INTRODUCTION TO MICROBIOLOGY

( VIRIOLOGY)
 SCOPE
• Introduction to Virology
• Virus Structure and Classification
• Replication of Viruses
• DNA Viruses of Medical Importance
• RNA Viruses of Medical Importance
• Specimen Collection and Documentation
 Introduction to Virology

• Nature of Viruses
 Obligate intracellular microbes (inert in extracellular
environment)
o There are more viruses in a liter of sea water than there are
humans
 They are not cells as seen with bacteria and other microbes
 Smallest infectious microbes(20-300nm in diameter-Parvoviruses
and Poxviruses).
 Contain a genome of either DNA or RNA, protected by a protein
sheet called Capsid
 Some have extra lipid-containing membrane called Envelope,
which indicates the host cell in which the virus was produced.
 Nature of Viruses

 An infectious virus particles/unit is called virion; it can only be

observed by an electron microscope
 Viruses do not reproduce by binary fission; they lack
metabolic systems to synthesize viral components so replicate
within a host cell by utilizing the host cell’s nucleic acids in an
elaborate process.
 Some viruse when they infect it is for life: HSV-1,HSV-
2,EBV,HHV-6,7,8,VZV etc
 Majority of viruses that infect has no impact on our well-
being or health
 Viruses & other Microbes
Microbes Growth Presence Presence Replicati Sensitivit
in Non- of Both of on by y to
living DNA & Ribosom Binary Antibioti
Media RNA es Fission cs

Bacteria YES YES YES YES YES

Mycoplas YES YES YES YES YES

ma
Rickettsia NO YES YES YES YES

Chlamydi NO YES YES YES YES

a
Viruses NO NO NO NO NO
 Culturing of Viruses
• The cultivation of viruses is complex and
includes three common methods
– Chicken egg culture
– Cell culture
– Animal inoculation
• READING ASSIGNMENT:
COMPARE THE CULTURING OF VIRUSES WITH
CULTURING OF OTHER MICROBES
 Viral Structure
• A Virion consists of a nucleic acid core surrounded by a sheet of protein
coat called the Capsid;
 Nucleic acid core + the capsid forms the substructure called Nucleocapsid
 Capsomeres are the defined number of structural units that make up the
capsid and are held together by non-covalent bonds
• Two types of structures seen in viruses- Helical & Icosahedral
 Symmetry is imparted to the capsid through the nature of the bonds
between the capsomeres
 Icosahedral (Cubic) Symmetry: has 20 faces(each is an equilateral
triangle),12 vertices and 30 edges with symmetrically arranged
capsomeres on the surface
-Hexamers:surface capsomeres that are bound to six other capsomeres
-Pentamers:capsomeres at the vertices bound to five other capsomeres
 Helical Symmetry has spirally arranged capsomeres
.
Icosahedral models seen: left to right, on
fivefold, threefold, and twofold axes of
rotational symmetry.
The helical structure of the rigid tobacco
mosaic virus rod.
 Viral Structure-contd
• A Complex Protein coat is also found around some viruses,
eg Poxviruses
• The Envelope is an extra lipoprotein coat found around some
viruses ,eg Influenza viruses;
 the viruses could have helical or icosahedral symmetry
 The envelope is acquired by the virion during the maturation
part of its replication as it buds out from the cellular
membrane
 Lipoprotein of the envelope is made up of lipids derived from
the host cell and proteins encoded by the virus
 Peplomers (peplos=envelope) are glycoprotein spikes seen on
the surface of the envelope
 Viral Structure-cont’d
 Matrix Protein is an envelope protein found in the inside layer of
the envelope and provides extra rigidity to the infectious
particle(virion).
 Some viral families have no matrix protein, eg Arenaviruses,
Bunyaviruses and Coronaviruses

