Download presentation
Presentation is loading. Please wait.
1
Microbiology: A Systems Approach, 2nd ed.
Chapter 6: An Introduction to the Viruses
2
6.1 The Search for the Elusive Virus
Viruses were too small to be seen with the first microscopes The cause of viral infections was unknown for years Louis Pasteur first proposed the term virus 1890s Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus Loeffler and Frosch discovered an animal virus that causes foot –and-mouth disease in cattle Many years of experimentation showed what we know today and by the 1950s virology had grown
3
6.2 The Position of Viruses in the Biological Spectrum
Can infect every type of cell Cannot exist independently from the host cell, so aren’t considered living things However, since they can direct life processes they are often considered more than lifeless molecules Referred to as infectious particles, either active or inactive 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
5
6.3 The General Structure of Viruses
Figure 6.1
6
Size Range Smallest infectious agents
Most are so small, they can only be seen with an electron microscope Animal viruses Proviruses- around 20 nm in diameter Mimiviruses- up to 450 nm in length Viewing viruses Special stains and an electron microscope Negative staining outlines the shape Positive staining shows internal details Shadowcasting technique
7
Figure 6.2
8
Viral Components: Capsids, Nucleic Acids, and Envelopes
Molecular structure- composed of regular, repeating subunits that give rise to their crystalline appearance Contain only those parts needed to invade and control a host cell External coating Capsid Envelope- in 13 of the 20 families of animal viruses If no envelope, called naked virus Core DNA RNA The capsid and the nucleic acid together are called the nucleocapsid Fully formed virus that is able to establish an infection in a host cell- virion
9
Figure 6.4
10
The Viral Capsid: The Protective Outer Shell
Constructed from identical subunits called capsomers Made up of protein molecules Two different types Helical Rod-shaped capsomers Assemble in to helical nucleocapsid
11
Figure 6.5
12
Figure 6.6
13
Icosahderal Three-dimensional, 20-sided figure with 12 evenly spaced corners Although they all display this symmetry, there are wide variations
14
Figure 6.7
15
Figure 6.8
16
Figure 6.9
17
Figure 6.10
18
The Viral Envelope Enveloped viruses take a bit of the host cell membrane in the form of an envelope In the envelope, some or all of the regular membrane proteins are replaced with viral proteins Some proteins form a binding layer between the envelope and the capsid Glycoproteins remain exposed as spikes (peplomers)- essential for attachment
19
Functions of the Viral Capsid/Envelope
Protects nucleic acids Help introduce the viral DNA or RNA into a suitable host cell Stimulate the immune system to produce antibodies that can protect the host cells against future infections
20
Nucleic Acids: At the Core of a Virus
Genome- the sum total of the genetic information carried by an organism Number of viral genes compared with a call- quite small They only have the genes necessary to invade host cells and redirect their activity Some viruses are exceptions to the rules re: DNA and RNA Parvoviruses contain single-stranded DNA Reoviruses contain double-stranded RNA
21
DNA Viruses ssDNA dsDNA linear circular
22
RNA Viruses Mostly single-stranded
Positive-sense RNA: genomes that are ready for immediate translation into proteins Negative-sense RNA: genomes have to be converted into the proper form to be made into proteins Segmented- individual genes exist on separate pieces of RNA
25
Other Substances in the Virus Particle
Can contain enzymes for specific operations within the host cell Polymerases to synthesize DNA and RNA Replicases to copy RNA
26
6.4 How Viruses are Classified and Named
Main criteria Structure Chemical composition Similarities in genetic makeup International Committee on the Taxonomy of Viruses, 2000 3 orders 63 famillies “-viridae” 263 genera “-virus” Some virologists use a species naming system, but it is not an official designation
29
6.5 Modes of Viral Multiplication
The host cell is absolutely necessary for viral multiplication
30
Figure 6.11
31
Multiplication Cycles in Animal Viruses
Adsorption Penetration Uncoating Synthesis Assembly Release
32
Adsorption Virus encounters susceptible host cells
Adsorbs specifically to receptor sites on the cell membrane Because of the exact fit required, viruses have a limited host range
33
Figure 6.12
34
Penetration Flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid Endocytosis: entire virus engulfed by the cell and enclosed in a vacuole or vesicle The viral envelope can also directly fuse with the host cell membrane
