Microbiology of Viruses PDF - Marjorie Kelly Cowan, Heidi Smith

Chapter 07 Viruses and Prions Microbiology A Systems Approach Seventh Edition Marjorie Kelly Cowan, Heidi Smith © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written co...

Chapter 07 Viruses and Prions Microbiology A Systems Approach Seventh Edition Marjorie Kelly Cowan, Heidi Smith © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 1  The Nature of Viruses Are they organisms, that is, are they alive? What role did viruses play in the evolution of life? What are their distinctive biological characteristics? How can a particle so small, simple, and seemingly insignificant be capable of causing disease and death? What is the connection between viruses and cancer? 2  Infective Biological Entities Best to describe viruses as active or inactive instead of alive or dead Considered not living because most viruses are unable to multiply independently from the host cell Able to direct certain life processes of cells, making them be considered more than inert and lifeless molecules Obligate intracellular parasites requiring to invade a specific host cell and use the cell’s genetic and metabolic machinery to make and release quantities of new viruses 3  Viruses Are Not Just Agents of Disease Viruses sometimes influence their host’s genetic makeup Responsible for shaping the way cells, tissues, bacteria, plants, and animals have evolved to their present forms Between 40% and 80% of the human genome may be remnants of ancient viral infections Viruses are part of our normal microbiome 4 Viruses on the Biological Spectrum Viruses can infect every type of cell: – Bacteria – Algae – Fungi – Protozoa – Plants – Animals Unique Properties of Viruses  Concept Check—Section 7.1 1. True/False: Is best to describe viruses as alive or dead. 2. What are the characteristics of viruses that distinguish them from cellular life? 7  Viral Size Range Ultramicroscopic size: – Smaller than the average bacterium – Electron microscopes are required to detect them Viral Size Range Viewing Viruses ?? Viral Components  Viral Components Capsid: – Shell (protein coat) surrounds the nucleic acid – Nucleocapsid: capsid and nucleic acid together – Capsid made up of subunits called Capsomere Envelope: – Usually a modified piece of the host cell membrane – not seen in all virus’ Naked viruses: – Consist only of a nucleocapsid Nonenveloped and Enveloped Viruses (a) BSIP/Universal Images Group/Getty Images; (b) Kathy Park Talaro Access the text alternative for these images 14  Viral Components Spikes: – Found on both naked and enveloped viruses – Project from either the nucleocapsid or envelope – Allow viruses to dock with their host cells Virion: – Fully formed virus able to establish infection in a host Viral Components The Viral Capsid The Viral Capsid The Viral Capsid The Viral Capsid  The Viral Capsid Complex capsids: Table 7.4 Complex Capsids Complex capsids, only found in the viruses that infect bacteria, may have multiple types of Found in proteins and take shapes that are not symmetrical. They are never enveloped. The bacteriophage, the one pictured on the right is a T4 bacteriophage. viruses that infect bacteria Have multiple types of proteins Take shapes that are not symmetrical Ami Images/Science Source Access the text alternative for these images 21  Nucleic Acids: At the Core of a Virus Genome: the full complement of DNA and RNA carried by a cell. Viruses contain either DNA or RNA but not both. – The number of viral genes is small compared to that of a cell – Possess only the genes necessary to invade host cells and redirect their activity Nucleic Acid  Concept Check True/False: Viruses can be seen with a light microscope. Viral capsids are constructed from ________ subunits.  Multiplication Cycles in Animal Viruses General phases in the life cycle of animal viruses: – Adsorption – Penetration – Uncoating – Synthesis – Assembly – Release from the host cell  Viral Multiplication RNA VIRUS – multiplication in the cytoplasm  Viral Multiplication DNA VIRUS – DNA enters the host nucleus  Adsorption Invasion begins when the virus encounters a susceptible host and adsorbs (attach) specifically to receptor sites on the cell membrane. Adsorption  Adsorption and Host Range Host range: – A virus can attach and thus invade its host cell only through making an exact fit with a specific host molecule (receptor) – Specificity. – Restricted host range: hepatitis B only infects liver cells of humans – Intermediate host range: poliovirus infects intestinal and nerve cells of primates – Broad host range: rabies virus infects various cells of all mammals  Penetration and Uncoating of Animal Viruses Endocytosis (a): entire virus is engulfed by the cell and enclosed in a vacuole or vesicle. Fusion (b): Outer membrane of virus fuses with the cell membrane of the host. (Note: naked virus cannot fuse – no outer membrane). Uncoating: enzymes in the vacuole dissolve the envelope and capsid, releasing the virus into the cytoplasm.  Entry of a Virus into Host Cell a.Endocytosis Naked or enveloped b. Fusion Enveloped only Entry of a Virus into Host Cell Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in presentation mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Slide Show mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http:// get.adobe.com/flashplayer.  Replication of DNA/Synthesis of Proteins and Assembly Viral nucleic acid takes control over the host’s synthetic and metabolic machinery to synthesize nucleic acid & proteins – Mechanism varies depending on whether the virus is a DNA or RNA virus – RNA viruses replicate in the cytoplasm – DNA viruses replicate in the nucleus – See slides 24/25 Materials produced are then Assembled  Release of Mature Viruses Viral budding or exocytosis is the mechanism of release for enveloped virus – Nucleocapsid binds to the membrane – A small pouch is formed – Pinching off of the pouch releases the virus with its envelope – Viruses are shed gradually; what happens to the host cell? ___________ - see next slide. Mechanism of Viral release of a naked virus is by ______; what happens to the host cell?