General Microbiology for Dental Students

Welcome back Growing Animal Viruses in the Laboratory These methods involve using living animals, embryonated eggs, or cell cultures. Viral Identification Viruses can’t be seen without the use of an electron microscope. Serological methods, such as Western blotting, are the...

Welcome back Growing Animal Viruses in the Laboratory These methods involve using living animals, embryonated eggs, or cell cultures. Viral Identification Viruses can’t be seen without the use of an electron microscope. Serological methods, such as Western blotting, are the most commonly used means of identification where, the virus is detected and identified by its reaction with antibodies. Observation of cytopathic effects (effects occurring upon viral growth in cell cultures) is also useful in identification. Virologists can identify and characterize viruses by using modern molecular methods such as PCR. Viruses and Cancer Several types of cancers can be caused by viruses. Viruses capable of inducing tumors in animals are called oncogenic viruses, or oncoviruses. The genetic material of oncogenic viruses integrates into the host cell’s DNA and replicates along with the host cell’s chromosome. Oncogenic Viruses are responsible for 10 % of human cancers.  Oncogenic viruses Papilloma virus (HPV) → cervical cancer Epstein-Barr virus (EBV) (HV4) → Burkitt’s lymphoma Hepatitis B and C virus (HBV) → liver cancer African Burkitt’s lymphoma Prions Small proteinaceous infectious particles (very resistant to inactivation by radiations). Inherited and transmissible by ingestion, transplant, and surgical instruments. Causes nine neurological diseases in animals called spongiform encephalopathies because large vacuoles develop in the brain): e.g. mad cow disease These diseases are caused by the conversion of a normal host glycoprotein called PrPC (for cellular prion protein) into an infectious form called PrPSc (for scrapie protein). Fragments of PrPSc molecules accumulate in the brain, forming plaques.  Prions Caused by altered protein: Mutation in normal PrPc gene (sporadic CJD) Creutzfeldt- Kuru infected Jakob disease boy. Papua, New ‘CJD’ Guinea Fatal familial insomnia ‘FFI’ Prion Animal Infections Deer infected with Sheep infected with “chronic wasting scrapie. disease.”CWD Bovine spongiform encephalopathy, “mad cow disease.”BSE  Bacteria (bacterium) Single-celled prokaryotes The most common shapes of bacteria are; ‘Bacillus’ (rod-like) ‘coccus’ (spherical or ovoid), and spiral (corkscrew or curved) Peptidoglycan (carbohydrate and protein complex) cell walls. Divide by Binary fission. For nutrition and energy production; use organic chemicals, inorganic chemicals, or photosynthesis. Many bacteria can “swim” by using moving appendages called flagella. Bacterial Cell Structure  The bacterial glycocalyx is a viscous (sticky), gelatinous polymer that is external to the cell wall and composed of polysaccharide, polypeptide, or both. Types; Slime layer Capsule Functions: Protect cells from dehydration and nutrient loss. Inhibit killing by white blood cells by phagocytosis contributing to pathogenicity. Attachment - formation of biofilms. Slime Layer Glycoproteins loosely attached to cell surface. Slime layers cause bacteria to adhere to solid surfaces and help protect the cell from drying out and loss of nutrients. In Streptococcus mutans, slime layer allows accumulation on tooth enamel Capsule Polysaccharides firmly attached to the cell wall. Capsules adhere to solid surfaces and to nutrients in the environment Capsules are important in contributing to bacterial virulence. Capsules can protect a bacterial cell from destruction by white blood cells (phagocytosis). Capsules make bacteria more slippery, helping the bacterium to escape engulfment by phagocytic cells. E.g., Streptococcus pneumoniae  Biofilm Photo courtesy of National Institutes of Health A biofilm is an aggregation of microorganisms growing on a solid substrate, in which cells are stuck to each other and/or to a surface. The glycocalyx is a very important component of biofilms. A glycocalyx that helps cells in a biofilm attach to their target environment and to each other is called an extracellular polymeric substance (EPS). Biofilms may form on living or non-living surfaces. An example of biofilms includes Streptococcus mutans causing dental caries.  Flagella Are long filamentous appendages that are responsible for movement. Not all prokaryotes have flagella. Bacterial cells can have one or more flagella. Flagella proteins are antigens –useful for distinguishing among strains within the species.  