose, and pectin.

• Algae have cell walls that contain cellulose and a variety of glycoproteins. Additional polysaccharide inclusions in their cell walls are used as an identification characteristic in algal taxonomy (see Chapter 8, Eukaryotic Microorganisms).

• The cell wall of fungi is a complex structure composed of chitin, glucans, and other polymers. Not all fungi have a cell wall but among those that do, the composition, properties, and form of it change during the cell cycle and during growth conditions (see Chapter 8, Eukaryotic Microorganisms).

MEDICAL HIGHLIGHTS

Penicillin and the Periplasmic Space

Although various mechanisms can lead to antibiotic-resistant strains, the periplasmic space is of clinical importance because some gram-negative bacteria secrete β-lactamase into this space. β-Lactamase is an enzyme responsible for degrading the penicillin group of antibiotic drugs, ultimately leading to penicillin resistance. The expression of β-lactamase can be either chromosomal or plasmid-mediated.

Bacterial Flagella

Bacterial flagella are long (3 to 12 micrometers [µm]) helical filaments about 12 to 30 nm in diameter, which they use for movement in their environment. The protein components are flagellins and are assembled to form a cylindrical structure with a hollow core. A flagellum consists of three parts (Figure 3.5):

Flagellins are immunogenic and represent a group of antigens called the H antigens. These antigens are characteristic of a species, strain, or variant of an organism. The species specificity of the flagellins is the result of differences in the primary structures of the proteins (see Chapter 2, Chemistry of Life). The flagella may be present at the poles of the cells as a single polar structure at one end of the bacterium (monotrichous), or at each end (amphitrichous). Flagella also may be present in tufts at one or both ends of the organism, in which case such organisms are lophotrichous. If the flagella are distributed over the general cell surface, they are peritrichous (Figure 3.6).

Bacterial Mobility

The thrust by which the flagella propel the bacterial cell is produced by either clockwise or counterclockwise rotation of the basal body driven by energy from a proton-motive force rather than directly from adenosine-5′-triphosphate (ATP). Bacterial flagella do not rotate at a constant speed; the speed of rotation depends on the cell’s generation of energy. Bacteria can change speed and direction of their flagellar movements and as a result are capable of various patterns of motility. Movements are referred to as runs (swims) or tumbles. A bacterium moving in one direction for a period of time is said to run, and when the direction changes the bacterium tumbles (Figure 3.7). Any movement of bacteria toward or away from a particular stimulus is called taxis. As bacteria run, they may show properties of chemotaxis (response to chemical stimuli) or phototaxis (response to light). Both require sophisticated sensory receptors located in the cell surface and periplasm.

Not all prokaryotes have flagella, but some are still capable of movement by the process of gliding. In general, gliding prokaryotes are filamentous or rod shaped in contact with a solid surface (e.g., filamentous cyanobacteria).

Pili

Pili (fimbriae) are another form of bacterial surface projection (Figure 3.8) and are more rigid than flagella. They are composed of pilin proteins. In some organisms, such as Shigella species and Escherichia coli, as many as 200 pili are distributed over a single cell surface. Pili come in two types: short, abundant common pili, and a smaller number (one to six) of very long sex pili. Sex pili attach male to female bacteria during conjugation. Pili of many enteric bacteria have adhesive properties that can attach to various epithelial surfaces and to surfaces of yeast and fungal cells. These adhesive properties of piliated bacteria play an important role as factors in the colonization of epithelial surfaces. Adhesion to host cells is dependent on specific interactions between the adhesins and molecules in the host cell membranes. For example, the adhesins of E. coli chemically interact with molecules of mannose (a sugar monomer) on the surface of intestinal epithelial cells.

Cilia

Cilia and eukaryotic flagella are structurally identical hairlike appendages, about 0.25 µm in diameter, made from microtubules (Figure 3.9). They are motile and either move the cell or move substances over or around the cell. Protozoans use cilia to collect food particles and for locomotion (see Chapter 8, Eukaryotic Microorganisms). The primary action of cilia in most animal species is to move fluid, mucus, or cells over their surface. The major difference between cilia and eukaryotic flagella is their length, which is much greater for flagella. Cilia can either be motile, constantly beating in one direction, or nonmotile, nonbeating sensors. Motile cilia rarely occur singly and they are usually present in large numbers on cell surfaces, beating in coordinated waves. Nonmotile cilia, on the other hand, generally are single structures per cell.

Microvilli

Microvilli are plasma membrane extensions on the surface of eukaryotic cells. Most often they occur in groups on the surface of absorptive and secretory epithelial cells such as in the kidney and intestine (Figure 3.10). They are approximately 0.08 µm in diameter, 1 µm long, and greatly increase the surface area of cells to facilitate absorption and secretion. Microvilli can also be found on the surface of white blood cells, where they aid in the migration of the cells.

