Anatomy Of Bacterial Cell - I PDF

Anatomy Of Bacterial Cell - I Bacterial cell structure Despite their simplicity, Bacteria contain a well-developed cell structure which is responsible for some unique biological activities and its pathogenic potential. The bacterial cell structure includes : surface appendages such as (Glycocalyx, F...

Anatomy Of Bacterial Cell - I Bacterial cell structure Despite their simplicity, Bacteria contain a well-developed cell structure which is responsible for some unique biological activities and its pathogenic potential. The bacterial cell structure includes : surface appendages such as (Glycocalyx, Flagella, Pili, Fimbriae) and cell wall, cell membrane, nucleoid, plasmids, cytoplasm and ribosomes. Glycocalyx : Glycocalyx is a glycoprotein and glycolipid covering of certain bacteria and surround the outside of the cell envelope. Glycocalyx can be found outside of cell wall in some species of G+ve and G-ve bacteria. The Glycocalyx is called as a capsule if it is firmly attached to the cell wall , and called as a slime layer if loosely attached. Most bacterial capsules are composed of polysaccharide. The sugar components of the polysaccharide vary from one species of bacteria to another. Since capsules don’t take up any stain, they can be demonstrated by negative staining techniques using India ink and Nigrosine. However, the capsule of Bacillus anthracis is composed of a polyglutamic acid, while Yersinia pestis produces a capsule consist of mixed amino acids. Capsules may be weakly antigenic to strongly antigenic, depending on their chemical complexity. Bacteria with capsules form it has a smooth colonies texture , while those without capsules form it has a rough colonies texture.  Capsules are often lost during in vitro culture. Capsules are not essential to cell viability, some strains within a species will produce a capsule while others do not. Function of Capsule : 1. Capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease by prevents phagocytosis. 2. Capsules contain water which protects the bacteria against desiccation. 3. Capsules protect cells from bacteriophages and most hydrophobic toxic materials. 4. Capsules help cells adhere to surfaces such as, catheter tools. 5. Capsules protect the cells from lysis by lysozyme. 6. Capsules can be a source of nutrients and energy to microbes, for example Streptococcus which colonizes teeth, ferments the sugar in the capsule and contribute to tooth decay. 7. Cause host cell toxicity. Flagella Some bacteria are motile and some are not. Almost all motile bacteria possess flagellum (singular) as the organ of movement. Such bacteria tend to move towards or away from the source of stimulus. These stimulus can be chemicals (chemotaxis), light (phototaxis), air (aerotaxis). Flagellum can never be seen directly with the light microscope but only after staining with special flagella stains that increase their diameter. Flagella are approximately (3- 20μm) in long and (12-30 nm) in diameter. The flagella structure is divided into 3 parts, filament that attaches to the hook attached to the basal body that arises from the plasma membrane. The flagella are semi-rigid, helical, hollow tubular structures. Flagella are appendages whip-like that move the bacteria towards nutrients and other attractants. They are composed of single kind of protein subunit called flagellin which is highly antigenic, this protein form long chains that give the flagellum a helical shape.  Bacteria move by flagella with average speed of 50μm/sec, the fastest bacteria is Vibrio cholerae that moves 200μm/sec. Bacterial cells may have one or many flagella, the numbers of flagella, their location, and arrangement on the bacterial cell surface are characteristic of a species and used for classification of bacteria. Common forms include : 1. Atrichous – bacteria without flagella (Spirochetes ). 2. Monotrichous – one flagellum at one pole ( Vibrio cholerae ) 3. Amphitrichous – one flagella at both ends ( Alkaligens faecalis ) 4. Cephalotrichous - tuft of flagella at both pole (Pseudomonas fluorescence ) 5. Peritrichous – flagella surrounding the entire surface of cell (Salmonella typhi). 6. Lophotrichous - tuft of flagella at one pole (Spirillum ) Functions of Flagella 1. They help an organism in movement. 2. They act as sensory organs to detect changes in pH and temperature. 3. Signal transduction 4. Helps the bacteria in adhesion by anchored in host tissues. Medical Important of Flagella 1. Role in pathogenesis : Escherichia coli and Pseudomonas aeruginosa are common causes of urinary tract infections. The flagella play a role in pathogenesis by propelling the bacteria up the urethra into the bladder. 2. Role in organisms identification : Some species of bacteria, ex. Salmonella species are identified in the clinical laboratory by the use of specific antibodies against flagellar proteins. 3. Flagella are generally as being important virulence factors because it contains protein flagillin. Pili : Pilus (singular) are hair-like filaments that extend from the cell surface and arranged in helical strands. They are shorter and straighter than flagella and are composed of subunits of a protein Pilin. They are (0.5μm) long and (10 nm) thick. They are found only in gram negative bacteria. Pili are longer than fimbriae and there are only a few per cell ranging from (1-10). Pili are more rigid than fimbriae. Medical Importance of Pili 1. The attachment of bacteria to specific receptors on the human cell surface is necessary step in the initiation of infection. Mutants of Neisseria gonorrhoeae that do not contain pili are nonpathogens. 