Chapter 3 Bacteria and Archaea ©McGraw-Hill Education Form and Function of Bacteria and Archaea How bacteria and archaea are different from eukaryotes: • The way their DNA is packaged: lack of nucleus and histones • The makeup of their cell wall: peptidoglycan and other unique chemicals • Their internal structures: lack of membrane-bound organelles ©McGraw-Hill Education The Structure of the Bacterial Cell All bacterial cells possess: • Cytoplasmic membrane • Cytoplasm • Ribosomes • Cytoskeleton • One (or a few) chromosome(s) Most bacterial cells possess: • Cell wall • A surface coating called a glycocalyx ©McGraw-Hill Education Structures Found in Some Bacteria Some, but not all bacterial cells possess: • Flagella, pili, and fimbriae • An outer membrane • Nanowires/nanotubes • Plasmids • Inclusions • Endospores • Microcompartments ©McGraw-Hill Education Structure of a Bacterial Cell Jump to long description ©McGraw-Hill Education Bacterial Shapes and Arrangements Most bacteria function as independent singlecelled, unicellular organisms: • Some act as a group in colonies or biofilms • Some communicate through nanotubes Bacteria have an average size of 1 μm (micron): • Cocci: circumference of 1 μm • Rods: length of 2 μm and a width of 1 μm Pleomorphism: variation in the size and shape of cells of a single species due to nutritional and genetic differences ©McGraw-Hill Education Bacterial Shapes Jump to long description ©McGraw-Hill Education Source: CDC/Janice Haney Carr (a); Source: CDC/Janice Haney Carr (b); Source: CDC/Janice Haney Carr (c); Source: USDA/Photo by De Wood. Digital colorization by Chris Pooley (d); ©VEM/Science Source (e); ©Eye of Science/Science Source (f) Bacterial Arrangements: Cocci Arrangement of cocci: • Single • Diplococci: pairs • Tetrads: groups of four • Staphylococci or micrococci: irregular clusters • Streptococci: chains • Sarcina: cubical packet of eight, sixteen, or more cells ©McGraw-Hill Education Arrangement of Cocci Jump to long description ©McGraw-Hill Education Bacterial Arrangements: Bacilli Arrangement of bacilli: • Single • Diplobacilli: pair of cells with ends attached • Streptobacilli: chain of several cells ©McGraw-Hill Education ©De Agostini/Getty Images External Structures Appendages: • Motility: flagella and axial filaments • Attachment points or channels: fimbriae, pili, and nanotubes/nanowires Flagellum: • Primary function is motility • Three distinct parts: • Filament • Hook • Basal body ©McGraw-Hill Education Flagellum of a Gram-Negative Cell Jump to long description ©McGraw-Hill Education Sarkar MK1, Paul K, Blair D., “Chemotaxis signaling protein CheY binds to the rotor protein FliN to control the direction of flagellar rotation in Escherichia coli,” PNAS May 18, 2010 vol. 107 no. 20 9370-9375 (b) Arrangement of Flagella Polar arrangement: flagella attached at one or both ends of the cell • Monotrichous: single flagellum • Lophotrichous: small bunches or tufts of flagella emerging from the same site • Amphitrichous: flagella at both poles of the cell Peritrichous arrangement: flagella are dispersed randomly over the surface of the cell ©McGraw-Hill Education Types of Flagellar Arrangements Jump to long description ©McGraw-Hill Education ©Science Photo Library/Alamy Stock Photo (a); Source: CDC/ Melissa Brower (b); ©Heather Davies/Science Source (c); ©Smith Collection/Gado/Getty Images (d) Fine Points of Flagellar Function Chemotaxis: movement of bacteria in response to chemical signals: • Positive chemotaxis: movement toward favorable chemical stimulus • Negative chemotaxis: movement away from a repellant • Run: rotation of flagellum counterclockwise, resulting in a smooth linear direction • Tumble: reversal of the direction of the flagellum, causing the cell to stop and change course ©McGraw-Hill Education Operation of Flagella Jump to long description ©McGraw-Hill Education Chemotaxis in Bacteria Jump to long description ©McGraw-Hill Education Periplasmic Flagella Spirochetes: corkscrew-shaped bacteria: • Possess an unusual, wriggly mode of locomotion due to periplasmic flagella Periplasmic flagella: • Also called axial filaments • Internal flagellum enclosed in the space between the cell wall and the cytoplasmic membrane ©McGraw-Hill Education Appendages for Attachment or Channel Formation: Fimbriae Fimbria/fimbriae: • Small, bristle-like fibers sprouting off the surface of many bacterial cells • Allow tight adhesion between fimbriae and epithelial cells, allowing bacteria