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medical virology introduction to basics

Medical VirologyIntroduction to Basics

Dr.T.V.Rao MD

Dr.T.V.Rao MD

history virology
History Virology
  • Smallpox was endemic in China by 1000BC. In response, the practice of variolation was developed. Recognizing that survivors of smallpox outbreaks were protected from subsequent infection, variolation involved inhalation of the dried crusts from smallpox lesions like snuff, or in later modifications, inoculation of the pus from a lesion into a scratch on the forearm of a child.

Dr.T.V.Rao MD

introduction to virology
Introduction to Virology
  • A virus is an obligate intracellular parasite containing genetic material surrounded by protein
  • Virus particles can only be observed by an electron microscope

Dr.T.V.Rao MD

introduction to virology5
Introduction to Virology
  • Recognizing the shape, size, and structure of different viruses is critical to the study of disease
    • Viruses have an inner core of nucleic acid surrounded by protein coat known as an envelope
    • Most viruses range in sizes from 20 – 250 nanometers

Dr.T.V.Rao MD

viral properties
Viral Properties
  • Viruses are inert (nucleoprotein ) filterable Agents
  • Viruses are obligate intracellular parasites
  • Viruses cannot make energy or proteins independent of a host cell
  • Viral genome are RNA or DNA but not both.
  • Viruses have a naked capsid or envelope with attached proteins
  • Viruses do not have the genetic capability to multiply by division.
  • Viruses are non-living entities

Dr.T.V.Rao MD

viruses are ultramicroscopic
Viruses are Ultramicroscopic

Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997

Dr.T.V.Rao MD

the size of viruses
The size of viruses

Dr.T.V.Rao MD

viral structure some terminology
VIRAL STRUCTURE – SOME TERMINOLOGY
  • virus particle = virion
  • protein which coats the genome = capsid
  • capsid usually symmetrical
  • capsid + genome = nucleocapsid
  • may have an envelope

Dr.T.V.Rao MD

virion
Virion
  • The complete infectious unit of virus particle
  • Structurally mature, extracellular virus particles.

Dr.T.V.Rao MD

virion11
Virion

envelope

Capsid

Viral core

Dr.T.V.Rao MD

virion structure

Lipid Envelope

Nucleic Acid

Protein

Capsid

Virion

Associated

Polymerase

Spike

Projections

Virion Structure

Dr.T.V.Rao MD

distinguishing characteristics of viruses
Distinguishing characteristics of viruses
  • Obligate intracellular parasites

• Extreme genetic simplicity

• Contain DNA or RNA

• Replication involves disassembly and reassembly

• Replicate by "one-step growth”

Dr.T.V.Rao MD

how are viruses named
How are viruses named?
  • Based on:

- the disease they cause

poliovirus, rabies virus

- the type of disease

murine leukemia virus

- geographic locations

Sendai virus, Coxsackie virus

- their discovers

Epstein-Barr virus

- how they were originally thought to be contracted

dengue virus (“evil spirit”), influenza virus (the “influence” of bad air)

- combinations of the above

Rous Sarcoma virus

Dr.T.V.Rao MD

slide15

Virus particle = virion

White, DO and Fenner, FJ.

Medical Virology, 4th Ed. 1994

Dr.T.V.Rao MD

5 basic types of viral structure
5 BASIC TYPES OF VIRAL STRUCTURE

nucleocapsid

icosahedralnucleocapsid

lipid bilayer

ICOSAHEDRAL

ENVELOPED ICOSAHEDRAL

helicalnucleocapsid

COMPLEX

nucleocapsid

lipid bilayer

glycoprotein spikes

= peplomers

HELICAL

ENVELOPED HELICAL

Dr.T.V.Rao MD

Adaptedfrom Schaechter et al., Mechanisms of Microbial Disease

icosahedral
Adeno-associated Virus (AAV)AdenovirusB19Coxsackievirus - ACoxsackievirus - BCytomegalovirus (CMV)Eastern Equine Encephalitis Virus (EEEV)EchovirusEpstein-Barr Virus (EBV)Hepatitis A Virus (HAV)Hepatitis B Virus (HBV)Hepatitis C Virus (HCV)Hepatitis Delta Virus (HDV)Hepatitis E Virus (HEV)