Functions: motility enables a bacterium to move toward a favorable environment or away from an adverse one. The movement of a bacterium toward or away from a particular stimulus is called taxis: chemical stimuli – chemotaxis; light stimuli – phototaxis  Fine, sticky, proteinaceous, hair-like bristles from the cell surface. May be hundreds per cell and can occur at the poles of the bacterial cell or can be evenly distributed over the entire surface of the cell. Function: adhesion to one another, to hosts, and to substances in environment, used to attach the bacteria to target cells in infection (microbes colonize mucous membranes and cause disease) or to surfaces, where they form a biofilm. When fimbriae are absent (because of genetic mutation), colonization cannot happen, and no disease takes place.  Rigid tubular structure longer than fimbriae but shorter than flagella. Bacteria typically only have one or two per cell. Function: involved in motility and DNA transfer by conjugation. Bacterial Cell Structure The cell wall A complex, semirigid structure responsible for the shape of the cell. Almost all prokaryotes have a cell wall that surrounds the underlying, fragile plasma (cytoplasmic) membrane and protects it and the interior of the cell from adverse changes in the outside environment. The cell wall (cont’d) Function: Prevent rupturing when the water pressure inside the cell is greater than that outside the cell, helps maintain the shape of a bacterium serves as a point of anchorage for flagella. Clinically; it contributes to the ability of some species to cause disease and is the site of action of some antibiotics. Some eukaryotes have cell walls, but they differ in composition from those of prokaryotes.  The cell wall (cont’d) Bacteria are divided into two major groups: Gram-positive and Gram-negative, based on their reaction to Gram staining. Gram-positive bacteria: (thick cell wall): ❑ thick peptidoglycan ❑ cell membrane Gram stained bacteria Gram-negative bacteria (thinner cell wall): ❑ outer cell membrane ❑ thin peptidoglycan layer ❑ cell membrane De Agostini Picture Library / Getty Images  Cell wall composition Mainly of Peptidoglycan (also known as murein). Composed of repeating units of disaccharides cross-linked by polypeptides. The disaccharide backbone (glycan portion of peptidoglycan) is made up of alternating monosaccharides derivatives of glucose: N- acetylglucosamine (NAG) and N-acetlymuramic acid (NAM) in equal amounts and short amino acid chain.  The penicillins interfere with the last step of bacterial cell wall synthesis (transpeptidation or cross-linkage exposing the osmotically less stable membrane.  Cell Wall of Gram Positive Bacteria Consists of several layers of peptidoglycan forming a thick, rigid structure. Running perpendicular to the peptidoglycan sheets is a group of molecules called teichoic acids and lipoteichoic acids. Teichoic acid gives the wall antigenic specificity allowing laboratory identification of Gram positive bacteria. Cell Wall of Gram- Negative Bacteria Composed of an outer membrane and thin peptidoglycan layer. Peptidoglycan makes up only 5 – 20% of the cell wall, and is not the outermost layer, but lies between the plasma membrane and an outer membrane.  Cell Wall of Gram- Negative Bacteria The periplasm is the region between the outer membrane and the plasma membrane. it is a gel-like fluid that contains high concentration of degradative enzymes and transport proteins.  The outer membrane of the gram-negative cell consists of lipopolysaccharides (LPS), lipoproteins, and phospholipids. LPS is a large, complex molecule that contains lipids and carbohydrates and consists of three components: Lipid A - Core polysaccharide - O polysaccharide. The outer membrane strong negative charge is an important factor in evading phagocytosis. The outer membrane provides a barrier to detergents, heavy metals, bile salts, certain dyes, antibiotics, and digestive enzymes such as lysozyme.  Gram- Negative Bacteria Outer Membrane LPS layer Lipid A When gram-negative bacteria die, they release lipid A, which functions as an endotoxin Lipid A is responsible for the symptoms associated with infections by gram-negative bacteria, such as fever, dilation of blood vessels, shock, and blood clotting. Core polysaccharide; is attached to lipid A and contains unusual sugars. Its role is to provide stability. O polysaccharide; is composed of sugar molecules. It is antigenic and is useful for distinguishing species.  The Porins in Gram negative Porins are found in the outer membranes of Gram-negative bacteria. Porins are proteins that form pores of a fixed diameter, that permit small molecules such as nucleotides, disaccharides, peptides, amino acids, vitamin B12, and iron to pass through the outer membrane. E.g., the omp C and omp F porins that form channels through outer membrane for passage of hydrophilic molecules.