Microbiology Ch 13 lecture_presentation

© 2015 Pearson Education, Inc.

Chapter 13


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



• 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


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)



• 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


Viralgenome

Partial genomeof E. coli

Figure 13.2 The relative sizes of genomes.



• 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



• 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


Figure 13.3 Some examples of plant, bacterial, and human hosts of viral infections.


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.



• 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


Figure 13.5 The shapes of virions.


Figure 13.6 The complex shape of bacteriophage T4.

Tail fibers

Base plate

Tail

Head



• 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


Figure 13.7 Enveloped virion.

Glycoproteins

Helical capsid

Matrix protein

Envelope

Enveloped virus with helical capsid




• Tell Me Why• Why are naked icosahedral viruses able to crystallize?


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




Classification of Viruses

• Tell Me Why• What characteristics of the genomes of parvoviruses

and of reoviruses make them very different from cells?


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


Viral Replication: Overview


Figure 13.8 The lytic replication cycle in bacteriophages.


Figure 13.9 Pattern of virion abundance in lytic cycle.


Viral Replication: Virulent Bacteriophages


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


Figure 13.10 Bacteriophage lambda.


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


Viral Replication: Temperate Bacteriophages


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


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


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


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


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


Viral Replication


• 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


Viral Replication


• 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


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


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



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


Envelopedvirion

Budding ofenveloped virus

Cytoplasmicmembraneof host

Viralglycoproteins

Viral capsid

3 4

5

2

1

Figure 13.14: The process of budding in enveloped viruses.


Figure 13.15 Pattern of virion abundance in persistent infections.


Viral Replication: Animal Viruses


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



Viral Replication

• Tell Me Why• Why are lysogenic and latent viral infections generally

longer lasting than lytic infections?


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


Figure 13.16 The oncogene theory of the induction of cancer in humans.



• Environmental factors that contribute to the

activation of oncogenes

• Ultraviolet light

• Radiation

• Carcinogens

• Viruses



• 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



• Tell Me Why• Why are DNA viruses more likely to cause neoplasias

than are RNA viruses?


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



• 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


Bacterial lawn

Viral plaques

Figure 13.17 Viral plaques in a lawn of bacterial growth on the surface of an agar plate.



• 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



• 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


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.



• 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


Figure 13.19 An example of cell culture.



• 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?


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


Are Viruses Alive?

• Tell Me Why• Why are viruses seemingly alive and yet not alive?


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


Figure 13.20 The RNA strand of the small potato spindle tuber viroid (PSTV).Genome of bacteriophage T7 PSTV


Figure 13.21 One effect of viroids on plants.



• 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


Prions: Overview


α-helices β-pleated sheet

Cellular PrP Prion PrP

Figure 13.22 The two stable, three-dimensional forms of prion protein (PrP).


Prions: Characteristics



• 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


VacuoleFigure 13.23 A brain showing the large vacuoles and spongy appearance typical in prion-induced diseases.


Prions: Diseases



• Tell Me Why• Why did scientists initially resist the idea of an infectious

protein?



Important topics

• General characteristics of viruses • Mechanism of reproduction in RNA viruses• Lytic versus the lysogenic replication cycle• Viroid vs. prion