Herpes Simplex Virus 1 (HHV1)Herpes Simplex Virus 2 (HHV2)Human Immunodeficiency Virus (HIV)Human T-lymphotrophic Virus (HTLV)Norwalk VirusPapilloma Virus (HPV)Polio virusRhinovirusRubella VirusSaint Louis Encephalitis VirusVaricella-Zoster Virus (HHV3)Western Equine Encephalitis Virus (WEEV)Yellow Fever Virus

Icosahedral

Dr.T.V.Rao MD

viral structure
Viral Structure
  • Varies in size, shape and symmetry
  • VIP for classification
  • 3 types of capsid symmetry:
    • Cubic (icosahedral)
      • Has 20 faces, each an equilateral triangle. Eg. adenovirus
    • Helical
      • Protein binds around DNA/RNA in a helical fashion eg. Coronavirus
    • Complex
      • Is neither cubic nor helical eg. poxvirus

Dr.T.V.Rao MD

the baltimore classification system
The Baltimore classification system

Based on genetic contents and replication strategies of viruses. According to the Baltimore classification, viruses are divided into the following seven classes:

1. dsDNA viruses

2. ssDNA viruses

3. dsRNA viruses

4. (+) sense ssRNA viruses (codes directly for protein)

5. (-) sense ssRNA viruses

6. RNA reverse transcribing viruses

7. DNA reverse transcribing viruses

where "ds" represents "double strand" and "ss" denotes "single strand".

Dr.T.V.Rao MD

virus classification i the baltimore classification
Virus Classification I- the Baltimore classification
  • All viruses must produce mRNA, or (+) sense RNA
  • A complementary strand of nucleic acid is (–) sense
  • The Baltimore classification has + RNA as its central point
  • Its principles are fundamental to an understanding of virus classification and genome replication, but it is rarely used as a classification system in its own right

Dr.T.V.Rao MD

virus classification ii the classical system
Virus classification II -the Classical system
  • This is a based on three principles -
    • 1) that we are classifying the virus itself, not the host
    • 2) the nucleic acid genome
    • 3) the shared physical properties of the infectious agent (e.g capsid symmetry, dimensions, lipid envelope)

Dr.T.V.Rao MD

virus classification iii the genomic system
Virus classification III -the genomic system
  • More recently a precise ordering of viruses within and between families is possible based on DNA/RNA sequence
  • By the year 2000 there were over 4000 viruses of plants, animals and bacteria - in 71 families, 9 subfamilies and 164 genera

Dr.T.V.Rao MD

viral structure overview

Nucleic acid

Capsid

Envelope protein

Membrane protein

Viral envelope**

Nucleocapsid

Viral Structure - Overview

Spike protein

Fig 1. Schematic overview of the structure of animal viruses

** does not exist in all viruses

Dr.T.V.Rao MD

slide27

Icosahedral capsids

a) Crystallographic structure of a simple icosahedral virus.

b) The axes of symmetry

Dr.T.V.Rao MD

icosahedral symmetry

5-FOLD

3-FOLD

2-FOLD

ICOSAHEDRAL SYMMETRY

Dr.T.V.Rao MD

icosahedral symmetry31

CAPSOMER

ICOSAHEDRAL SYMMETRY

= PENTON (pentamer)

Dr.T.V.Rao MD

icosahedral symmetry32

CAPSOMER

CAPSOMER

= HEXON

ICOSAHEDRAL SYMMETRY

= PENTON

Dr.T.V.Rao MD

slide33

Adenovirus

Dr.T.V.Rao MD

slide34

12 PENTONS

240 HEXONS

Adenovirus

Dr.T.V.Rao MD

slide35

Helical symmetry

Dr.T.V.Rao MD

helical
Helical
  • California Encephalitis VirusCoronavirusHantavirusInfluenza Virus (Flu Virus)Measles Virus ( Rubeola)Mumps VirusPara influenza VirusRabies VirusRespiratory Syncytial Virus(RSV)

Dr.T.V.Rao MD

slide37
Helical symmetry

How to assemble

Dr.T.V.Rao MD

helical symmetry
Helical symmetry

In 1955, Fraenkel, Conrat, and Williams demonstrated that tobacco mosaic virus (TMV) spontaneously formed when mixtures of purified coat protein and its genomic RNA were incubated together.

