Bacterial Structure (2) PDF

Bacterial structure 1 Asst. Prof. Dr. Ayhan MEHMETOĞLU [email protected]  Learning objectives Student should be able to define typical bacteria. Bacterial morphology. Cell wall structure, peptidoglycans. Cel...

Bacterial structure 1 Asst. Prof. Dr. Ayhan MEHMETOĞLU [email protected]  Learning objectives Student should be able to define typical bacteria. Bacterial morphology. Cell wall structure, peptidoglycans. Cell wall in gram positive and gram negative bacteria. Cell membrane, mesosome, ribosome, genetic material. Specialized Structures Outside the Cell Wall : capsule, pili, flagella, glycocalex. 2 Structure of Bacterial Cells Bacteria are classified by shape into three basic groups: cocci, bacilli, and spirochetes. The cocci are round, the bacilli are rods, and the spirochetes are spiral-shaped. Some bacteria are variable in shape and are said to be pleomorphic (many-shaped). 3  Bacterial morphology A: Cocci in clusters (e.g., Staphylococcus; A-1); chains (e.g., Streptococcus; A-2); in pairs with pointed ends (e.g., Streptococcus pneumoniae; A-3); in pairs with kidney bean shape (e.g., Neisseria; A-4). B: Rods (bacilli): with square ends (e.g., Bacillus; B-1); with rounded ends (e.g., Salmonella; B-2); club-shaped (e.g., Corynebacterium; B-3); fusiform (e.g., Fusobacterium; B-4); comma-shaped (e.g., Vibrio; B-5). C: Spirochetes: relaxed coil (e.g., Borrelia; C-1); tightly coiled (e.g., Treponema; C-2). 4 Structure 5  Cell Wall The cell wall is the outermost component common to all bacteria (except Mycoplasma). Some bacteria have surface features external to the cell wall, such as a capsule, flagella, and pili, which are less common components and are discussed below. It is composed of an inner layer of peptidoglycan and an outer membrane that varies in thickness and chemical composition depending upon the bacterial type. The peptidoglycan provides structural support and maintains the 6 characteristic shape of the cell. Also called murein or mucopeptide found only in bacterial walls PEPTIDOGLYCAN Provides rigid support to the cell Important in maintaining the characteristic shape of the cell  Allows the cell to withstand media of low osmotic pressure Peptidoglycan The structure of peptidoglycan is composed of glycan chains which are connected to each other through peptide bonds. Cell wall is stronger and thicker in gram positives Gram (+) bacterial cell wall constitutes 50% (40-80%) of dry weight Peptidoglycan Gram (-) bacterial cell wall constitutes 5-10% of dry weight  10 Copyright from Ingraham JL, Maaløe O, Neidhardt FC. Growth of the Bacterial Cell. Sinauer Associates; 1983.)  In Gram positive bacteria, the cell wall is composed of a BACTERIAL thick peptidoglycan layer in CELL WALL which glycan strings are tightened with teichoic acid strings.  In Gram negative bacteria, The Cell Wall thin peptidoglycan layer (between composed of, an outer periplasmic space cytoplasmic membrane membrane and outer membrane) and several enzymes such as proteases, hydrolytic enzymes, nucleases, hyaluronidases, collagenases. 14  Composed of glycan chains which are connected to each other through peptide bonds. Glycan chain is composed of repeating sugars, N-acetyl-muramic acid Structure of (NAMA) and N-acetyl glucosamine Peptidoglycan (NAGA) connected by a beta 1,4- glycoside bond. These bonds are the target region of lysozyme. Gram positive Cell Wall Structure of Peptidoglycan in Gram positive Bacteria  Is an enzyme present in human tears, saliva and mucus. Can cleave the glycosyl bonds between N- acetyl-muramic acid and N-acetyl glucosamine. Lysozyme Helps natural resistance of the host against bacterial infection. Is especially very active against gram positive bacteria.  Transpeptidation Transpeptidase PBP Endopeptidase Carboxypeptidase Penicillin binding proteins (PBP) are major targets for beta-lactam antibiotics  Beta lactam antibiotics (penicillin) show molecular similarity and bind with PBP EFFECT Irreversibly binds to the active site of the transpeptidase enzyme ON CELL By binding with transpeptidases WALL (PBP), Penicillin prevents the enzyme from binding with D-ala Peptidoglycan synthesis is inhibited.  Present in the cell wall of gram positive bacteria only Activates the same pathways that lipopolysaccharide activates in gram negative TEICHOIC bacteria ACID These are phage receptors in gram negative bacteria. Composed of polymers of either glycerol phosphate or ribitol phosphate  Teichoic Acid The medical importance of teichoic acids lies in their ability to induce septic shock when caused by certain gram-positive bacteria; i.e., they activate the same pathways as does endotoxin (LPS) in gram-negative bacteria. Teichoic acids also mediate the attachment of staphylococci to mucosal cells. 22 Gram negative Cell Wall  It’s inner and outer layers that form bilayered membrane are different from each other. OUTER MEMBRANE The outermost phospholipid layer contains a lot of lipopolysaccharide molecules.  