Lecture 10 - Virology PDF

Because learning changes everything. ® Chapter 06 Viruses and Other Acellular Infectious Agents Prescott’s Microbiology Twelfth edition Joanne Willey, Kathleen Sandman, Dorothy Wood BIOL1030...

Because learning changes everything. ® Chapter 06 Viruses and Other Acellular Infectious Agents Prescott’s Microbiology Twelfth edition Joanne Willey, Kathleen Sandman, Dorothy Wood BIOL1030 Biochemistry & Microbiology Ms. Aleksandra Belovanovic © 2023 McGraw Hill, LLC. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill, LLC.  Outline 1. Define the terms virology, bacteriophage, and phage 2. Virion Structure 3. Viral Life Cycles 4. Several Types of Viral Infections 5. Emerging Infectious Diseases and SARS-CoV-2 6. Similarities and Differences Between Viruses and Cells © McGraw Hill, LLC 2  1. Viruses Virology—the study of viruses. Viruses—infectious agents unique in their simple, acellular organization and pattern of multiplication. Major causes of disease. In 2020, the entire world learned the impact viruses have on human health as S A R S-CoV-2 reached pandemic proportions. Associated with a number of plant, animal, and human diseases. Can only reproduce using the metabolic machinery of the host cell. May have a DNA or RNA genome. Obligate intracellular parasites Cannot reproduce outside a living cell Are either active or inactive, instead of living or nonliving © McGraw Hill, LLC 3  History Invention of the electron microscope allowed these infectious agents to be seen for the first time. French chemist Louis Pasteur suggested that something smaller than a bacterium was the cause of rabies. Pasteur used the word “virus,” Latin for poison. © McGraw Hill, LLC 4  Extracellular or Intracellular Viruses Extracellular viruses Inactive Cannot reproduce outside of living cells. Intracellular viruses Commandeer host cells and use them to synthesize viral components from which mature progeny viruses are assembled and released. © McGraw Hill, LLC 5  Viruses Can Infect All Cell Types Most are eukaryotic viruses. Infect plants, animals, protists, and fungi. Bacteriophages (phages)—infect bacteria. Few archaeal viruses has been identified. © McGraw Hill, LLC 6  2. The Structure of Viruses Each type has at least two parts (capsid and core). Virion—mature virus particle. Virions range in size. 20 nm in diameter Size of rod-shaped bacterial cell (1.5 × 0.5 μm). Most viruses must be viewed with an electron microscope. Contents Nucleocapsid—composed of nucleic acid (D N A or R N A) and a protein coat (capsid). Enveloped viruses—lipid membrane. Nonenveloped viruses (naked viruses) © McGraw Hill, LLC 7  Size and Virion Morphology of Selected Viruses Access the text alternative for slide images. © McGraw Hill, LLC 8  Capsids Large macromolecular structures which serve as protein coat of virus. Made of protein subunits called protomers. Protect viral genetic material and aid in its transfer between host cells. Nonenveloped viruses construct a capsid. Adenovirus and T-even Bacteriophage Capsid is constructed from many copies of one protein. Enveloped viruses require nucleocapsid proteins and extra proteins to anchor to the membrane. Capsids are helical, icosahedral, or complex. Tobacco Mosaic and Influenza Viruses Figure 20.1 © McGraw Hill, LLC 9  Helical Capsids Shaped like hollow tubes with protein walls. Protomers self assemble into rigid tube. Size of capsid is influenced by promoters and genome. (a) Photo Researchers/Science History Images/Alamy Stock Photo © McGraw Hill, LLC 10  Icosahedral Capsids Most efficient way to enclose a space. An icosahedron is a regular polyhedron with 20 equilateral triangular faces and 12 vertices. Capsomers—Ring or knob-shaped units made of 5 or 6 protomers. Pentamers (pentons)—5 promoter capsomers. Hexamers (hexons)—6 promoter capsomers. Biophoto Associates/Science Source © McGraw Hill, LLC 11  Viral Envelopes and Enzymes Many viruses are bound by an outer, flexible, membranous layer called an envelope. Animal virus envelopes (lipids and carbohydrates) usually arise from host cell plasma or organelle membranes. Heather Davies/Science Source Envelope proteins, which are viral encoded, may project from the envelope surface as spikes or peplomers. Spikes