TMV, a filamentous virus

Dr.T.V.Rao MD

slide39

Enveloped helical virus

Enveloped icosahedral virus

Dr.T.V.Rao MD

properties of naked viruses
Properties of naked viruses
  • Stable in hostile environment
  • Not damaged by drying, acid, detergent, and heat
  • Released by lysis of host cells
  • Can sustain in dry environment
  • Can infect the GI tract and survive the acid and bile
  • Can spread easily via hands, dust, fomites, etc
  • Can stay dry and still retain infectivity
  • Neutralizing mucosal and systemic antibodies are needed to control the establishment of infection
naked viruses non enveloped
Naked viruses( Non Enveloped )
  • Adeno-associated Virus (AAV)AdenovirusB19Coxsackievirus - ACoxsackievirus - BEchovirusHepatitis A Virus (HAV)Hepatitis E Virus (HEV)Norwalk Virus
complex symmetry
COMPLEX SYMMETRY

surface view

cross section

POXVIRUS FAMILY

White, DO and Fenner, FJ.

Medical Virology, 4th Ed. 1994

Dr.T.V.Rao MD

envelope
ENVELOPE
  • OBTAINED BY BUDDING THROUGH A CELLULAR MEMBRANE (except poxviruses)
  • POSSIBILITY OF EXITING CELL WITHOUT KILLING IT
  • CONTAINS AT LEAST ONE VIRALLY CODED PROTEIN
    • ATTACHMENT PROTEIN
  • LOSS OF ENVELOPE RESULTS IN LOSS OF INFECTIVITY

Dr.T.V.Rao MD

classification nucleic acid
CLASSIFICATION NUCLEIC ACID
  • RNA or DNA
  • segmented or non-segmented
  • linear or circular
  • single-stranded or double-stranded
  • if single-stranded RNA
    • is genome mRNA (+) sense or complementary to mRNA (-) sense

Dr.T.V.Rao MD

genome
Genome
  • The genome of a virus can be either DNA or RNA
  • DNA-double stranded (ds): linear or circular

Single stranded (ss) : linear or circular

  • RNA- ss:segmented or non-segmented

ss:polarity+(sense) or polarity –(non-sense)

ds: linear (only reovirus family)

Dr.T.V.Rao MD

slide46

DNA

RNA

double-stranded

single-stranded

double-stranded

single-stranded

linear

circular

linear

circular

linear

linear (circular)*

single

single

multiple

single

single

multiple

single

multiple

(+)sense

(-)sense

single

multiple

single

multiple

Dr.T.V.Rao MD

viral genome strategies
Viral genome strategies
  • dsDNA (herpes, papova, adeno, pox)
  • •ssDNA (parvo)
  • •dsRNA (reo, rota)
  • •ssRNA (+) (picorna, toga, flavi, corona)
  • •ssRNA (-) (rhabdo, paramyxo, orthomyxo,
  • bunya, filo)
  • •ssRNA (+/-) (arena, bunya)
  • •ssRNA (+RTase) (retro, lenti)

Dr.T.V.Rao MD

slide48

DNA VIRUSES

DOUBLE STRANDED

SINGLE STRANDED

COMPLEX

NON-ENVELOPED

ENVELOPED

PARVOVIRIDAE

POXVIRIDAE

ENVELOPED

NON-ENVELOPED

HERPESVIRIDAE

HEPADNAVIRIDAE

CIRCULAR

LINEAR

All families shown are icosahedral except for poxviruses

PAPILLOMAVIRIDAE

POLYOMAVIRIDAE

(formerly grouped together as the PAPOVAVIRIDAE)

