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Transcript
Introduction to Microbiology Microbiology is the study of microorganisms, a large and diverse group of microscopic organisms which must be viewed with a microscope that exist as single cells or cell clusters; it also includes viruses, which are microscopic but not cellular . Importance of microbiology The importance of microbiology includes: used in biomedical research, creation of medicines, environmental applications and new research tools. Bacteria are important for fixing N2 in a usable form for plants. Bacteria and some fungi are important in decomposition and recycling of materials. Industry applications of microbiology: waste management, food industry, mining, medicine, research and biotechnology. Anatomy of bacteria The bacterial cell is a prokaryote cell which is simpler, and therefore smaller, than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. Nuclear material of prokaryotic cell consist of a single chromosome which is in direct contact with cytoplasm. Here the undefined nuclear region in the cytoplasm is called nucleoid. A prokaryotic cell has two architectural regions: • On the outside, flagelig and pilli project from the cell’s surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells; • Enclosing the cell is the cell envelope — generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea). The cell wall consists of peptidoglycan in.bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Extra cellular structures *Capsule The cell capsule is a very large structure this a gelatinous structure is present in some bacteria outside the cell wall. It is considered a virulence factor because it enhances the ability of bacteria to cause disease . The capsule may be polysaccharide as in pneumococci , meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci . The capsule is antigenic. The capsule has antiphagocytic function so it determines the virulence of many bacteria. It also plays a role in attachment of the organism to mucous membranes Demonstration of Capsule 1. India ink staining: the capsule appears as a clear halo around the bacterium as the ink can't penetrate the capusule. 2. Serological methods: Capsular material is antigenic and can be demonstrated by mixing it with a specific anticapsular serum. When examined under the microscope, the capsule appears 'swollen' due to an increase in its refractivity. This phenomenon is called as capsule swelling reaction or Quellung phenomenon. 3. Special capsule staining: These techniques employ copper salts as mordants for staining the capsule . For Vaccination capsular material is effective against some organisms (e.g., H. influenzae type b and S. pneumoniae). *Flagella Flagella are the organelles of cellular motility. They arise from cytoplasm and extrude through the cell wall. They are long and thick thread-like appendages, protein in nature called flagellin. Three types of arrangement are known: monotrichous (single polar flagellum), lophotrichous (multiple polar flagella), and peritrichous (flagella distributed over the entire cell). staining with basic fuchsin makes the flagella visible in the light microscope *Fimbriae (pill) They are short and thin hair like filaments, formed of protein called pilin (antigenic). Fimbriae are responsible for attachment of bacteria to specific receptors of human cell (adherence). There are special types of pili called (sex pili) involved in conjunction. *Cilia Cilia are especially notable on the single-celled protozoans. They are composed of extensions of the cell membrane that contain microtubules, they move materials like these present in respiratory system that consists of mucus-secreting cells lining the trachea and bronchi, and ciliated epithelial cells that move the mucus ever-upward . *Cell Wall The bacterial cell wall is strength layer composed of a substance variously referred to as murein, mucopeptide, or peptidoglycan (all are synonyms). In addition to giving osmotic protection, the cell wall plays an essential role in cell division as well as serving as a primer for its own biosynthesis. Various layers of the wall are the sites of major antigenic determinants of the cell surface, and one component—the lipopolysaccharide of gram-negative cell walls—is responsible for the nonspecific endotoxin activity of gram-negative bacteria. The Peptidoglycan Layer Peptidoglycan is a complex polymer consisting of three parts: a backbone, composed of alternating N-acetylglucosamine and Nacetylmuramic acid; a set of identical tetrapeptide side chains attached to N-acetylmuramic acid; and a set of identical peptide cross-bridges . The tetrapeptide side chains of all species, however, have certain important features in common. Most have L-alanine at position 1 (attached to N-acetylmuramic acid), D-glutamate or substituted Dglutamate at position 2, and D-alanine at position 4. Position 3 is the most variable one: Most gram-negative bacteria have diaminopimelic acid at this position, to which is linked the lipoprotein cell wall component. Gram-positive bacteria usually have L-lysine at position 3; however, some may have diaminopimelic acid or another amino acid at this position. Diaminopimelic acid is a unique element of bacterial cell walls. It is never found in the cell walls of Archaea or eukaryotes. Diaminopimelic acid is the immediate precursor of lysine in the bacterial biosynthesis of that amino acid The fact that all peptidoglycan chains are cross-linked means that each peptidoglycan layer is a single giant molecule. In grampositive bacteria, there are as many as 40 sheets of peptidoglycan, comprising up to 50% of the cell wall material; in gram-negative bacteria, there appears to be only one or two sheets, comprising 5–10% of the wall material. Special Components of Gram-Positive Cell Walls Most gram-positive cell walls contain considerable amounts of 1teichoic acid containing glycerophosphate or ribitol phosphate residues. These polyalcohols are connected by phosphodiester linkages and usually have