Biofilms

A biofilm is a microbial community enclosed by an extracellular, mostly polysaccharide polymeric matrix (Figure 3.11). Noncellular materials such as mineral crystals, corrosive particles, blood, and other substances can also be found in the biofilm matrix. The kind of inclusions depends on the environment where a biofilm develops. Biofilms develop when free-floating microorganisms attach to a surface. The first colony initially adheres to a given surface by van der Waals forces (see Chapter 2, Chemistry of Life). If not immediately separated from that surface, the organisms can anchor themselves more permanently via adhesion molecules such as pili. Once adhered these microbes then provide various adhesion sites for other new incoming cells, resulting in the building of a matrix that holds the biofilm together. Microbial species that cannot attach to certain surfaces on their own can often attach to the biofilm matrix or to earlier colonized organisms. The biofilm then grows through a combination of cell division and recruitment. The cells on the surface of the biofilm are different from the ones within the matrix and the behavior of the embedded cells may change within the thickness of the biofilm.

Biofilms can form on a wide variety of surfaces including living tissues, indwelling medical devices and catheters, industrial or clean water system piping, natural aquatic systems, and rocks and pebbles, and on the surface of stagnant pools of water. Moreover, biofilms are important components of the food chains formed by the living matter of rivers and streams.

Microbial biofilms can cause equipment damage and therefore can be the source of product contamination and microbial infections. They do represent a concern in the food industry because they can arise from raw materials, surfaces, people, animals, and even the air. Once food or a surface in a food-processing plant is contaminated, microbial colonies and eventually biofilms can form. Cleaning materials commercially used to clean countertops will kill planktonic or single cells of bacteria, but might be unable to penetrate a biofilm. For example, chlorination of a biofilm is often unsuccessful because the agent only affects the outer layers, whereas the inner layers of the biofilm remain healthy and are able to reestablish the biofilm.

Biofilms also provide an ideal environment for the exchange of plasmids (extrachromosomal bacterial DNA; see Chapter 6). Certain plasmids encode resistance to antimicrobial agents and their inclusion within a biofilm provides an opportunity to promote and spread bacterial resistance to antimicrobial agents. Repeated use of antimicrobial agents actually increases the possibility of resistance to biocides (see Chapter 19, Physical and Chemical Methods of Control).

On the other hand, microbial processes at the surface of the biofilms can give opportunities for positive industrial and environmental effects. Biofilms can be used in:

Research investigations exploring new uses for biofilm processes now include scientists from a variety of disciplines such as engineers, microbiologists, biochemists, computer scientists, statisticians, and physicists, to mention a few.

Cytoplasm, Cytoskeleton, Cell Organelles, and Inclusions

Other than a plasma membrane, cell wall, appendages, and glycocalyx, all of which play a specific role in the functioning and protection of cells, specific chemical processes (metabolism) occur within the cell. All metabolic processes in a cell are aided either by cell organelles or other structures and chemicals within the matrix of a cell.

Cytoskeleton

It was previously thought that the cytoskeleton was present only in eukaryotic cells, but more recent research shows that homologs of cytoskeletal proteins present in eukaryotic cells are also found in prokaryotes. Speculations indicate that constituents of today’s eukaryotic skeleton (tubulin and actin) evolved from prokaryotic precursor proteins closely related to today’s bacterial proteins. The studies indicate that the existence of a shape-preserving cytoskeleton is universally present in bacteria.

Structurally, the cytoskeleton (Figure 3.12) is not rigid but is dynamic and capable of rapid reorganization. This is essential for intracellular transport of vesicles and large molecules, as well as for the rearrangement of organelles. Furthermore, the cytoskeleton forms a spindle apparatus during cell division, which is discussed later in this chapter. The cytoskeleton is composed of actin filaments, intermediate filaments, and microtubules:

• Actin filaments (microfilaments) are made from the globular protein actin, which polymerizes in a helical fashion to form microfilaments. They are about 7 nm in diameter and provide mechanical support for the cell, determine the cell shape, and enable cell movements.

• Intermediate filaments are 8 to 11 nm in diameter and are the more stable components of the cytoskeleton. They organize the internal tridimensional structure of the cell. Intermediate filament proteins include keratins, vimentins, neurofilaments, and lamins.

• Microtubules are hollow cylinders with a diameter of about 24 nm and varying lengths, made of tubulin proteins. They serve as structural components within cells and take part in many cellular processes such as mitosis, cytokinesis, and vesicular transport.

Centrioles and centrosomes are also part of the cytoskeleton and are essential during cell division in eukaryotic cells. A centriole is a barrel-shaped microtubular structure present in most animal cells and cells of fungi and algae, but not frequent in plants. When two centrioles are arranged perpendicularly and are surrounded by additional proteins, they form the centrosome. The centrosome is known as the microtubule-organizing center and plays an important role in the microtubule organization of a cell. During the process of cell division centrioles form the mitotic spindle on which the chromosomes pull apart.