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Prokaryotic Cell Architecture(Bacteria) Structurally, a Bacterial Cell Prokaryotic Cell Architecture(bacteria) Structurally, a bacterial cell (Figure below) has three architectural regions: appendages (attachments to the cell surface) in the form of flagella and pili (or fimbriae); a cell envelope consisting of a capsule, cell wall and plasma membrane; and a cytoplasmic region that contains the cell chromosome (DNA) and ribosomes and various sorts of inclusions. Schematic drawing of a typical bacterial cell. Appendages Flagella Flagella are filamentous protein structures attached to the cell surface that provide the swimming movement for most motile prokaryotes. The diameter of a procaryotic flagellum is about 20 nanometers, well-below the resolving power of the light microscope. The flagellar filament is rotated by a motor apparatus in the plasma membrane allowing the cell to swim in fluid environments. Bacterial flagella are powered by proton motive force (chemiosmotic potential) established on the bacterial membrane, rather than ATP hydrolysis which powers eukaryotic flagella. About half of the bacilli and all of the spiral and curved bacteria are motile by means of flagella. Very few cocci are motile, which reflects their adaptation to dry environments and their lack of hydrodynamic design. Salmonella enterica. The enteric are motile by means of peritrichous flagella. Flagella may be variously distributed over the surface of bacterial cells in distinguishing patterns, but basically flagella are either polar (one or more 1 flagella arising from one or both poles of the cell) or peritrichous (lateral flagella distributed over the entire cell surface). Flagellar distribution is a genetically-distinct trait that is occasionally used to characterize or distinguish bacteria. For example, among Gram-negative rods, pseudomonads have polar flagella to distinguish them from enteric bacteria, which have peritrichous flagella. Fimbriae and Pili Fimbriae and Pili are interchangeable terms used to designate short, hair-like structures on the surfaces of prokaryotic cells. Like flagella, they are composed of protein. Fimbriae are shorter and stiffer than flagella, and slightly smaller in diameter. Generally, fimbriae have nothing to do with bacterial movement (there are exceptions, e.g. twitching movement on Pseudomonas). There are two types of Pili: Common Pili (often called fimbriae) are usually involved in specific adherence (attachment) of prokaryotes to surfaces in nature. In medical situations, they are major determinants of bacterial virulence . The F or sex pilus, mediates DNA transfer during conjugation and apparently stabilizes mating bacteria during the process of conjugation. Gram-positive wall is a uniformly thick layer external to the plasma membrane. It is composed mainly of peptidoglycan (murein). The Gram-negative wall appears thin and multilayered. It consists of a relatively thin peptidoglycan sheet between the plasma membrane and a phospholipid-lipopolysaccharide outer membrane. The space between the inner (plasma) and outer membranes (wherein the peptidoglycan resides) is called the periplasm. Capsules Most bacteria contain some sort of a polysaccharide layer outside of the cell wall polymer. In a general sense, this layer is called a capsule. A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall. A less discrete structure or matrix which embeds the cells is a called a slime layer or a biofilm. A type of capsule found in bacteria called a glycocalyx or microcapsule is a very thin layer of tangled polysaccharide fibers on the cell surface. Capsules have several functions and often have multiple functions in a particular organism. Like fimbriae, capsules, slime layers, and glycocalyx often 2 mediate adherence of cells to surfaces. Capsules also protect bacterial cells from engulfment by predatory protozoa or white blood cells (phagocytes), or from attack by antimicrobial agents of plant or animal origin. Capsules in certain soil bacteria protect cells from perennial effects of drying or desiccation. Capsular materials (e.g. dextrans) may be overproduced when bacteria are fed sugars to become reserves of carbohydrate for subsequent metabolism. A classic example of biofilm construction in nature is the formation of dental plaque mediated by the oral bacterium, Streptococcus mutans. The bacteria adhere specifically to the pellicle of the tooth by means of a protein on the cell surface. The bacteria grow and synthesize a dextran capsule which binds them to the enamel and forms a biofilm some 300-500 cells in thickness. The bacteria are able to cleave sucrose (provided by the animal diet) into glucose plus fructose. The fructose is fermented as an energy source for bacterial growth. The glucose is polymerized into an extracellular