Chapter 13 Viruses, Viroids, and Prions Part 1. General Characteristics of Viruses Very small in size –Need an electron microscope to visualize and determine.

Chapter 13

Viruses, Viroids, and Prions

Part 1

General Characteristics of Viruses

• Very small in size– Need an electron microscope to visualize and

determine viral sizes– Passes through microbial filters (filterable

agent)– Range from 20 - 100 nm in length

General Characteristics of Viruses

• Inert outside living host cells

• Obligatory intracellular parasites– Viral nucleic acids only active inside a living

host cell – Take over host’s metabolic machinery to

multiply itself

• Not all of them cause disease – e.g. TT virus (TTV) discovered in 1997 is a

harmless symbiont (found in 2% of healthy humans)

General Characteristics of Viruses

• Contain either DNA or RNA, not both – Can be single-stranded or double-stranded

• Contain a protein coat that surrounds DNA or RNA – Some are enclosed by an envelope (composed

of lipids, proteins, and carbohydrate)

• Multiply Inside living cells by using the host’s synthesizing machinery

General Characteristics of Viruses

• Directs synthesis of specialized structures that can transfer the viral nucleic acid to other cells

• Hard to treat – Most antiviral drugs that would interfere with

viral multiplication would also interfere with the functioning of the host cell

General Characteristics of Viruses

• Host range: spectrum of host cells the virus can infect

• Most viruses infect only specific types of cells in one host species (species specific)

• Host range is determined by specific host attachment sites (viral receptors) and the availability of cellular factors within the (potential) host

General Characteristics of Viruses

• Host receptor site for bacteriophage (phage)– part of the cell wall (bacterial), or sometimes

part of the fimbriae or flagella

• Host receptor site for animal viruses– On the plasma membranes

• Viruses are classified by differences in the structures of their protein coat

Viral Structure

• Virion: a complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat– Vehicle of transmission from one host cell to

another– Structures of protein coat used for viral

classification

Viral Structure

• Viral nucleic acid (either DNA or RNA) – Can be single-stranded or double-stranded– Can be linear or circular– Can be in several separate segments (e.g.

influenza virus)

• Capsid: the protein coat of a virus that surrounds the nucleic acid – Each capsid composed of protein subunit

(capsomeres)

Viral Structure

– Arrangement of capsomeres characteristic of a particular type of virus

– Structure of capsid determined by the viral nucleic acid

– In a nonenveloped virus capsid protects nucleic acid from nuclease enzymes in biological fluids and promotes the virus’s attachment to susceptible host cells

Viral Structure

• Envelope: an outer covering surrounding the capsid of some viruses– combination of lipid, proteins, and

carbohydrates– Some animal virus take host cell’s plasma

membrane as envelope when they are released from a host cell by an extrusion process

– Some envelopes may be covered by spikes (carbohydrate-protein complexes used for attachment to a host)

Viral Structure

• Mutation of viral surface proteins allows viruses to escape antibodies made in an infected host– Cause reinfection with the same virus– e.g. Influenzavirus (changes in its spikes)

General morphology

• Based on capsid architecture, viruses may be classified into several different morphological types– Use electron microscopy and X-ray

crsytallography

• Helical viruses– Resemble long rods; may be rigid or flexible– e.g. Rabies and Ebola viruses

Helical virus

General morphology

• Polyhedral (many-sided) viruses– Many animal, plant, and bacterial viruses– Capsid is in the shape of an icosahedron (a

regular polyhedron with 20 triangular faces and 12 corners)

– e.g. Adenovirus and poliovirus

Polyhedral virus

General morphology

• Enveloped viruses– Roughly spherical– enveloped helical or enveloped polyhedral

viruses– e.g. enveloped helical viruses: Influenzavirus– e.g. enveloped polyhedral (icosahedral) virus:

herpes simplex virus

Enveloped virus

General morphology

• Complex viruses– Viruses with complicated structures– e.g. Bacterial viruses (bacteriophages) and

poxviruses (have several coats around the nucleic acid)

Complex virus

Taxonomy of Viruses

• Oldest classification based on symptomatology

• International Committee on the Taxonomy of Viruses (ICTV) group viruses into families based on:– Nucleic acid type– Strategy for replication– Morphology

Taxonomy of Viruses

• Virus species: a group of viruses sharing the same genetic information and ecological niche (host range)

• Order names end in -ales

• Family names end in -viridae

• Genus names end in -virus

Taxonomy of Viruses

• No specific epithets (species) used for viruses; use descriptive common names– Subspecies are designated by a number

• Example– Herpesviridae>Herpesvirus>Human herpes

virus 1 (HHV 1), HHV 2, HHV 3– Retroviridae>Lentivirus>Human

immunodeficiency virus 1 (HIV 1), HIV 2

Isolation, Cultivation, and Identification of Viruses

• Viruses must be grown in living cells– Viruses that use bacterial cells as host easier to

grow using bacterial cultures than animal or plant viruses

• Growing Bacteriophages in the Laboratory– Grow in either suspensions of bacteria in liquid

medie or in bacterial cultures on solid media– Solid media allows to detect and count viruses

using plaque method

Growing bacteriophages in the lab

• Plaque: a clearing in a bacterial lawn resulting from lysis by phages– Each plaque theoretically is formed from a

single virus in the initial viral suspension

• Plaque-forming units (pfu): the concentration of viral suspensions measured by the number of plaques

Growing animal viruses in the lab

• In living animals– Some animal viruses can only be cultured in

living animals (mice, rabbits, and guinea pigs)– Some human viruses cannot be grown in other

animals, or can be grown but do not cause disease e.g. HIV 1 can infect Chimpanzees but show no symptoms of the disease

– Simian AIDS (in green monkey) and feline AIDS provide a model for studying human AIDS

Growing animal viruses in the lab

• In Embryonated Eggs– Inexpensive form of host and fairly convenient– Virus is injected near the one most appropriate

for its growth– Viral growth is signaled by the death of the

embryo, or by the formation of typical pocks or lesions on the egg membranes

– Used to grow viruses for some vaccine (need to watch out for allergic reaction to egg proteins)

Growing animal viruses in the lab

• In cell cultures (use animal and plant cells)– Replaced embryonated eggs as the preferred

type of growth medium for many viruses– More convenient to work with than whole

animals or embryonated eggs– Primary cell lines: derived from tissue slices,

tend to die out after only a few generations– Continuous cell lines: transformed cells that can

be maintained through an indefinite number of generations; also known as immortal cell lines

Cell Culture

Growing animal viruses in the lab

– Some viruses have never been successfully cultivated in cell culture

– Maintenance of cell culture lines requires trained technicians

– Look for cytopathic effect (CPE) formed on the monolayer of cells infected with virus

Cytopathic Effect (CPE)

Viral identification• Cytopathic effects (CPE) in cell culture

• Serological tests– Detect antibodies against viruses in patient– Use antibodies to identify viruses in neutralization

tests, viral hemagglutination, and Western blot

• Nucleic acids– RFLPs (restriction fragment length

polymorphisms) – PCR (West Nile encephalitis outbreak)