35
Figure 6.13
36
Uncoating Enzymes in the vacuole dissolve the envelope and capsid
The virus is now uncoated
37
Synthesis Free viral nucleic acid exerts control over the host’s synthetic and metabolic machinery DNA viruses- enter host cell’s nucleus where they are replicated and assembled DNA enters the nucleus and is transcribed into RNA The RNA becomes a message for synthesizing viral proteins (translation) New DNA is synthesized using host nucleotides RNA viruses- replicated and assembled in the cytoplasm
38
Assembly Mature virus particles are constructed from the growing pool of parts
39
Release Nonenveloped and complex viruses are released when the cell lyses or ruptures Enveloped viruses are liberated by budding or exocytosis Anywhere from 3,000 to 100,000 virions may be released, depending on the virus Entire length of cycle- anywhere from 8 to 36 hours
40
Figure 6.15
41
Damage to the Host Cell and Persistent Infections
Cytopathic effects- virus-induced damage to the cell that alters its microscopic appearance Inclusion bodies- compacted masses of viruses or damaged cell organelles
42
Figure 6.16
44
Important for the diagnosis of viral infections
Some viral infections maintain a carrier relationship The cell harbors the virus and is not immediately lysed Persistent infections- from a few weeks to the remainder of the host’s life Some viruses remain in a chronic latent state, periodically becoming activated Some viruses enter their host cell and permanently alter its genetic material, leading to cancer Oncogenic viruses Their effect is called transformation Oncoviruses- mammalian viruses capable of initiating tumors
45
Viruses that Infect Bacteria
Bacteriophage Most contain dsDNA Often make the bacteria they infect more pathogenic for humans
46
T-even Phages Icosahedral capsid head containing DNA
Central tube surrounded by a sheath Collar Base plate Tail pins Fibers Similar stages as animal viruses Adsorb to host bacteria The nucleic acid penetrates the host after being injected through a rigid tube inserted through the bacterial membrane and wall Entry of the nucleic acid causes the cessation of host cell DNA replication and protein synthesis The host cell machinery is then used for viral replication and synthesis of viral proteins As the host cell produces new parts, they spontaneously assemble
47
Figure 6.17
48
Figure 6.18
49
Figure 6.19
50
Lysogeny: The Silent Virus Infection
Temperate phages- special DNA phages that undergo adsorption and penetration but are not replicated or released immediately Instead the viral DNA enters an inactive prophage stage Lysogeny: the cell’s progeny will also have the temperate phage DNA Lysogenic conversion: when a bacterium acquires a new trait from its temperate phage
51
Figure 6.20
53
6.6 Techniques in Cultivating and Identifying Animal Viruses
Primary purposes of viral cultivation To isolate and identify viruses in clinical specimens To prepare viruses for vaccines To do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells Using Live Animal Inoculation Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits Occasionally invertebrates or nonhuman primates are used Animal is exposed to the virus by injection Using Bird Embryos Enclosed in an egg- nearly perfect conditions for viral propagation Chicken, duck, and turkey are most common Egg is injected through the shell using sterile techniques
54
Figure 6.21
55
Using Cell (Tissue) Culture Techniques
Most viruses are propagated in some sort of cell culture The cultures must be developed and maintained Animal cell cultures are grown in sterile chambers with special media Cultured cells grow in the form of a monolayer Primary or continuous
56
Figure 6.22
57
6.7 Medical Importance of Viruses
Most common cause of acute infections that do not result in hospitalization Most do not cause death but those that do can have very high mortality rates Others can lead to long-term debility
58
6.8 Other Noncellular Infectious Agents
Spongiform encephalopathies Chronic, persistent diseases Long period of latency Deposition of protein fibrils in the brain tissue- prions Satellite viruses Defective forms of viruses Dependent on other viruses for replication Viroids Parasitize plants Very small Composed only of naked strands of RNA
59
6.9 Treatment of Animal Viral Infections
Because they are not bacteria, antibiotics are ineffective Antiviral drugs block virus replication by targeting one of the steps in the viral life cycle Interferon shows potential for treating and preventing viral infections Vaccines stimulate immunity
Similar presentations