to colonize and infect host tissues ©McGraw-Hill Education ©Eye of Science/Science Source Appendages for Attachment or Channel Formation: Pili and Nanotubes Pilus/pili: • Used in conjugation between bacterial cells • Well characterized in gram-negative bacteria • Type IV pilus can transfer genetic material, act like fimbriae and assist in attachment, and act like flagella and make a bacterium motile ©McGraw-Hill Education Conjugating Process ©McGraw-Hill Education ©L. Caro/SPL/Science Source S Layer and Glycocalyx S layer: • Single layers of thousands of copies of a single protein linked together like chain mail • Only produced when bacteria are in a hostile environment Glycocalyx: • Coating of repeating polysaccharide or glycoprotein units • Slime layer: loose, protects against loss of water and nutrients • Capsule: more tightly bound, denser, and thicker; produce a sticky (mucoid) character to colonies on agar ©McGraw-Hill Education Position of Bacterial S Layer ©McGraw-Hill Education ©Russell Kightley/Science Source Encapsulated Bacteria ©McGraw-Hill Education CDC (a); ©Michael Abbey/Science Source (b) Specialized Functions of the Glycocalyx Capsules: • Formed by many pathogenic bacteria • Have greater pathogenicity • Protect against phagocytosis Biofilms: • Plaque on teeth protects bacteria from becoming dislodged • Responsible for persistent colonization of plastic catheters, IUDs, metal pacemakers, and other implanted medical devices ©McGraw-Hill Education Biofilm Formation Jump to long description ©McGraw-Hill Education ©Scimat/Science Source (b) The Cell Envelope Lies outside the cytoplasm Composed of two or three basic layers that each perform a distinct function, but together act as a single protective unit: • Cell wall • Cytoplasmic membrane • Outer membrane (in some bacteria) ©McGraw-Hill Education Comparison of Gram-Positive and Gram-Negative Cell Envelopes Jump to long description ©McGraw-Hill Education ©Dr. Kari Lounatmaa/Science Source (gram-positive cell.); ©Dennis Kunkel Microscopy, Inc./Medical Images (gram-negative cell) The Cell Wall Helps determine the shape of a bacterium Provides strong structural support to keep the bacterium from bursting or collapsing because of changes in osmotic pressure: • Certain drugs target the cell wall, disrupting its integrity and causing cell lysis (disintegration or rupture) of the cell Gains its relative rigidity from peptidoglycan ©McGraw-Hill Education Peptidoglycan Compound composed of a repeating framework of long glycan (sugar) chains cross-linked by short peptide (protein) fragments Provides a strong but flexible support framework Jump to long description ©McGraw-Hill Education Gram-Positive Cell Wall Thick, homogenous sheet of peptidoglycan: • 20 to 80 nm in thickness Contains teichoic acid and lipoteichoic acid: • Function in cell wall maintenance and enlargement • Contribute to the acidic charge on the cell surface ©McGraw-Hill Education Gram-Negative Cell Wall Single, thin sheet of peptidoglycan: • 1 to 3 nm in thickness Thinness gives gram-negative cells greater flexibility and sensitivity to lysis ©McGraw-Hill Education Steps in a Gram Stain Jump to long description ©McGraw-Hill Education ©McGraw-Hill Education Nontypical Cell Walls: Acid-Fast Bacteria Mycobacterium and Norcardia: contain peptidoglycan and stain gram-positive, but bulk of cell wall is composed of unique lipids Mycolic acid: • Very-long-chain fatty acid • Found in the cell walls of acid-fast bacteria • Contributes to the pathogenicity of the bacteria • Makes bacteria highly resistant to certain chemicals and dyes ©McGraw-Hill Education Nontypical Cell Walls: Archaea Some have cell walls composed entirely of polysaccharides Others have cell walls made of pure protein All lack true peptidoglycan structure Some lack a cell wall entirely ©McGraw-Hill Education Mycoplasmas and Other Cell-Wall-Deficient Bacteria Mycoplasmas: • Naturally lack a cell wall • Sterols in the cell membrane stabilize the cell against lysis • Mycoplasma pneumoniae: “walking pneumonia” L forms: • Some bacteria that naturally have a cell wall but lose it during part of their life cycle • Role in persistent infections • Resistant to antibiotics ©McGraw-Hill Education The Gram-Negative Outer Membrane Similar in composition to most membranes, except it contains specialized polysaccharides and proteins Lipopolysaccharide: • Signaling molecules and receptors • Endotoxin Porin proteins: • Special membrane channels that only allow certain