The lipopolysaccharide (LPS) of the outer membrane of the cell wall of gram- negative bacteria is endotoxin. It is responsible for many of the features of disease, such as fever and shock (especially hypotension), caused by these organisms. Lipopolysaccharide (LPS) It is called endotoxin because it is an integral part of the cell wall, in contrast to exotoxins, which are freely released from the bacteria. The pathologic effects of endotoxin are similar irrespective of the organism from which it is derived. 25  Composition of LPS The LPS is composed of three distinct units: A phospholipid called lipid A, which is responsible for the toxic effects A core polysaccharide of five sugars linked to lipid A An outer polysaccharide consisting of up to 25 repeating units of three to five sugars. This outer polymer is the important somatic, or O antigen of several gram-negative bacteria that is used to identify certain 26 organisms in the clinical laboratory.  Lipopolysaccharides Endotoxin causes septic shock.  Porin proteins Outer membrane contains protein structures called porins that are channels for several substances. They are abbreviated as OMP (outer membrane porin). These are phage receptors in gram negative bacteria.  Acid-Fast and Associated Cell Wall Cell Wall Structure of Mycobacteria, Nocardia Rich in lipid Contain mycolic acid  Mycobacteria, e.g., Mycobacterium tuberculosis, have an unusual cell wall, resulting in their inability to be Gram- stained. Cell Walls These bacteria are said to be acid-fast, of Acid- because they resist decolorization with acid-alcohol after being stained with Fast carbolfuchsin. Bacteria This property is related to the high concentration of lipids, called mycolic acids, in the cell wall of mycobacteria. 30  Cell wall-deficient (CWD) bacteria, are strains of bacteria that lack cell walls Bacteria may survive by losing cell wall L-form structures Bacteria when exposed to lysozyme or antibiotics acting on the cell wall. Dr. Ayham Abulaila 32  Cytoplasmic Membrane Just inside the peptidoglycan layer of the cell wall Composed of a phospholipid bilayer like eukaryotic cells but eukaryotic membranes contain sterols, whereas prokaryotes generally do not. Except for Mycoplasma. The membrane has four important functions: 1. Active transport of molecules into the cell 2. Energy generation by oxidative phosphorylation 3. Synthesis of precursors of the cell wall 33 4. Secretion of enzymes and toxins. 34  This invagination of the cytoplasmic membrane is important during cell division, when it functions as the origin of the transverse septum that divides Mesosome the cell in half and as the binding site of the DNA that will become the genetic material of each daughter cell. 35  The cytoplasm has two distinct areas when investigated in the electron microscope: 1. An amorphous matrix that Cytoplasm contains ribosomes, nutrient granules, metabolites, and plasmids 2. An inner, nucleoid region composed of DNA. 36  Ribosomes Site of protein synthesis Differ from eukaryotic ribosomes in size and chemical composition. Bacterial ribosomes are 70S in size, with 50S and 30S subunits, whereas eukaryotic ribosomes are 80S in size, with 60S and 40S subunits. 37  Important note The differences in both the ribosomal RNAs and proteins constitute the basis of the selective action of several antibiotics that inhibit bacterial, but not human, protein synthesis. 38  Granules The cytoplasm contains several different types of granules that serve as storage areas for nutrients and stain characteristically with certain dyes. 39  Nucleoid The nucleoid is the area of the cytoplasm in which DNA is located. The DNA of prokaryotes is a single, circular molecule Its molecular weight (MW) is approximately 2 x 109 and contains about 2000 genes. (By contrast, human DNA has approximately 100,000 genes.) Because the nucleoid contains no nuclear membrane, no nucleolus, no mitotic spindle, and no histones, there is little resemblance to the eukaryotic nucleus. One major difference between bacterial DNA and eukaryotic DNA is that bacterial DNA has no introns whereas eukaryotic DNA does. 40  Plasmids Plasmids are extrachromosomal, double-stranded, circular DNA molecules that are capable of replicating independently of the bacterial chromosome. Although plasmids are usually extrachromosomal, they can be integrated into the bacterial chromosome. Plasmids occur in both gram-positive and gram-negative bacteria, and several different types of plasmids can exist in one cell. 41  Transmissible plasmids can be transferred from cell to cell by conjugation They are large (MW 40–100 million), Transmissible since they contain about a dozen genes plasmids responsible for synthesis of the sex pilus and for the enzymes required for transfer. They are usually present in a few (one to three) copies per cell. 42  Non-transmissible plasmids are small (MW 3–20 million), since Non- they do not contain the transfer transmissible genes; they are frequently present in many (10–60) copies per cell. plasmids 43  Plasmids carry the genes for the following functions and structures of medical importance: 1. Antibiotic resistance, which is mediated by a variety of enzymes 2. Resistance to heavy metals such as mercury (the active component of some Plasmids antiseptics, such as Merthiolate and Mercurochrome) and silver, which is genes mediated by a reductase enzyme 3. Resistance to ultraviolet light, which is mediated by DNA repair enzymes 4. Pili (fimbriae), which mediate the adherence of bacteria to epithelial cells 5. Exotoxins, including several 44 enterotoxins.  