2. The sex pilus is a specialized kind of pilus, acts to join bacterial cells for transfer of DNA from one cell to another by a process called conjugation. 3. Pili are responsible for virulence in the pathogenic strains of bacteria, including Escherichia coli and Vibrio cholera , this is because enhance ability of bacteria to bind to body tissues. Fimbriae : Fimbria (Singular), are filamentous structures that extend from the surface of a cell and can have many functions. Fimbria composed of protein Fimbrillin. Fimbriae are found in both gram negative and gram positive bacteria. Fimbriae shorter in length as compared to pili and Less rigid. Number of fimbriae ranging from (200-400) per cell. Medical Importance of fimbriae: 1. They act as adhesins and allow bacteria to colonize cells. For example, Shigella dysenteriae uses its fimbriae to attach to the intestine and then produces a toxin that causes diarrhea. 2. Fimbriae can detect chemical signals and are important in bacterial cell communication. 3. Fimbriae of Streptococcus pyogenes are coated with M protein, which acts as an important virulence factor by adhering to host cells and resisting phagocytosis. ‫نفسيتك من رجع‬ ‫الدوام‪..‬‬ Anatomy Of Bacterial Cell - II Cell envelope: The bacterial "cell envelope" is a complex multilayered structure enclosing the cytoplasm. Cell envelope represents the outermost layer of the bacterial cell , it consist of several chemically and functionally distinct layers. The most prominent of which are the : 1. Cell wall 2. Cytoplasmic membrane 3. Capsule if present. General functions of bacterium cell envelope : 1. The protection of the cell from hostile environment. 2. Communication with the environment. 3. Maintenance of cellular shape. 4. Stability and rigidity of the cell. 5. Allowing appropriate metabolism. 6. Growth of the cell. 7. Division of the cell. Cell wall: The cell wall an essential structure for viability and bacterial survival. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan also called murein or mucopeptide. Peptidoglycan is a polymer consisting of : 1. Sugars : Polysaccharide backbone consisting of alternating N-Acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in equal amounts. 2. Amino acids : Peptide cross-bridges, links peptidoglycan subunits together.  Bacterial cell walls are different from the cell walls of plants and fungi which are made of cellulose and chitin, respectively. The presence of amino acids helps render the bacterial cell wall resistant to host peptidases such as those in the intestine. Function of Peptidoglycan: 1. Responsible for the rigidity of the bacterial cell wall. 2. Structural role in the bacterial cell wall. 3. Determines the shape of the cell. 4. Involved in binary fission during bacterial cell reproduction. 5. Prevents osmotic lysis result of solute concentration via active transport. Most bacteria are classified according to their response to the Gram-staining procedure based on the structural differences in their cell walls , as : 1. Gram-positive (G+ve) , bacteria that retain the crystal violet dye , because of a thick layer of peptidoglycan and are colored with violet (blue). 2. Gram negative (G-ve) , bacteria do not retain the crystal violet dye , because of a thin layer of peptidoglycan and are colored with purple (red). Cell wall of Gram positive bacteria ( G+ ve ) : Gram positive bacteria have thick , multilayered of peptidoglycan in their cell walls that are exterior to the cytoplasmic membrane. In (G+ve), the cell wall mainly consists of : 1. Peptidoglycan 2. Teichoic acid the peptidoglycan in most (G+ve) species of bacteria is covalently linked to anionic polymers called (teichoic acid), which is essentially a polymer of substituted glycerol units linked by phosphodiester bonds. The techoic acids are major cell surface antigens. Teichoic acids play crucial role in : 1. Cell shape determination. 2. Regulation of cell division. Cell wall of Gram negative bacteria ( G- ve ) : The cell wall in (G-ve) have a more complex cell wall structure composed of two membrane (outer and inner). The two membrane separated by the periplasmic space , which contains degradative enzymes and transport proteins. Cell wall have a thin peptidoglycan layer, which itself is surrounded by an outer membrane. The major constituent of the outer membrane of cell wall of (G-ve) bacteria contains two components : 1. lipoprotein. 2. lipopolysaccharide (LPS) , which is antigenic and toxic. The polysaccharide portion of (LPS) called (O- polysaccharide) is antigenic , therefore can be used to identify different species.  