chemicals to penetrate ©McGraw-Hill Education Cytoplasmic Membrane Structure A lipid bilayer with proteins embedded Regulates transport of nutrients and wastes Selectively permeable: special carrier mechanisms for passage of most molecules ©McGraw-Hill Education Differences in Cell Envelope Structure Outer membrane of gram-negative bacteria contributes an extra barrier: • Resistant to certain antimicrobial chemicals • More difficult to inhibit or kill than gram-positive bacteria Alcohol-based compounds dissolve lipids in the outer membrane and therefore damage the cell: • Alcohol swabs used to cleanse the skin before certain medical procedures Treatment of infections caused by gram-negative bacteria requires drugs that can cross the outer membrane ©McGraw-Hill Education The Cytoplasm 70 to 80% water Complex mixture of sugars, amino acids, and salts Serves as a pool for building blocks for cell synthesis or sources of energy ©McGraw-Hill Education Bacterial Chromosomes and Plasmids The hereditary material of most bacteria exists in the bacterial chromosome DNA is aggregated in the nucleoid Plasmids: • Nonessential pieces of DNA • Confer protective traits such as drug resistance and toxin and enzyme production ©McGraw-Hill Education Ribosomes Site of protein synthesis ©McGraw-Hill Education Bacterial Ribosome Jump to long description ©McGraw-Hill Education Inclusion Bodies and Microcompartments Used for food storage Pack gas into vesicles for buoyancy Store crystals of iron oxide with magnetic properties Bacterial microcompartments: • Outer shells made of protein, arranged geometrically • Packed full of enzymes designed to work together in biochemical pathways ©McGraw-Hill Education The Cytoskeleton Some bacteria produce long polymers of protein similar to eukaryotic cells for the cytoskeleton: • Arranged in helical ribbons around the cell • Contribute to cell shape • Have also been identified in archaea • Unique to non-eukaryotic cells – may be a potential target for antibiotic development ©McGraw-Hill Education Bacterial Endospores Dormant bodies Produced by Bacillus, Clostridia, and Sporosarcina Vegetative cell: metabolically active Sporulation: induced by environmental conditions Endospores resist extremes of heat, drying, freezing, radiation, and chemicals that would kill vegetative cells ©McGraw-Hill Education ©Science Source Sporulation Process in Bacillus Species Jump to long description ©McGraw-Hill Education ©Science Source The Medical Significance of Bacterial Endospores Bacillus anthracis: agent of anthrax Clostridium tetani: cause of tetanus Clostridium perfringens: cause of gas gangrene Clostridium botulinum: cause of botulism Clostridium difficile: “C. diff,” a serious gastrointestinal disease ©McGraw-Hill Education Archaea: The Other “Prokaryotes” Considered a third cell type in a separate superkingdom More closely related to domain Eukarya than bacteria: • Share rRNA sequences not found in bacteria • Protein synthesis and ribosomal subunit structures are similar ©McGraw-Hill Education Archaea Differ from Other Cell Types Extremophiles: • Some live at extremely high or low temperatures • Some need extremely high salt or acid concentrations to survive • Some live on sulfur or methane Some live on the human body and may be capable of causing human disease ©McGraw-Hill Education Comparison of Three Cellular Domains Characteristic Bacteria Chromosomes Single or a few, circular Single, circular Types of ribosomes 70S 70S but structure is similar to 80S 80S Contains unique ribosomal RNA signature sequences + + + Eukarya(-like) protein synthesis − + + Cell wall made of peptidoglycan + − − Cytoplasmic membrane lipids Fatty acids with ester linkages Long-chain, branched hydrocarbons with ether linkages Fatty acids with ester linkages Sterols in membrane − (some exceptions) − + Nucleus and membrane-bound organelles No No Yes Flagellum Bacterial flagellum Archaellum Eukaryotic flagellum ©McGraw-Hill Education Archaea Eukarya Multiple, linear Chapter 5 Viral Structure and Multiplication ©McGraw-Hill Education The Position of Viruses in the Biological Spectrum Viruses infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals Seawater can contain 10 million viruses per milliliter For many years, the cause of viral infections was unknown: • Louis Pasteur hypothesized that rabies was caused by a “living thing” smaller than bacteria • He also proposed the term virus, which is Latin for “poison” ©McGraw-Hill Education The Viral