Transposons(jumping genes) Transposons are pieces of DNA that move readily from one site to another, either within or between the DNAs of bacteria, plasmids, and bacteriophages. They can code for drug-resistant enzymes, toxins, or a variety of metabolic enzymes Not capable of independent replication 45  CAPSULE Specialized Structures FLAGELLA Outside the PILI (FIMBRIAE) Cell Wall GLYCOCALYX (SLIME LAYER) 46  The capsule is a gelatinous layer covering the entire bacterium. It is composed of polysaccharide, except in the anthrax bacillus, polymerized D-glutamic acid The sugar components of the polysaccharide Capsule vary from one species of bacteria to another and frequently determine the serologic type within a species. For example, there are 84 different serologic types of Streptococcus pneumoniae, which are distinguished by the antigenic differences of the sugars in the polysaccharide capsule. 47 Capsule  1. It is a determinant of virulence of many bacteria, since it limits the ability of phagocytes to engulf the bacteria. The 2. Specific identification of an organism. In the presence capsule is of the homologous antibody, the capsule will swell greatly. quellung reaction. important 3. Capsular polysaccharides are used as the antigens in certain vaccines. For example, the purified capsular for four polysaccharides of 23 types of S. pneumoniae are present in the current vaccine. reasons 4. The capsule may play a role in the adherence of bacteria to human tissues, which is an important initial step in causing infection. 49  Flagella are long, whiplike appendages that move the bacteria toward nutrients and other attractants, a process called chemotaxis. Flagellated bacteria have a characteristic number and location of flagella: Flagella Some bacteria have one, and others have many; in some, the flagella are located at one end, and in others, they are all over the outer surface. Only certain bacteria have flagella; many rods do, but most cocci do not Spirochetes move by using a flagellum like structure called the axial filament, which wraps around the spiral-shaped cell to produce an undulating motion. 50  In laboratory some bacteria are identified based on their flagella (H) antigens. 51  Flagella are 1. Some species of motile bacteria, e.g., E. coli and Proteus species, are common causes of medically urinary tract infections. Flagella may play a role in pathogenesis by propelling the important bacteria up the urethra into the bladder. for two 2. Some species of bacteria, e.g., Salmonella species, are identified in the clinical reasons: laboratory by the use of specific antibodies against flagellar proteins. 52  Pili are hairlike filaments that extend from the cell surface. They are shorter and straighter than flagella. They are found mainly on gram-negative organisms. Pili Pili have two important roles: (Fimbriae) They mediate the attachment of bacteria to specific receptors on the human cell surface. Mutants of Neisseria gonorrhoeae that do not form pili are nonpathogens. A specialized kind of pilus, the sex pilus, forms the attachment between the donor and the recipient bacteria during conjugation. 53  The glycocalyx is a polysaccharide coating that is secreted by many bacteria. It covers surfaces like a film and allows the Glycocalyx bacteria to adhere firmly to various (Slime structures, e.g., skin, heart valves, and catheters. It also mediates adherence of Layer) certain bacteria, such as Streptococcus mutans, to the surface of teeth. This plays an important role in the formation of plaque, the precursor of dental caries. 54  These highly resistant structures are formed in response to adverse conditions by two genera of medically important gram-positive rods: The genus Bacillus, which includes the agent of anthrax, and the genus Clostridium, which includes the agents of tetanus and botulism. Spores Spore formation (sporulation) occurs when nutrients, such as sources of carbon and nitrogen, are depleted The spore forms inside the cell and contains bacterial DNA, a small amount of cytoplasm, cell membrane, peptidoglycan, very little water, and most importantly, a thick, keratin-like coat 55  Endospore: Central; B. Anthracis, Terminal; C. Tetani, Subterminal; C.botulinum Close to the terminal 56 57  58 Review of Medical Microbiology and Immunology, Fourteenth Editi, Copyright © 2016 by McGraw Review of Medical Microbiology and Immunology, Fourteenth Editi, Copyright © 2016 by McGraw 59  For any question [email protected] Dr. Ayham Abulaila 60