________ Release of Mature Enveloped Viruses  VIRAL REPLICATION https://www.youtube.com/watch? v=uIut0oVWCEg  Damage to the Host Cell and Persistent Infections Cytopathic Effects (CPEs): – Virus-induced damage to the cell that alters its microscopic appearance – Cells can become disoriented, undergo major changes in shape or size, or develop intracellular damage  Damage to the Host Cell Cytopathic effects (cont’d): – Inclusion bodies: Compacted masses of viruses or damaged cell organelles in the nucleus or cytoplasm – Syncytia (singular, syncytium): Fusion of multiple host cells into single large cells containing multiple nuclei Cytopathic Effects Damage to the Host Cell  Persistent Infections A carrier relationship that develops in some cells. – Cell harbors the virus and is not immediately lysed – Provirus: viral DNA incorporated into the DNA of the host – Chronic latent state: periodic activation after a period of viral inactivity Ex. HSV, HIV  Viruses and Cancer Oncogenic viruses: – Experts estimate that up to 20% of human cancers are caused by viruses & bacteria. – http://blogs.discovermagazine.com/80beats/2012/05/09/16- of-cancers-are-caused-by-viruses-or-bacteria/ #.WnYQU2eWzIU Transformation: – Virus carries genes that directly cause cancer – Virus produces proteins that induce a loss of growth regulation in the cell Viruses and Cancer  Viruses That Infect Bacteria Bacteriophage: – Discovered in 1915 by Frederick Twort and Felix d’Herelle – Parasitize every known bacterial species – Often make the bacteria they infect more pathogenic for humans  Bacteriophage “T-even” bacteriophage infect E. coli – Most widely studied bacteriophage – Go through similar stages as animal viruses ** Attach, penetrate, synthesis, assembly, release. **Difference between animal, bacterial virus – whole bacteriophage does not enter the cell – nucleic acid injected. Bacteriophage Life Cycle Bacteriophage Life Cycle Bacteriophage Life Cycle Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in presentation mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Slide Show mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http:// get.adobe.com/flashplayer.  Bacteriophage Life Cycle – 2 choices Lytic phase/cycle: life cycle of bacteriophage that ends in destruction of the bacterial cell or Lysogenic cycle: bacteriophage becomes incorporated into the host cell DNA.  Bacteriophage Life Cycle https://www.youtube.com/watch? v=hFwA0aBX5bE  Lysogeny: The Silent Virus Infection Temperate phages (virus) – Have the ability to undergo adsorption and penetration but do not immediately undergo replication or release – Prophage: phage DNA in the inactive state – Induction: activation of a prophage in a lysogenic cell to undergo the lytic cycle  Lysogeny Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in presentation mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Slide Show mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http:// get.adobe.com/flashplayer.  The Danger of Lysogeny in Human Diseases Occasionally, phage genes in the bacterial chromosome cause the production of toxins or enzymes that cause pathology in the human Lysogenic conversion: – The acquisition of a new trait from a temperate phage – Responsible for the diphtheria toxin, cholera toxin, and botulism toxin Comparison of Bacteriophage and Animal Virus Multiplication  Concept Check List and describe the main steps of animal virus replication. What is the difference between DNA virus replication and RNA virus replication? True/False: Viruses that infect animal cells can never infect human cells. Describe the differences between the lytic and lysogenic phase of viral replication.  Techniques in Cultivating and Identifying Animal Viruses In vivo methods: viral cultivation in lab animals (mice, rats, hamsters) or embryonic bird tissues. In vitro methods: viral cultivation in cell or tissue culture.  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. Using Bird Embryos  Viruses and Human Health Impossible to measure the number of viral infections worldwide. Most common cause of acute infections that do not result in hospitalization: – Colds, chickenpox, influenza, herpes, warts Viral infections that only occur in certain regions: – Dengue fever, Rift Valley fever, Yellow fever  Viruses and Human Health Some have high mortality rates: – Rabies – Ebola Other viral infections lead to long-term disability: – Polio – Neonatal rubella  Viruses and Human Health Viruses mutate at a rapid rate. Difficult to design therapies against viruses. WHY? Scientists focus on developing vaccines against viruses since so few antiviral drugs are available and antibiotics are ineffective.  Viruses and Human Health Interferon (IFN): – Naturally occurring human cell product – Used with some success in preventing and treating viral infections  Interferon Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in presentation mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Slide Show mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http:// get.adobe.com/flashplayer.  Concept Check True/False: Viral infections are more common than bacterial infections. True/False: Viral mutation rates are low and viruses are easy to track in a population. Doctors and scientists most often rely on _________ to treat or prevent viral infections: – Antibiotics – Antiviral drugs – Vaccines – Herbal remedies