Debate Two sides of the debate: • Since viruses are unable to multiply independently from the host cell, they are not living things and should be called infectious molecules • Even though viruses do not exhibit most of the life processes of cells, they can direct them, and thus are certainly more than inert and lifeless molecules Viruses are better described as active or inactive rather than alive or dead ©McGraw-Hill Education The Vital Role of Viruses in Evolution Infect cells and influence their genetic makeup Shape the way cells, tissues, bacteria, plants, and animals have evolved 8% of the human genome consists of sequences that come from viruses 10 to 20% of bacterial DNA contains viral sequences Obligate intracellular parasites: • Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses ©McGraw-Hill Education Properties of Viruses (1) Are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals Are ultramicroscopic in size, ranging from 20 nm up to 1,000 nm (diameter) Are not cells; structure is very compact and economical Do not independently fulfill the characteristics of life Basic structure consists of protein shell (capsid) surrounding nucleic acid core ©McGraw-Hill Education Properties of Viruses (2) Nucleic acid can be either DNA or RNA, but not both Nucleic acids can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA Molecules on virus surfaces give them high specificity for attachment to host cell Multiply by taking control of host cell’s genetic material and regulating the synthesis and assembly of new viruses Lack enzymes for most metabolic processes Lack machinery for synthesizing proteins ©McGraw-Hill Education How Viruses Are Classified and Named For many years, animal viruses were classified on the basis of their hosts and the diseases they caused Newer classification systems emphasize the following: • Hosts and diseases they cause • Structure • Chemical composition • Similarities in genetic makeup ©McGraw-Hill Education Virus Size Range Smallest infectious agents Smallest viruses: parvoviruses around 20 nm in diameter Largest viruses: herpes simplex virus around 150 nm in length Some cylindrical viruses can be relatively long (800 nm) but are so narrow in diameter (15 nm) that their visibility is limited without an electron microscope ©McGraw-Hill Education Size Comparison of Viruses with a Eukaryotic Cell (Yeast) and Bacteria Jump to long description ©McGraw-Hill Education Viral Architecture Is Best Observed with Special Stains and Electron Microscopy Jump to long description ©McGraw-Hill Education Source: CDCl/Dr. F. A. Murphy (a); ©Phototake (b); ©A.B. Dowsette/SPL/Science Source (c) Viral Components (1) Viruses bear no resemblance to cells and lack any of the protein-synthesizing machinery found in cells Viral structure is composed of regular, repeating subunits that give rise to their crystalline appearance The structure contains only those parts needed to invade and control a host cell: • External coating • Core containing one or more nucleic acid strains of DNA or RNA • Sometimes one or two enzymes ©McGraw-Hill Education Viral Components (2) Capsid: protein shell that surrounds the nucleic acid: • Nucleocapsid: the capsid together with the nucleic acid • Naked viruses consist only of a nucleocapsid. Envelope: external covering of a capsid, usually a modified piece of the host’s cell membrane Spikes can be found on naked or enveloped viruses: • Project from the nucleocapsid or the envelope • Allow viruses to dock with host cells Virion: a fully formed virus that is able to establish an infection in a host cell ©McGraw-Hill Education Structure of Viruses Jump to long description ©McGraw-Hill Education Viral Capsid Capsid: • Most prominent feature of viruses • Constructed from identical protein subunits called capsomeres • Capsomeres spontaneously self-assemble into the finished capsid Two different types: • Helical • Icosahedral ©McGraw-Hill Education Viral Envelope Enveloped viruses: • Take a bit of the cell membrane when they are released from a host cell Enveloped viruses can bud from: • Cell membrane • Nuclear envelope • Endoplasmic reticulum More flexible than the capsid so enveloped viruses are pleomorphic ©McGraw-Hill Education Helical Capsid Structure Helical Capsids Naked Enveloped The simpler helical capsids have rod-shaped capsomeres that bond together to form a series of hollow discs resembling a bracelet. During the formation of the nucleocapsid, these discs link with other discs to form