ADENOVIRIDAE

Dr.T.V.Rao MD

Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

dna viruses
DNA viruses

From Principles of Virology Flint et al ASM Press

Dr.T.V.Rao MD

slide50

RNA VIRUSES

SINGLE STRANDED

SINGLE STRANDED

DOUBLE STRANDED

positive sense

negative sense

ENVELOPED

NONENVELOPED

ENVELOPED

NONENVELOPED

HELICAL

ICOSAHEDRAL

HELICAL

ICOSAHEDRAL

ICOSAHEDRAL

ORTHOMYXOVIRIDAE

FLAVIVIRIDAE

CORONAVIRIDAE

PICORNAVIRIDAE

REOVIRIDAE

PARAMYXOVIRIDAE

CALICIVIRIDAE

ASTROVIRIDAE

TOGAVIRIDAE

RHABDOVIRIDAE

RETROVIRIDAE

FILOVIRIDAE

BUNYAVIRIDAE

ARENAVIRIDAE

Dr.T.V.Rao MD

Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

rna viruses
RNA viruses

From Principles of Virology Flint et al ASM Press

Dr.T.V.Rao MD

basic steps in viral life cycle
BASIC STEPS IN VIRAL LIFE CYCLE
  • ADSORPTION
  • PENETRATION
  • UNCOATING AND ECLIPSE
  • SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN
  • ASSEMBLY (maturation)
  • RELEASE

Dr.T.V.Rao MD

virus replication

1

Virus attachment

and entry

1

2

Uncoating of virion

3

Migration of

genome nucleic

5

acid to nucleus

4

Transcription

4

2

5

Genome replication

6

Translation of virus

3

mRNAs

7

Virion assembly

7

8

Release of new

virus particles

6

8

Virus Replication

Dr.T.V.Rao MD

adsorption
ADSORPTION
  • TEMPERATURE INDEPENDENT
  • REQUIRES VIRAL ATTACHMENT PROTEIN
  • CELLULAR RECEPTORS

Dr.T.V.Rao MD

penetration enveloped viruses

PENETRATION - ENVELOPED VIRUSES

FUSION WITH PLASMA MEMBRANE

ENTRY VIA ENDOSOMES

Dr.T.V.Rao MD

penetration
PENETRATION

herpesviruses, paramyxoviruses, HIV

Dr.T.V.Rao MD

penetration enveloped viruses58

PENETRATION - ENVELOPED VIRUSES

FUSION WITH PLASMA MEMBRANE

ENTRY VIA ENDOSOMES, FUSION WITH ACIDIC ENDOSOME MEMBRANE

Dr.T.V.Rao MD

slide60

H+

Dr.T.V.Rao MD

virus uptake via endosomes
VIRUS UPTAKE VIA ENDOSOMES
  • CALLED
    • VIROPEXIS / ENDOCYTOSIS / PINOCYTOSIS

Dr.T.V.Rao MD

penetration non enveloped viruses
PENETRATIONNON-ENVELOPED VIRUSES

entry directly across plasma membrane:

Dr.T.V.Rao MD

slide63

H+

Dr.T.V.Rao MD

replicative cycle
Replicative cycle
  • As obligate intracellular parasites, Virus must enter and replicate in living cells in order to “reproduce” themselves. This “growth cycle” involves specific attachment of virus, penetration and uncoating, nucleic acid transcription, protein synthesis, maturation and assembly of the virions and their subsequent release from the cell by budding or lysis

Dr.T.V.Rao MD

uncoating
UNCOATING
  • NEED TO MAKE GENOME AVAILABLE
  • ONCE UNCOATING OCCURS, ENTER ECLIPSE PHASE
  • ECLIPSE PHASE LASTS UNTIL FIRST NEW VIRUS PARTICLE FORMED

Dr.T.V.Rao MD

synthesis of viral nucleic acid and protein
SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN
  • MANY STRATEGIES
  • NUCLEIC ACID MAY BE MADE IN NUCLEUS OR CYTOPLASM
  • PROTEIN SYNTHESIS IS ALWAYS IN THE CYTOPLASM

Dr.T.V.Rao MD

assembly and maturation
ASSEMBLY AND MATURATION
  • NUCLEUS
  • CYTOPLASM
  • AT MEMBRANE

Dr.T.V.Rao MD

release
RELEASE
  • LYSIS
  • BUDDING THROUGH PLASMA MEMBRANE
  • NOT EVERY RELEASED VIRION IS INFECTIOUS

Dr.T.V.Rao MD

transmission of viruses
Transmission of Viruses
  • Respiratory transmission
    • Influenza A virus
  • Faecal-oral transmission
    • Enterovirus
  • Blood-borne transmission
    • Hepatitis B virus
  • Sexual Transmission
    • HIV
  • Animal or insect vectors
    • Rabies virus

Dr.T.V.Rao MD

slide73

The commonest forms oftransmission are via...

