microbiology ch 13 lecture_presentation
Post on 09-Jan-2017
1.217 Views
Preview:
TRANSCRIPT
© 2015 Pearson Education, Inc.
Chapter 13
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Viruses
• Minuscule, acellular, infectious agents having either DNA or RNA
• Cause infections of humans, animals, plants, and bacteria
• Cause most of the diseases that plague the industrialized world
• Cannot carry out any metabolic pathway
• Neither grow nor respond to the environment
• Cannot reproduce independently
• Recruit the cell's metabolic pathways to increase their numbers
• No cytoplasmic membrane, cytosol, organelles
• Have extracellular and intracellular state
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Extracellular state• Called virion• Protein coat (capsid) surrounding nucleic acid• Nucleic acid and capsid, also called nucleocapsid• Some have phospholipid envelope• Outermost layer provides protection and recognition
sites for host cells
• Intracellular state• Capsid removed• Virus exists as nucleic acid
© 2015 Pearson Education, Inc.
Figure 13.1 Virions, complete virus particles, include a nucleic acid, a capsid, and in some cases an envelope.
Capsid (sectionedto show interior)
Nucleic acid(viral genome)
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Genetic Material of Viruses• Show more variety in their genomes than do cells
• Primary way scientists categorize and classify viruses
• May be DNA or RNA, but never both
• Can be dsDNA, ssDNA, dsRNA, ssRNA
• May be linear and segmented or single and circular
• Much smaller than genomes of cells
© 2015 Pearson Education, Inc.
Viralgenome
Partial genomeof E. coli
Figure 13.2 The relative sizes of genomes.
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Hosts of Viruses• Most viruses infect only particular host's cells
• Due to affinity of viral surface proteins for complementary
proteins on host cell surface
• May be so specific they infect only a particular kind of
cell in a particular host
• Generalists – infect many kinds of cells in many different
hosts
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Hosts of Viruses• All types of organisms are susceptible to viral attack
• A bacteriophage (phage) is a virus that infects bacteria
• Plant viruses infect many food crops
• Introduced through abrasions of the cell wall or by
plant parasites
• Fungal viruses are not well studied
• Appear to have no extracellular state
© 2015 Pearson Education, Inc.
Figure 13.3 Some examples of plant, bacterial, and human hosts of viral infections.
© 2015 Pearson Education, Inc.
Red blood cell(10,000 nm in diameter)
Bacterialribosomes(25 nm)
Poliovirus(30 nm)
Bacteriophage MS2(24 nm)
Bacteriophage T4(50 nm x 225 nm)
Tobacco mosaic virus(15 nm x 300 nm)
Smallpox virus(200 nm x 300 nm)
E. coli (bacterium)(1000 nm x 3000 nm)
Figure 13.4 Sizes of selected virions.
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Capsid Morphology• Capsids
• Provide protection for viral nucleic acid
• Means of attachment to host's cells
• Composed of proteinaceous subunits called capsomeres
• Capsomere may be made of single or multiple types
of proteins
© 2015 Pearson Education, Inc.
Figure 13.5 The shapes of virions.
© 2015 Pearson Education, Inc.
Figure 13.6 The complex shape of bacteriophage T4.
Tail fibers
Base plate
Tail
Head
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• The Viral Envelope• Acquired from host cell during viral replication or release
• Envelope is portion of membrane system of host
• Composed of phospholipid bilayer and proteins
• Some proteins are virally coded glycoproteins (spikes)
• Envelope proteins and glycoproteins often play role in
host recognition
© 2015 Pearson Education, Inc.
Figure 13.7 Enveloped virion.
Glycoproteins
Helical capsid
Matrix protein
Envelope
Enveloped virus with helical capsid
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Characteristics of Viruses
• Tell Me Why• Why are naked icosahedral viruses able to crystallize?
© 2015 Pearson Education, Inc.
Classification of Viruses
• International Committee on Taxonomy of Viruses
determines virus classification
• Viruses classified by nucleic acid, presence of
envelope, shape, and size
• Relationship among viruses not well understood
• No kingdoms, divisions, or classes have been defined
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Classification of Viruses
• Tell Me Why• What characteristics of the genomes of parvoviruses
and of reoviruses make them very different from cells?