a continuous helix into which the nucleic acid strand is coiled. The nucleocapsids of naked helical viruses are very rigid and tightly wound into a cylinder-shaped package. An example is the tobacco mosaic virus, which attacks tobacco leaves. Enveloped helical nucleocapsids are more flexible and tend to be arranged as a looser helix within the envelope. This type of morphology is found in several enveloped human viruses, including influenza, measles, and rabies. Naked Capsids ©McGraw-Hill Education Enveloped Capsids ©Science Source, Source: CDC/Dr. Fred Murphy Icosahedral Capsid Structure Icosahedral Capsids These capsids form an icosahedron (eye″-koh-suh-hee′-drun)—a threedimensional, 20-sided figure with 12 evenly spaced corners. The arrangements of the capsomeres vary from one virus to another. Some viruses construct the capsid from a single type of capsomere, while others may contain several types of capsomeres. There are major variations in the number of capsomeres; for example, a poliovirus has 32, and an adenovirus has 252 capsomeres. Naked Adenovirus is an example of a naked icosahedral virus. In the photo you can clearly see the spikes, some of which have broken off. Enveloped Two very common viruses, hepatitis B virus and the herpes simplex virus, possess enveloped icosahedrons. Naked Capsids ©McGraw-Hill Education Enveloped Capsids ©Dr. Linda M. Stannard, University of Cape Town/Science Source, ©Dr. Linda M. Stannard, University of Cape Town/Science Source (hep B virus); ©Eye of Science/Science Source Complex Capsid Structure Complex Capsids ©McGraw-Hill Education Complex capsids, only found in the viruses that infect bacteria, may have multiple types of proteins and take shapes that are not symmetrical. They are never enveloped. The one pictured on the right is a T4 bacteriophage. ©AmiImages/Science Source Nucleic Acids: At the Core of a Virus Genome: the sum total of the genetic information carried by an organism Viruses contain DNA or RNA, but not both The number of viral genes is quite small compared with that of a cell: • Four genes in hepatitis B virus • Hundreds of genes in some herpesviruses • Possess only the genes needed to invade host cells and redirect their activity ©McGraw-Hill Education Variety in Viral Nucleic Acid DNA viruses: Single-stranded (ss) or double-stranded (ds; linear or circular) RNA viruses: can be double-stranded, but more often single-stranded: • Positive-sense RNA: ready for immediate translation • Negative-sense RNA: must be converted before translation can occur • Segmented: individual genes exist on separate pieces of RNA • Retroviruses: carry their own enzymes to create DNA out of their RNA ©McGraw-Hill Education Viral Nucleic Acid Virus Name Disease It Causes Variola virus Smallpox Herpes simplex II Genital herpes Parvovirus Erythema infectiosum (skin condition) DNA Viruses Examples Double-stranded DNA Single-stranded DNA RNA Viruses–Examples Single-stranded (+) sense Poliovirus Poliomyelitis Single-stranded (−) sense Influenza virus Influenza Double-stranded RNA Rotavirus Gastroenteritis Single-stranded RNA + reverse transcriptase HIV AIDS ©McGraw-Hill Education Other Substances in the Virus Particle Enzymes for specific operations within their host cell: • Polymerases that synthesize DNA and RNA • Replicases that copy RNA • Reverse transcriptase synthesizes DNA from RNA Completely lack the genes for synthesis of metabolic enzymes Some viruses carry away substances from their host cell: • Arenaviruses pack along host ribosomes • Retroviruses borrow the host’s tRNA molecules ©McGraw-Hill Education Lytic Replication Cycle in Animal Viruses General phases of the animal lytic viral replication cycle: • Adsorption(Attachment) • Penetration • Uncoating • Synthesis • Assembly • Release The length of the replication cycle varies from 8 hours in polioviruses to 36 hours in herpesviruses ©McGraw-Hill Education Adsorption(Attachment) A virus can invade its host cell only through making an exact fit with a specific host molecule Host range: the limited range of cells that a virus can infect: • Hepatitis B: liver cells of humans • Poliovirus: intestinal and nerve cells of primates • Rabies: various cells of all mammals Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus Tropisms: specificities of viruses for certain tissues ©McGraw-Hill Education Viral Attachment Process Jump to long description ©McGraw-Hill Education Penetration and Uncoating The flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid Penetration through endocytosis happens when an entire virus is engulfed by the cell and enclosed in a vacuole or vesicle Direct fusion of the viral envelope with the host cell membrane: • Envelope merges directly with the cell membrane, liberating the nucleocapsid into the cell’s interior ©McGraw-Hill Education Penetration by Animal Viruses Jump to long description ©McGraw-Hill Education Synthesis: Replication and Protein Production DNA viruses: • Enter the host cell’s nucleus and are replicated and assembled there RNA viruses: • Replicated and assembled in the cytoplasm Retroviruses turn their RNA genomes into DNA ©McGraw-Hill Education Assembly and Release Assembly: virus is put together using “parts” manufactured during the synthesis process Release: the number of viruses released by infected cells is variable, controlled by: • Size of the virus • Health of the host cell Poxvirus-infected cell: 3,000 to 4,000 virions Poliovirus-infected cell: 100,000 virions Immense potential for rapid viral proliferation ©McGraw-Hill Education Maturation and Release of Enveloped Viruses ©McGraw-Hill Education ©Chris Bjornberg/Science Source (b) Lytic Cycle of Animal Viruses (1) 1. Adsorption(Attachment) • The virus encounters a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane • The membrane receptors that viruses attach to are usually proteins that the cell requires for its normal function • Glycoprotein spikes on the envelope (or on the capsid of naked viruses) bind to the cell membrane receptors 2. Penetration and Uncoating • In this example, the entire virus is engulfed (endocytosed) by the cell and enclosed in a vacuole or vesicle • When enzymes in the vacuole dissolve the envelope and capsid, the virus is said to be uncoated, a process that releases the viral nucleic acid into the cytoplasm ©McGraw-Hill Education Life Cycle of Animal Viruses (2) 3. Synthesis: Replication and Protein Production • Viral nucleic acid begins to synthesize the building blocks for new viruses • Some viruses come equipped with the necessary enzymes for synthesis of viral components; others utilize those of the host • Proteins for the capsid, spikes, and viral enzymes are synthesized on the host’s ribosomes using its amino acids ©McGraw-Hill Education Life Cycle of Animal Viruses (3) 4. Assembly • Mature virus particles are constructed from the growing pool of parts • Capsid is first laid down as an empty shell that will serve as a receptacle for the nucleic acid strand • Viral spikes are inserted into the host’s cell membrane so they can be picked up as the virus buds off with its envelope 5. Release • Assembled viruses leave their host in one of two ways: • Nonenveloped and complex viruses that reach maturation in the cell nucleus or cytoplasm are released when the cell lyses or rupture • Enveloped viruses are liberated by budding from the membranes of the cytoplasm, nucleus, endoplasmic reticulum, or vesicles • During this process, the nucleocapsid binds to the membrane, which curves completely around it and forms a small pouch • Pinching off the pouch releases the virus with its envelope ©McGraw-Hill Education Lysogenic Cycle Persistent Infections Some cells maintain a carrier relationship: cell harbors the virus and is not immediately lysed: • Can last from a few weeks to the remainder of the host’s life • Can remain latent in the cytoplasm Provirus: • Viral DNA incorporated into the DNA of the host • Measles virus Chronic latent state: • Periodically become activated under the influence of various stimuli • Herpes simplex and herpes zoster viruses ©McGraw-Hill Education Damage to the Host Cell Cytopathic effects (CPEs): virus-induced damage to the cell that alters its microscopic appearance Types of CPEs include: • Gross changes in shape and size • Development of intracellular changes • Inclusion bodies: compacted masses of viruses or damaged cell organelles in the nucleus and cytoplasm • Syncytia: fusion of multiple damaged host cells into single large cells containing multiple nuclei (giant cells) Accumulated damage from a virus infection kills most host cells ©McGraw-Hill Education Cytopathic Changes Jump to long description ©McGraw-Hill Education Source: CDC (a); Courtesy Massimo Battaglia, INeMM CNR, Rome Italy (b) Viruses and Cancer (1) Experts estimate that 13% of cancers are caused by viruses Transformation: the effect of oncogenic, or cancer-causing viruses: • Some viruses carry genes that directly cause cancer • Other viruses