involved in viral attachment to host cell. Surface proteins can have enzymatic activity needed for entry or exit from host cell. Used for identification of virus. CDC/Dr. Fred Murphy/Sylvia Whitfield Access the text alternative for slide images. © McGraw Hill, LLC 12  Classification System of Viruses Viruses mutate rapidly and are not living organisms, they are difficult to classify and name A classification system similar to that used for living organisms Differs because it includes only the taxonomic levels of order—family, genus, and species—not the higher levels of kingdom, phylum, and class Over 2,500 species of viruses have been identified Viral genomes are structurally diverse A virus may have single- or double-stranded D N A or R N A The size of viral genome varies 4,000 nucleotides to 2 million nucleotides Genomes can be linear or circular Some R N A viruses have segmented genomes © McGraw Hill, LLC 13  3. One-Step Growth Curves Early experiments in 1939 by Delbruck and Ellis with bacteriophage T4 and E. coli host cells. Illustrated an eclipse period where no new virus was present in host cells. Also showed burst size— the number of new viruses produced per infected cell. First steps and evidence to understanding viral life cycle steps. Access the text alternative for slide images. © McGraw Hill, LLC 14  Viral Multiplication - Replication Mechanism used depends on viral structure and genome. Five steps: Attachment (adsorption) Penetration (entry into the host) Synthesis Maturation (assembly) Virion release Access the text alternative for slide images. © McGraw Hill, LLC 15  Attachment (Adsorption) Viruses require a host cell in which to multiply. Ligand (on virion) attaches to a receptor (on host). The SARS-CoV-2 spike protein is the viral ligand that attaches to the human receptor called ACE2. Receptor determines host preference: Tropism—viruses will bind to specific tissue receptors. In plants, no receptors have been found, instead damage of the host cell is required for entry. © McGraw Hill, LLC 16  Penetration (Entry Into the Host) After attachment, the virus’s genome or entire nucleocapsid enters the cytoplasm. In some cases, only the genome enters leaving the capsid attached to the outside of the cell. Other times, the genome is still enclosed. Three methods used: Fusion of the viral envelope with host cell’s plasma membrane. Endocytosis Release of nucleic acid. © McGraw Hill, LLC 17  Synthesis This is the step that differs the most between viruses. Genome dictates the events. dsDNA follows typical synthesis. Transcription and translation by host. RNA viruses Virus must carry in or synthesize the proteins necessary to complete synthesis. Viral replication complexes—enclose machinery needed for genome replication. Tightly regulated gene expression and protein synthesis. © McGraw Hill, LLC 18  Maturation (Assembly) Late proteins are involved in assembly. Assembly process is complex. Baseplate, tail fibers, and head components of bacteriophage T4 are assembled separately. Access the text alternative for slide images. © McGraw Hill, LLC 19  Virion Release Mechanisms Two mechanisms: Host cell lysis (nonenveloped viruses) Release by budding (enveloped viruses) (a) Lee D. Simon/Science Source; (b) Courtesy of Susan Brumfield and Mark Young Access the text alternative for slide images. © McGraw Hill, LLC 20  4. Lytic and Lysogenic Cycles Bacteriophages—Viruses that infect bacterial cells Lysogenic bacteria—infected bacteria There are two types of bacteriophage life cycles. The lytic cycle (Virulent phage) Viral reproduction occurs. The host cell undergoes lysis. Hundreds of virus particles are released. The lysogenic cycle (Temperate phage) Viral reproduction does not occur immediately but may occur in the future. Virus becomes integrated into the host genome and may reenter lytic cycle. This is known as latency, and the latent viral DNA is called a prophage. Diphtheria is caused by a prophage-carrying bacterium, which produces a toxin that damages the lining of the upper respiratory tract, restricting breathing. © McGraw Hill, LLC 21  Lytic and Lysogenic Cycles Access the text alternative for slide images. © McGraw Hill, LLC 22  Reproduction of Animal Viruses Retrovirus HIV Retroviruses (HIV, the virus that causes AIDS) Contain reverse transcriptase. Carries out RNA → cDNA reverse transcription. cDNA becomes integrated into host DNA. Replicated as host DNA replicates. HIV may remain latent for years. Viral DNA is transcribed; new viruses are produced. © McGraw Hill, LLC Figure 20.3 23  Reproduction of Animal Viruses Flu Virus Spikes of a Seasonal Flu and Bird Flu Virus A flu virus has an H (hemagglutinin) spike and an N (neuraminidase) spike. H spike allows the virus to bind to the receptor. Sixteen different types N spike attacks host plasma membranes. Allows mature viruses to exit the cell Nine different types Each type of spike can occur in different varieties. Our immune system only recognizes H spikes and N spikes it has been exposed Figure 20A to. © McGraw Hill, LLC 24  5. Emerging Infectious Diseases and SARS-CoV-2 Emerging viruses are new or previously uncommon illnesses. Examples are coronavirus (COVID), Zika, Middle East respiratory syndrome (MERS), AIDS, West Nile encephalitis, Ebola hemorrhagic fever, and avian influenza (bird flu). Several types of events can cause emergence of viruses. A virus may extend its range. Example: West Nile was transported to the United States and took hold in birds and mosquitoes. A genetic mutation may occur. Example: Influenza strains H5N1, H1N1, and H7N9 were created through mutation of flu viruses, which only infected animals. It is necessary to obtain flu vaccine each year due to the rapidly mutating flu virus. © McGraw Hill, LLC 25  Emerging Viruses Illustrated Figure 20.4 © McGraw Hill, LLC 26  Zika Virus Zika virus is spread by a species of Aedes mosquitoes, the same mosquitoes that spread dengue and chikungunya viruses. Once a person is infected, they can pass the virus back to new mosquitoes when bitten, spreading the infection. Zika can spread from mother to fetus, causing microencephaly. Figures 20B and 20C (Left) Source: Centers for Disease Control and Prevention. “Facts about Microcephaly.” Accessed February 11, 2020. https://www.cdc.gov/ncbddd/birthdefects/microcephaly.html. (Right) Source: Centers for Disease Control and Prevention. "Potential Range of Aedes aegypti and Aedes albopictus in the United States, 2017" https://www.cdc.gov/mosquitoes/mosquito-control/professionals/range.html © McGraw Hill, LLC 27  Coronaviruses and COVID-19 Coronavirus was first recorded in humans in the 1950s. SARS-CoV first appeared in China in 2002, though no human cases have been reported since 2004. M E R S-CoV first appeared in Jordan in 2012. SARS-CoV-2, the respiratory virus that causes COVID-19, originated in 2019 in Wuhan, China. Originally called 2019-nCoV and Novel Coronavirus 2019 Single-stranded animal RNA virus High mutation rates, like Ebola and influenza Exterior envelope made from the plasma membrane of the host cell and broken down by soaps and alcohol- based sanitizers Embedded in membrane is the “crown,” a series of glycoproteins: S protein, M protein, E protein Figure 20.5 © McGraw Hill, LLC 28  Life Cycle of SARS-CoV-2 Figure 20.6 © McGraw Hill, LLC 29  6. Similarities and Differences Between Viruses and Cells Similarities Genetic Material Reproduction Evolutionary Adaptation Differences Cellular Structure Metabolism Size © McGraw Hill, LLC 30  Viruses Do More than Cause Disease Typically thought to be major causes of disease, but there are other important uses. Vital members of the aquatic ecosystem. Can be used to destroy cancer cells. Bacteriophages in human guts may regulate bacterial microbiome. Important model organisms. © McGraw Hill, LLC 31  Take Home Message This material is a complement to the chapters on the three domains of life. The items in this chapter fall outside of the three domains, but are very important members of the microbial world. Understanding what each member of the acellular entities is, how they function, and how they replicate can round out your greater understanding of the microbial world. Try to think of how these entities fit into the overall scheme of microbes on Earth—how are they similar, how are they different. © McGraw Hill, LLC 32 Because learning changes everything. ® www.mheducation.com © 2023 McGraw Hill, LLC. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill, LLC.