Lecture IVe- Survey of Microorganisms-Eukaryotes: Viruses PDF

Lecture IVe- Survey of Microorganisms- Eukaryotes: VIRUSES MTN Cabasa...

Lecture IVe- Survey of Microorganisms- Eukaryotes: VIRUSES MTN Cabasan 1 CASE A woman brings her 8-month-old daughter into the urgent care clinic where you work. The baby has a runny nose and a fever of 39°C. The child’s ears and lungs are free of infection. The woman is annoyed because she has already taken her daughter to see her pediatrician. She requested an antibiotic, but the doctor refused to prescribe it. She once again requests that the baby receive antibiotics. Will you prescribe the medication the mother wants? Explain why or why not—and how you would discuss this with the mother. 2 1  5) VIRUSES 3 Objectives: 1)Differentiate a virus from a bacterium 2)Describe the chemical and physical structure of both an enveloped and a nonenveloped virus. 3) Define viral species and give an example of viruses. 4) Differentiate virus, viroid, and prion. 4 2  Viruses Contain a single type of nucleic acid, either DNA or RNA. Contain a protein coat (sometimes itself enclosed by an envelope of lipids, proteins, and carbohydrates) that surrounds the nucleic acid. Multiply inside living cells by using the synthesizing machinery of the cell. Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells. 5 Viruses have few or no enzymes of their own for metabolism; for example, they lack enzymes for protein synthesis and ATP generation. To multiply, viruses must take over the metabolic machinery of the host cell. 6 3  Gerard J. Tortora(2018) Microbiology: an introduction 7 Host range of a virus the spectrum of host cells the virus can infect: invertebrates, vertebrates, plants, protists, fungi, and bacteria Viruses that infect bacteria are called bacteriophages, or phages. 8 4  Host range of a virus Determined by: the virus’s requirements for its specific attachment to the host cell & availability within the potential host of cellular factors required for viral multiplication. For the virus to infect the host cell, the outer surface of the virus must chemically interact with specific receptor sites on the surface of the cell. The two complementary components are held together by weak bonds, such as hydrogen bonds. The combination of many attachment and receptor sites leads to a strong association between host cell and virus. 9 10 5  Viral Structure A virion is a complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat outside a host cell. Viruses are classified by their nucleic acid and by differences in the structures of their coats 11 Nucleic Acid Viral genes are encoded by either DNA or RNA— but never both. The genome of a virus can be single-stranded or double-stranded. Thus, there are viruses with: - double-stranded DNA - single-stranded DNA - double stranded RNA - single-stranded RNA Depending on the virus, the nucleic acid can be linear or circular. In some viruses (such as the influenza virus), the nucleic acid is in several separate segments 12 6  Capsid and Envelope The nucleic acid of a virus is protected by a protein coat called the capsid The structure of the capsid is determined by the viral nucleic acid and accounts for most of the mass of a virus, especially of small ones. Each capsid is composed of protein subunits called capsomeres. In some viruses, the proteins composing the capsomeres are of a single type; in other viruses, several types of protein may be present. 13 In some viruses, the capsid is covered by an envelope, which usually consists of some combination of lipids, proteins, and carbohydrates. Some animal viruses are released from the host cell by an extrusion process that coats the virus with a layer of the host cell’s plasma membrane; that layer becomes the viral envelope. In many cases, the envelope contains proteins determined by the viral nucleic acid and materials derived from normal host cell components. Depending on the virus, envelopes may or may not be covered by spikes, which are carbohydrate-protein complexes that project from the surface of the envelope. Some viruses attach to host cells by means of spikes. 14 7  Viruses whose capsids aren’t covered by an envelope are known as nonenveloped viruses. The capsid of a nonenveloped virus protects the nucleic acid from nuclease enzymes in biological fluids and promotes the virus’s attachment to susceptible host cells. 15 16 8  General Morphology- by electron microscopy and X-ray crystallography 1) Helical Viruses Helical viruses resemble long rods that may be rigid or flexible. The viral nucleic acid is found within a hollow, cylindrical capsid that has a helical structure. The viruses that cause rabies and Ebola are helical viruses. 17 General Morphology- by electron microscopy and X-ray crystallography 2) Polyhedral Viruses Many animal, plant, and bacterial viruses are polyhedral, or many sided, viruses. The capsid of most polyhedral viruses is in the shape of an icosahedron(a), a regular polyhedron with 20 triangular faces and 12 corners. The capsomeres of each face form an equilateral triangle. An example of a polyhedral virus in the shape of an icosahedron is the adenovirus (b). Another icosahedral virus is the poliovirus. 18 9  General Morphology- by electron microscopy and X-ray crystallography 3) Enveloped Viruses Enveloped viruses; roughly spherical When helical or polyhedral viruses are enclosed by envelopes, they are called enveloped helical or enveloped polyhedral viruses. An example of an enveloped helical virus is the influenza virus. An example of an enveloped polyhedral (icosahedral) virus is the human herpes virus. 19 General Morphology- 4) Complex Viruses Bacteriophage Some bacteriophages have capsids to which additional structures are attached: capsid (head) - is polyhedral and that the tail sheath is helical. The head contains the nucleic acid. Poxviruses- don’t contain clearly identifiable capsids but do have several coats around the nucleic acid 20 10  Gerard J. Tortora(2018) Microbiology: an introduction 21 Gerard J. Tortora(2018) Microbiology: an introduction 22 11  Gerard J. Tortora(2018) Microbiology: an introduction 23 24 12 25 26 13  Viroids infectious pieces of RNA that cause some plant diseases. short pieces of naked RNA, only 300 to 400 nucleotides long, with no protein coat. Some viroids, called virusoids, are enclosed in a protein coat. Virusoids cause disease only when the cell is infected by a virus. Linear and circular potato spindle tuber viroid (PSTV). Plant viruses must enter plant hosts through wounds or with invasive parasites, such as insects. 27 Prions infectious proteins Prion diseases involve the degeneration of brain tissue. Prion diseases are the result of an altered protein; the cause can be a mutation in the normal gene for PrPC or contact with an altered protein (PrPSc) 28 14