produce proteins that induce a loss of growth regulation, leading to cancer ©McGraw-Hill Education Viruses and Cancer (2) Transformed cells: • Increased rate of growth • Changes in their chromosomes • Changes in cell’s surface molecules • Capacity to divide indefinitely Oncoviruses: mammalian viruses capable of initiating tumors: • Papillomaviruses • Herpesviruses • Hepatitis B virus • HTLV-I ©McGraw-Hill Education Viruses That Infect Bacteria Bacteriophage: “bacteria eating”: • Most contain double-stranded DNA, but some RNA types exist as well • Every bacterial species is parasitized by various specific bacteriophages • The bacteria they infect are often more pathogenic for humans ©McGraw-Hill Education T-Even Bacteriophage Infect E. coli Structure: • Icosahedral capsid containing DNA • Central tube surrounded by a sheath • Collar • Base plate • Tail pins • Fibers Jump to long description ©McGraw-Hill Education Events in the Lytic Cycle of T-even Bacteriophages (1) ©McGraw-Hill Education Events in the Lytic Cycle of T-even Bacteriophages (2) ©McGraw-Hill Education Lysogenic Cycle: The Silent Virus Infection Temperate phages: • • Undergo adsorption and penetration Do not undergo replication or release immediately Viral DNA enters an inactive prophage state: • • • Inserted into bacterial chromosome Copied during normal bacterial cell division Lysogeny: a condition in which the host chromosome carries bacteriophage DNA Induction: prophage in a lysogenic cell becomes activated and progresses directly into viral replication and the lytic cycle ©McGraw-Hill Education The Role of Lysogeny in Human Disease Occasionally, phage genes in the bacterial chromosome cause the production of toxins or enzymes that the bacterium would not otherwise have Lysogenic conversion: when a bacterium acquires a new trait from its temperate phage: • Corynebacterium diphtheriae – diphtheria toxin • Vibrio cholerae – cholera toxin • Clostridium botulinum – botulinum toxin ©McGraw-Hill Education Techniques in Cultivating and Identifying Animal Viruses Viruses require living cells as their “medium”: • In vivo: laboratory-bred animals and embryonic bird tissues • In vitro: cell or tissue culture methods Primary purposes of viral cultivation: • Isolate and identify viruses in clinical specimens • Prepare viruses for vaccines • Do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells ©McGraw-Hill Education Using Live Animal Inoculation Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits are the usual choices for viral cultivation Occasionally, invertebrates such as insects or nonhuman primates are used Because viruses exhibit host specificity, certain animals can propagate viruses more readily than others ©McGraw-Hill Education Using Bird Embryos Bird eggs containing embryos: • Intact and self-supporting unit • Sterile environment • Contain their own nourishment Chicken, duck, and turkey eggs are the most common choices for inoculation Viruses are injected through the eggshell by drilling a small hole or making a small window ©McGraw-Hill Education Chicken Egg Used to Culture a Virus ©McGraw-Hill Education Using Cell (Tissue) Culture Techniques Isolated animal cells are grown in vitro in cell or tissue culture rather than in an animal or egg Cell culture, or tissue culture: • Grown in sterile chambers with special media that contain the correct nutrients for cells to survive • Cells form a monolayer, or single, confluent sheet of cells that supports viral multiplication • Allows for the close inspection of culture for signs of infection ©McGraw-Hill Education Detection of Viral Growth in Culture Observation of degeneration and lysis of infected cells Plaques: areas where virus-infected cells have been destroyed show up as clear, well-defined patches in the cell sheet: • Visible manifestation of cytopathic effects (CPEs) ©McGraw-Hill Education Normal and Infected Cell Culture Jump to long description ©McGraw-Hill Education Source: Bakonyi T, Lussy H, Weissenböck H, Hornyák A, Nowotny N. Emerging Infectious Diseases, Vol. 11, No. 2, Feb. 2005. Viroids Virus-like agents that parasitize plants About one-tenth the size of an average virus Composed of naked strands of RNA, lacking a capsid or any other type of coating Significant pathogens in economically important plants: tomatoes, potatoes, cucumbers, citrus trees, chrysanthemums ©McGraw-Hill Education Potato Infected With a Virus ©McGraw-Hill Education
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