dextran polymer that cements the bacteria to tooth enamel and becomes the matrix of dental plaque. The dextran slime can be depolymerized to glucose for use as a carbon source, resulting in production of lactic acid within the biofilm (plaque) that decalcifies the enamel and leads to dental caries or bacterial infection of the tooth. Figure 16 (Left) Dental plaque revealed by a harmless red dye. Figure 13. Bacterial capsules outlined by India ink viewed by light microscopy. The Cell Envelope The cell envelope is a descriptive term for the several layers of material that envelope or enclose the protoplasm of the cell. The cell protoplasm (cytoplasm) is surrounded by the plasma membrane, a cell wall and a capsule. The cell wall itself is a layered structure in Gram-negative bacteria. All cells have a plasma membrane, which is the essential and definitive 3 characteristic of a "cell". Almost all prokaryotes have a cell wall to prevent damage to the underlying protoplast. Outside the cell wall, foremost as a surface structure, may be a polysaccharide capsule or glycocalyx. Figure 12. Profiles of the cell envelope the Gram-positive and Gram- negative bacteria. Cell Wall Most prokaryotes have a rigid cell wall. The cell wall is an essential structure that protects the cell protoplast (the region bound by and including the membrane) from mechanical damage and from osmotic rupture or lysis. Bacteria usually live in relatively dilute environments such that the accumulation of solutes inside the cell cytoplasm greatly exceeds the total solute concentration in the outside environment. Thus, the osmotic pressure against the inside of the plasma membrane may be the equivalent of 10-25 atmospheres. Since the membrane is a delicate, plastic structure, it must be restrained by an outside wall made of porous, rigid material that has high tensile strength. Such a material is murein, the ubiquitous component of bacterial cell walls. Bacterial murein is a unique type of peptidoglycan. Peptidoglycan is a polymer of sugars (a glycan) cross-linked by short chains of amino acids (peptide). All bacterial peptidoglycan contain N-acetylmuramic acid, which is the definitive component of murein.The profiles of the cell surface of bacteria, as seen with the electron microscope, are drawn in Figure 12. In Gram-positive Bacteria (those that retain the purple crystal violet dye when subjected to the Gram-staining procedure) the cell wall is thick (15-80 nanometers), consisting of several layers of peptidoglycan. Running perpendicular to the peptidoglycan sheets are a group of molecules called teichoic acids which are unique to the Gram-positive cell wall. Peptidoglycan is a polysaccharide consisting of alternating amino sugars (N-acetylglucosamine (NAG) and N- acetyl muramic acid (NAM)) linked by Beta 1-4 bonds like those in cellulose. The Beta 1-4 bond can be broken by an enzyme called lysozyme that is present in your tears, saliva, blood, other bodily fluids and egg white. These polysaccharide chains are held together by peptide cross-links . These peptides that they may contain D-amino acids. 4 Antibiotics like penicillin and cephalosporin inhibit the enzymes that synthetise peptidoglycan. This is why these antibiotics are not affective against the eucaryotic cells of fungi and parsites. Figure 17. Structure of the Gram-positive bacterial cell wall. The wall is relatively thick and consists of many layers of peptidoglycan interspersed with teichoic acids that run perpendicular to the peptidoglycan sheets. In the Gram-negative Bacteria (which do not retain the crystal violet in the Gram-stain procedure) the cell wall is relatively thin (10 nanometers) and is composed of a single layer of peptidoglycan surrounded by a membranous structure called the outer membrane. The outer membrane of Gram-negative bacteria invariably contains a unique component, lipopolysaccharide (LPS or endotoxin), which is toxic to animals. In Gram-negative bacteria the outer membrane is usually considered as part of the cell wall. Figure 18. Structure of the Gram-negative cell wall. The wall is relatively thin and contains much less peptidoglycan than the Gram-positive wall. Also, teichoic acids are absent. However, the Gram negative cell wall consists of an outer membrane that is outside of the peptidoglycan layer. The outer membrane is attached to the peptidoglycan sheet by a unique group of lipoprotein molecules. Of special interest as a component of the Gram-negative cell wall is the outer membrane, a discrete bilayered structure on the outside of the peptidoglycan 5 sheet (see Figure 12 above and Figure 19 below). For the bacterium, the outer membrane is first and foremost a permeability barrier, but primarily due to its lipopolysaccharide content, it possesses many interesting
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