The lipid portion of (LPS) called ( lipid-A ) is imbedded in the membrane and is toxic for the mucosal membrane of the intestine of humans and animals (also called endotoxin). The outer membrane has special channels, consisting of protein molecules called ( porins ) that permit the passive diffusion of low-molecular-weight hydrophilic compounds such as : sugars , amino acids , ions. Large antibiotic molecules penetrate the outer membrane relatively slowly, which accounts for the relatively high antibiotic resistance of (G-ve) bacteria. The permeability of the outer membrane varies widely from one gram-negative species to another. For example , in Pseudomonas aeruginosa , which is extremely resistant to antibacterial agents, the outer membrane is (100 times) less permeable than that of Escherichia coli. The cell wall of G+ve bacteria, which is simply structured, is therefore more permeable to a series of antibiotics than is the G-ve cell wall that are resistant to antibiotics. Resistance of bacteria can depend on alterations of every layer of the cell envelope. Porins : are barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse. They are present in the outer membrane of G-ve bacteria and some G+ve of mycobacteria. It has several functions in the bacterial cell, such as : 1. Allow passive diffusion, they act as specific channels for some types of molecules of various sizes and charges across the membrane. 2. For survival, certain required nutrients and substrates must be transported into the cells. 3. Transferring toxins and waste out of the cell to ensure that they do not accumulate inside the cell. 4. Porins can regulate permeability and prevent lysis by limiting the entry of detergents into the cell.  In G-ve bacteria, the inner membrane is the major permeability barrier. The outer membrane is more permeable to hydrophilic substances due to the presence of porins. Generally, only substances less than 600 Daltons in size can diffuse through. Therefore (G-ve) are more resistant against antibiotics because of their impenetrable cell wall. Medical importance of bacterial cell wall : 1. Important sites for attack by antibiotics. 2. They provide ligands for adherence and receptor sites for drugs or viruses. 3. They cause symptoms of disease in human and animal. 4. They provide for immunological variation among strains of bacteria. 5. Various layers of the wall are the sites of major antigenic determinants of the cell surface. Functions of bacteria cell wall: 1. Non- selectively permeable. 2. Helps to provide fixed shape to the cell. 3. plays an essential role in cell division. 4. Protects from chemicals and other harsh condition of environment. 5. It facilitates movement of gases and water and other substances into or outside the cell. 6. Helps the bacteria to escape from phaogocytosis by host immune cells. 7. Protection against mechanical stress from insects and pathogens. 8. To giving osmotic protection (helps in osmotic-regulation). 9. Prevents water loss from the cell. 10. The physiological and biochemical activity of the cell wall. 11. Helps in cell-cell communication.  β-lactam antibiotics are a class of broad-spectrum antibiotics, consisting of all antibiotic agents that contain β - lactam ring in their molecular structures. This includes : penicillin , cephalosporins , monobactams and carbapenems. Most β- lactam antibiotics work by inhibiting cell wall (peptidoglycan cross-links) biosynthesis in bacteria. Bacteria often develop resistance to β-lactam antibiotics by synthesizing a β- lactamase enzyme that attacks the β-lactam ring. To overcome this resistance, β- lactam antibiotics are often given with β-lactamase inhibitors such as clavulanic acid. The enzyme lysozyme, found in human tears , also digests the cell wall of bacteria and defense against eye infections. Cytoplasmic membrane : Also called (plasma membrane or cell membrane). It is a biological membrane that separates the interior of a cell from its outside environment. composed of phospholipid with embedded proteins, the molecules of which form two parallel surface called ( lipid bilayer ) such that the polar phosphate groups are on the outside of the bilayer and the nonpolar lipid chains are on the inside. The phospholipids are amphoteric molecules with a polar hydrophilic glycerol (head) attached via an ester bond to two nonpolar hydrophobic fatty acid (tails), which naturally form a bilayer in aqueous environments. Cytoplasmic Membrane Functions : 1.Selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells. 2. Energy generating functions. 3. Secretion of extra-cytoplasmic proteins. 4. Excretion of hydrolytic exoenzymes. 5. Bearing the enzymes that function in the biosynthesis of DNA, cell wall, membrane lipids. 6. Protect the cell from its surroundings. 7. plays a role in anchoring the cytoskeleton to provide shape to the cell 8. The membrane maintains the cell potential. Other organelles in bacterial cell:  Nucleoid : is the part of cell which contains DNA material. It includes enzymes and proteins that transcribe RNA and DNA. Nucleoids also assist in cell development and growth.  Plasmid : Plasmids are small circular DNA fragments found in the cytoplasm that contain code responsible for antibiotic resistance. Plasmids can be transferred between bacteria, even from one bacterial species to another.  Cytoplasm : The cytoplasm is mostly water, but within it are the bacterial inclusions such : nucleoid, plasmids, ribosomes and storage granules as well as the components necessary for bacterial metabolism.  Ribosomes : Ribosomes give the cytoplasm of bacteria a granular appearance. Ribosomes have a similar function in translating the genetic message in messenger RNA into the production of peptide sequences (proteins).  Mesosome : Some contain pigments and enzymes needed for photosynthesis, others contain aerobic respiration enzymes.