other sugars and D-alanine attached .There are two types of teichoic acids: wall teichoic acid (WTA), covalently linked to peptidoglycan, and membrane teichoic acid(MTA), covalently linked to membrane glycolipid. Because the latter are intimately associated with lipids, they have been called lipoteichoic acids (LTA) and 2teichuronic acid, which may account for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell. In addition .The teichuronic acids are similar polymers, but the repeat units include sugar acids (such as N-acetylmannosuronic or D-glucosuronic acid) instead of phosphoric acids. They are synthesized in place of teichoic acids when phosphate is limiting. some gram-positive walls may contain polysaccharide molecules. Special Components of Gram-Negative Cell Walls Outer Membrane The outer membrane is chemically distinct from all other biological membranes. It is a bilayered structure ; it has special channels, consisting of protein molecules called porins, that permit the passive diffusion of low-molecular-weight hydrophilic compounds like sugars, amino acids, and certain ions. Lipopolysaccharide (LPS) The LPS of gram-negative cell walls consists of a complex glycolipid, called lipid A, to which is attached a polysaccharide made up of a core and a terminal series of repeat units . Lipid A consists of phosphorylated glucosamine disaccharide units to which are attached a number of long-chain fatty acids .Hydroxymyristic acid, a 14 fatty acid, is always present and is unique to this lipid. The polysaccharide cores similar in all gram-negative species that have two characteristic sugars, ketodeoxyoctanoic acid (KDO) and a heptose. LPS, which is extremely toxic to animals, has been called the endotoxin of gram-negative bacteria because it is firmly bound to the cell surface and is released only when the cells are lysed. When LPS is split into lipid A and polysaccharide, all of the toxicity is associated with the former. The O antigen is highly immunogenic in a vertebrate animal. Lipoprotein Molecules of an unusual lipoprotein cross-link the outer membrane and peptidoglycan layers .The lipoprotein contains 57 amino acids, representing repeats of a 15-amino-acid sequence. The Periplasmic Space The space between the inner and outer membranes, called the periplasmic space, contains the peptidoglycan layer and a gel-like solution of proteins. Cell Membranes The boundary of the cell, sometimes called the plasma membrane, separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. This membrane is very selective about what it allows to pass through; this characteristic is referred to as “selective permeability.” For example, it allows oxygen and nutrients to enter the cell while keeping toxins and waste products out. The plasma membrane is a double phospholipid membrane, or a lipid bilayer, with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer surfaces of the membrane. Membrane transport *Passive Transport Across the Cell Membrane Passive transport describes the movement of substances down a concentration gradient and does not require energy use the following . • Bulk flow is the collective movement of substances in the same direction in response to a force, such as pressure. Blood moving through a vessel is an example of bulk flow. • Simple diffusion is the net movement of substances from an area of higher concentration to an area of lower concentration. • Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma membrane. . • Osmosis is the diffusion of water molecules across a selectively permeable membrane. • Dialysis is the diffusion of solutes across a selectively permeable membrane. *Active Transport Across the Cell Membrane Active transport is the movement of solutes against a gradient and requires the expenditure of energy, usually in the form of ATP. Active transport is achieved through : Protein Pumps • Transport proteins in the plasma membrane transfer solutes such as small ions (Na ,K), amino acids, and monosaccharides. • The proteins involved with active transport are also known as ion pumps. * Protein pumps are catalyses in the splitting of ATP to ADP + phosphate, so they are called ATPase enzyme. * The sodium-potassium pump actively moves sodium out of the cell and potassium into the cell. and are essential in transmission of nerve impulses and in muscular contractions. *Vesicular Transport • Vesicles or other bodies in the cytoplasm move macromolecules or large particles across the plasma membrane. Types of vesicular transport include: 1. Exocytosis, which describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. This process is common when a cell produces substances for export. 2. Endocytosis, which describes the capture of a substance outside the cell when the plasma membrane merges to engulf it. The substance subsequently enters the cytoplasm enclosed in a vesicle. There are three kinds of endocytosis: • Phagocytosis or cellular eating, occurs when the dissolved materials enter the cell. The plasma membrane engulfs the solid material, forming a phagocytic vesicle. • Pinocytosis or cellular drinking occurs when the plasma membrane folds inward to form a channel allowing dissolved substances to enter the cell. When the channel is closed, the liquid is encircled within a pinocytic vesicle. • Receptor-mediated endocytosis occurs when specific molecules in the fluid surrounding the cell bind to specialized receptors in the plasma membrane. As in pinocytosis, the plasma membrane folds inward and the formation of a vesicle follows. Eubacteria Lacking Cell Walls These are microorganisms that lack cell walls (commonly called mycoplasmas and comprising the class Mollicutes) and do not synthesize the precursors of peptidoglycan. They are enclosed by a unit membrane, the plasma membrane They resemble the L forms that can be generated from many species of bacteria (notably gram-positive eubacteria); unlike L forms, however, mycoplasmas never revert to the walled state, and there are no antigenic relationships between mycoplasmas and eubacterial L forms.