INHALED DROPLETSin sneezing of coughingfor example the COMMON COLDor INFLUENZA VIRUSES.

Dr.T.V.Rao MD

slide74

or by...

drinking water oreating raw food, for example,HEPATITIS A and POLIOVIRUS.

Dr.T.V.Rao MD

slide75

The commonest forms oftransmission are also via...

sexual intercourse for exampleHIV and HEPATITIS B and...

Dr.T.V.Rao MD

slide76

also...

vertical transmission -from mother to baby for exampleHIV, HEPATITIS B and RUBELLA...

Dr.T.V.Rao MD

slide77

also...

bites of vector arthropods such asmosquitoes for example YELLOW FEVER,RIFT VALLEY FEVER and DENGUE.

Dr.T.V.Rao MD

slide78

Most viral infections...

do not lead to such seriouscomplications and the host...

Dr.T.V.Rao MD

slide79

get well after a period of sicknessto be immune for the rest of their lives.Examples are MEASLES INFECTION,RUBELLA or German measles,MUMPS and many others...

Dr.T.V.Rao MD

a bacteriophage
A bacteriophage
  • A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid. The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA ('ss-' or 'ds-' prefix denotes single-strand or double-strand) along with either circular or linear arrangement.

Dr.T.V.Rao MD

classification of bacteriophages
Classification of Bacteriophages
  • The dsDNA tailed phages, or Caudovirales, account for 95% of all the phages reported in the scientific literature, and possibly make up the majority of phages on the planet. However, other phages occur abundantly in the biosphere, with different virions, genomes and lifestyles. Phages are classified by the International Committee on Taxonomy of Viruses (ICTV) according to morphology and nucleic acid.

Dr.T.V.Rao MD

sub viral agents
Sub-viral agents
  • Satellites
    • Contain nucleic acid
    • Depend on co-infection with a helper virus
    • May be encapsidated (satellite virus)
    • Mostly in plants, can be human e.g. hepatitis delta virus
    • If nucleic acid only = virusoid
  • Viroids
    • Unencapsidated, small circular ssRNA molecules that replicate autonomously
    • Only in plants, e.g. potato spindle tuber viroid
    • Depend on host cell polII for replication, no protein or mRNA
  • Prions
    • No nucleic acid
    • Infectious protein e.g. BSE

Dr.T.V.Rao MD

viroids prions
Viroids & Prions
  • Viroids
    • ss RNA genome and the smallest known pathogens.
    • Affects plants
  • Prions
    • Infectious particles that are entirely protein.
    • No nucleic acid
    • Highly heat resistant
    • Animal disease that affects nervous tissue
    • Affects nervous tissue and results in
      • Bovine spongiform encepahltits (BSE) “mad cow disease”,
      • scrapie in sheep
      • kuru & Creutzfeld-Jakob Disease (CJD) in humans

Dr.T.V.Rao MD

viroids
Viroids
  • Viroids are small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites.
dependovirus virusoids
Dependovirus /Virusoids
  • Viroids are small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites.
prions
(Prions)
  • Prions are rather ill-defined infectious agents believed to consist of a single type of protein molecule with no nucleic acid component. Confusion arises from the fact that the prion protein & the gene which encodes it are also found in normal 'uninfected' cells. These agents are associated with diseases such as Creutzfeldt-Jakob disease in humans, scrapie in sheep & bovine spongiform encephalopathy (BSE) in cattle.
slide88

Programme Created by Dr.T.V.Rao MD for Medical and Paramedical Students in the Developing World

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Dr.T.V.Rao MD