© 2015 Pearson Education, Inc.
Viral Replication
• Dependent on hosts' organelles and enzymes to produce new virions
• Lytic replication• Viral replication usually results in death and lysis of host
cell• Five stages of lytic replication cycle
• Attachment• Entry• Synthesis• Assembly• Release
© 2015 Pearson Education, Inc.
Viral Replication: Overview
© 2015 Pearson Education, Inc.
Figure 13.8 The lytic replication cycle in bacteriophages.
© 2015 Pearson Education, Inc.
Figure 13.9 Pattern of virion abundance in lytic cycle.
© 2015 Pearson Education, Inc.
Viral Replication: Virulent Bacteriophages
© 2015 Pearson Education, Inc.
Viral Replication
• Lysogeny• Modified replication cycle
• Infected host cells grow and reproduce normally for
generations before they lyse
• Temperate phages
• Prophages – inactive phages
• Lysogenic conversion
• Results when phages carry genes that alter phenotype of
a bacterium
© 2015 Pearson Education, Inc.
Figure 13.10 Bacteriophage lambda.
© 2015 Pearson Education, Inc.
Figure 13.11 The lysogenic replication cycle in bacteriophages.
12 3Attachment
Lambdaphage
Entry
Lyticcycle
Release
Assembly
Synthesis
Induction
Prophagein chromosome
Lysogeny
Replication ofchromosomeand virus;cell division
Further replications andcell divisions
12 3
4
5
6
7
8
© 2015 Pearson Education, Inc.
Viral Replication: Temperate Bacteriophages
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Same basic replication pathway as bacteriophages
• Differences result from
• Presence of envelope around some viruses
• Eukaryotic nature of animal cells
• Lack of cell wall in animal cells
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Attachment of animal viruses
• Chemical attraction between viral protein and cell receptor
• Animal viruses do not have tails or tail fibers
• Have glycoprotein spikes or other attachment molecules
that mediate attachment
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Entry and uncoating of animal viruses
• At least three different mechanisms by which animal
viruses enter a cell
• Direct penetration
• Membrane fusion
• Endocytosis
• Viruses that enter cell with capsid intact are uncoated
© 2015 Pearson Education, Inc.
Figure 13.12 Three mechanisms of entry of animal viruses.
Phage genomeinside capsid
Capsid
Viral genomeReceptors oncytoplasmic membrane
Direct penetration
Viralglycoproteins
Receptors oncytoplasmicmembraneof host
Envelope
Viralglycoproteinsremain incytoplasmicmembrane Endocytosis
ViralgenomeUncoatingcapsid
Cytoplasmic membraneof host engulfs virus(endocytosis)
Viral genomeUncoatingcapsid
Membrane fusion
12 3
1 23
4
56
4
32
1
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Synthesis of DNA viruses of animals
• Each type of animal virus requires different strategy,
depending on its nucleic acid
• DNA viruses often enter the nucleus
• RNA viruses often replicate in the cytoplasm
• Must consider
• How mRNA is synthesized
• What serves as template for nucleic acid replication
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Synthesis of DNA viruses of animals
• dsDNA viruses
• Similar to replication of cellular DNA
• Viral genome replicated in the nucleus
• Viral proteins are made in the cytoplasm
• Some exceptions
• Poxvirus replication occurs in the cytoplasm
• Hepatitis B viruses replicate DNA from an RNA
intermediary
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Synthesis of DNA viruses of animals
• ssDNA viruses
• Cells do not use ssDNA
• Parvoviruses have ssDNA genomes
• Host enzymes produce DNA strand
complementary to viral genome to form dsDNA
molecule
• dsDNA used for viral replication and transcription
© 2015 Pearson Education, Inc.
Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses.
+ssRNAvirus
Receptors oncytoplasmicmembrane of host
–ssRNA
Transcriptionby viral RNApolymerase
Complementary –ssRNAto act as template
Furthertranscription
Copies of+ssRNA
Positive-sense ssRNA virus
Assembly
Translationof viralproteins,genome actsas mRNA
–ssRNAvirus
+ssRNA
Transcription byRNA-dependentRNA transcriptase
Furthertranscription
Copies of–ssRNA
Assembly
Negative-sense ssRNA virus
Translationof viralproteins
Complementary+ssRNA to actas templateand as mRNA
–ssRNATranscriptionby viral RNApolymeraseto makecomplementaryRNA strands
+ssRNAacts asmRNA
Unwinding
dsRNA
dsRNAvirus
Translationof viralproteins
Assembly
Double-stranded RNA virus
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Synthesis of RNA viruses of animals
• Retroviruses
• Do not use their genomes as mRNA
• Use DNA intermediary transcribed by viral reverse
transcriptase as template to produce viral genomes
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Assembly and release of animal viruses
• Most DNA viruses assemble in nucleus
• Most RNA viruses develop solely in cytoplasm
• Number of viruses produced depends on type of virus and
size and initial health of host cell
• Enveloped viruses are often released by budding
• Enables some viruses to cause persistent infections
• Naked viruses are released by exocytosis or lysis
© 2015 Pearson Education, Inc.
Envelopedvirion
Budding ofenveloped virus
Cytoplasmicmembraneof host
Viralglycoproteins
Viral capsid
3 4
5
2
1
Figure 13.14: The process of budding in enveloped viruses.
© 2015 Pearson Education, Inc.
Figure 13.15 Pattern of virion abundance in persistent infections.
© 2015 Pearson Education, Inc.
Viral Replication: Animal Viruses
© 2015 Pearson Education, Inc.
Viral Replication
• Replication of Animal Viruses• Latency of animal viruses
• When animal viruses remain dormant in host cells
• Viruses are called latent viruses or proviruses
• May be prolonged for years with no viral activity
• Some latent viruses do not become incorporated into host
chromosome
• Incorporation of provirus into host DNA is permanent
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Viral Replication
• Tell Me Why• Why are lysogenic and latent viral infections generally
longer lasting than lytic infections?
© 2015 Pearson Education, Inc.
The Role of Viruses in Cancer
• Cell division is under strict genetic control• Genes dictate that some cells can no longer divide at all
• Cells that can divide are prevented from unlimited division
• Genes for cell division are "turned off," or genes inhibiting division are "turned on"
• Neoplasia • Uncontrolled cell division in multicellular animal
• Mass of neoplastic cells is tumor
• Benign versus malignant tumors• Malignant tumors also called cancers
• Metastasis occurs when tumors spread
© 2015 Pearson Education, Inc.
Figure 13.16 The oncogene theory of the induction of cancer in humans.
© 2015 Pearson Education, Inc.
The Role of Viruses in Cancer
• Environmental factors that contribute to the
activation of oncogenes
• Ultraviolet light
• Radiation
• Carcinogens
• Viruses
© 2015 Pearson Education, Inc.
The Role of Viruses in Cancer
• Viruses cause 20–25% of human cancers
• Some carry copies of oncogenes as part of their genomes
• Some promote oncogenes already present in host
• Some interfere with tumor repression
• Specific viruses are known to cause ~15% of human cancers
• Burkitt's lymphoma
• Hodgkin's disease
• Kaposi's sarcoma
• Cervical cancer
© 2015 Pearson Education, Inc.
The Role of Viruses in Cancer
• Tell Me Why• Why are DNA viruses more likely to cause neoplasias
than are RNA viruses?
© 2015 Pearson Education, Inc.
Culturing Viruses in the Laboratory
• Viruses cannot grow in standard microbiological
media
• Are cultured inside host cells
• Three types of media for culturing viruses
• Media consisting of mature organisms
• Embryonated eggs
• Cell cultures
© 2015 Pearson Education, Inc.
Culturing Viruses in the Laboratory
• Culturing Viruses in Mature Organisms• Culturing viruses in bacteria
• Phages are grown in bacteria in liquid cultures or on agar
plates
• Lysis of bacteria produces plaques
• Allows estimation of phage numbers by plaque assay
© 2015 Pearson Education, Inc.
Bacterial lawn
Viral plaques
Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate.
© 2015 Pearson Education, Inc.
Culturing Viruses in the Laboratory
• Culturing Viruses in Mature Organisms• Culturing viruses in plants and animals
• Numerous plants and animals have been used to culture
viruses
• Laboratory animals can be difficult and expensive to
maintain
• Ethical concerns
© 2015 Pearson Education, Inc.
Culturing Viruses in the Laboratory
• Culturing Viruses in Embryonated Chicken Eggs• Inexpensive
• Among the largest of cells
• Free of contaminating microbes
• Contain a nourishing yolk
• Fertilized chicken eggs are often used
• Embryonic tissues provide ideal site for growing viruses
• Some vaccines are prepared in chicken cultures
© 2015 Pearson Education, Inc.
Injection intochorioallantoicmembrane
Air sac
Injection intochorioallantois
Injection intoembryo
Injection intoamnion
Injection intoyolk sac
Figure 13.18 Inoculation sites for the culture of viruses in embryonated chicken eggs.
© 2015 Pearson Education, Inc.
Culturing Viruses in the Laboratory
• Culturing Viruses in Cell (Tissue) Culture• Cells are isolated from an organism and grown on a
medium or in a broth
• Cell cultures sometimes inaccurately called "tissue
cultures"
• Two types of cell cultures
• Diploid cell cultures
• Continuous cell cultures
© 2015 Pearson Education, Inc.
Figure 13.19 An example of cell culture.
© 2015 Pearson Education, Inc.
Culturing Viruses in the Laboratory
• Tell Me Why• HIV replicates only in certain types of human cells, and
one early problem in AIDS research was culturing those
cells. Why are scientists now able to culture HIV?
© 2015 Pearson Education, Inc.
Are Viruses Alive?
• Some microbiologists consider them complex
pathogenic chemicals
• Others consider them to be the least complex
living entities
• Use sophisticated methods to invade cells
• Have the ability to take control of their host cell
• Are able to replicate themselves
© 2015 Pearson Education, Inc.
Are Viruses Alive?
• Tell Me Why• Why are viruses seemingly alive and yet not alive?
© 2015 Pearson Education, Inc.
Other Parasitic Particles: Viroids and Prions
• Characteristics of Viroids• Extremely small, circular pieces of RNA that are
infectious and pathogenic in plants
• Similar to RNA viruses but lack capsid
• May appear linear because of hydrogen bonding
• Viroidlike agents affect some fungi
© 2015 Pearson Education, Inc.
Figure 13.20 The RNA strand of the small potato spindle tuber viroid (PSTV).Genome of bacteriophage T7 PSTV
© 2015 Pearson Education, Inc.
Figure 13.21 One effect of viroids on plants.
© 2015 Pearson Education, Inc.
Other Parasitic Particles: Viroids and Prions
• Characteristics of Prions• Proteinaceous infectious agents
• Cellular PrP
• Made by all mammals
• Normal, functional structure has α-helices
• Prion PrP
• Disease-causing form has β-sheets
• Prion PrP causes cellular PrP to refold into prion PrP
© 2015 Pearson Education, Inc.
Prions: Overview
© 2015 Pearson Education, Inc.
α-helices β-pleated sheet
Cellular PrP Prion PrP
Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP).
© 2015 Pearson Education, Inc.
Prions: Characteristics
© 2015 Pearson Education, Inc.
Other Parasitic Particles: Viroids and Prions
• Characteristics of Prions• Prion diseases
• Spongiform encephalopathies
• Large vacuoles form in brain
• Characteristic spongy appearance
• BSE, vCJD, kuru
• Transmitted by ingestion, transplantation, or contact of
mucous membranes with infected tissues
• Prions are destroyed by incineration or autoclaving in
concentrated sodium hydroxide
© 2015 Pearson Education, Inc.
VacuoleFigure 13.23 A brain showing the large vacuoles and spongy appearance typical in prion-induced diseases.
© 2015 Pearson Education, Inc.
Prions: Diseases
© 2015 Pearson Education, Inc.
Other Parasitic Particles: Viroids and Prions
• Tell Me Why• Why did scientists initially resist the idea of an infectious
protein?
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
Important topics
• General characteristics of viruses • Mechanism of reproduction in RNA viruses• Lytic versus the lysogenic replication cycle• Viroid vs. prion
top related