 READING ASSIGNMENT: ILLUSTRATE THE DIFFERENCE

SCHEMATICALLY BETWEEN AN ENVELOPED VIRUS WITH
ICOSAHEDRAL SYMMETRY AND WITH A HELICAL SYMMETRY
 Chemical Composition of Viruses
• Viral Nucleic Acid (viral genome)
 All are Haploid(contain one copy of each gene)
 Only Retrovirus genomes are diploid
 Genome is either DNA or RNA
 Could be double stranded (ds) or single stranded (ss)
 All DNA viruses are haploid and ds ,except Parvoviruses which
are ss; they may be circular or Linear .
 RNA viruses maybe circular or linear; some are segmented.
 ss genomic RNA may show polarity or sense; either plus (+)
or minus (-)
 Plus sense is same as mRNA but minus sense (Negative
sense)is complementary to mRNA
 Ambisense genome: some RNA viruses have part plus sense
and part negative sense
 Chemical Composition of Viruses-cont’d
• Viral Proteins
 Structural Proteins-these are virus coded and are part of the
virion
 The capsid is a structural protein protecting the inner nucleic
acid; icosahedral viruses or viruses with cubic symmetry form an
icosahedral capsid
 Non-structural Proteins which are involved with replication
processes ;mostly as enzymes fueling the transcription of mRNA
from DNA.
 Viral Glycoproteins-occurs as projections or spikes protruding
from the envelope of enveloped viruses.
 Viral envelope: lipids-comprising cellular lipids and viral proteins.
 Environmental Effect on Viral Infectivity
• Viruses are highly susceptible to inactivation by physical and
chemical agents
 Temperature
–high temperatures adversely affects viral infectivity due to
protein denaturation;
-denatured surface proteins leads to loss of cellular attachment and
hence no uncoating
- rate of loss of infectivity reduces in the face of ambient
temperatures, is slower but significant.
• Preservation of viral Infectivity
 Storage of viral preparations at low temperatures
-4 oC: from wet ice or refrigerator for a day or two
- -70 oC—the temp of frozen carbon dioxide (dry ice)
- -196 oC- the temp of liquid nitrogen
 Enveloped viruses are more heat-labile than nonenveloped viruses
 Environmental Effect on Viral Infectivity-cont’d

Ionic environment
pH
Lipid solvents-eg chloroform
Detergents-eg sodium deoxycholate
- Enveloped viruses easily destroyed by
lipid solvents and detergents
 Disease Causation
• All living things-plants, animals & insects, other
microbes and humans are susceptible
• Infection via inhalation, ingestion, inoculation,
sexual intercourse, mother –to-baby in utero
• Some human cancers are caused by viruses
 carcinoma of the cervix (Human papillomavirus),
 liver cancer (hepatitis B and C),
 Kaposi sarcoma (human herpesvirus 8)
 Burkitts lymphoma (Epstein-Barr virus).
 Disinfection and inactivation of viruses

Most are inactivated at 56 °C for 30

Heat
minutes or at 100 °C for a few seconds
Variable; enveloped viruses are rapidly
Drying
inactivated.
Ultra-violet irradiation Inactivates viruses
Organic solvents (Chloroform, Ether, Enveloped viruses are inactivated; those
Alcohol) without are resistant.
Viruses are inactivated by formaldehyde,
Oxidizing and reducing agents
chlorine, iodine and hydrogen peroxide
Phenols Most viruses are susceptible in vitro
 Classification & Nomenclature of Viruses
• Classification:
 Biological & Physico-Chemical characteristics
 Genome type
 Capsid symmetry
 Envelope Presence or Absence
 Ether sensitivity
 Physical type of nucleic acid(ss,ds,segmented,etc)
 Size of virus size
 Molecular weight of nucleic acid
 Family
• Nomenclature:
i. International Committee on Taxonomy of Viruses (ICTV)
 Order > Family> Subfamily > Genus > Species (Type Species)
 ICTV-cont’d
 Type Species or Species: signifies the virus that initially led to
the creation or definition of the group of virus bearing the
character semblance
 Genus: a group of species sharing certain defined common
characters
 Subfamily: a group of genera sharing certain defined common
characters
 Family: a group of genera or subfamilies sharing certain
defined common characters
 Order: a group of families sharing certain defined common
characters
 Eg : Order Mononegravirales, family Paramyxoviridae,
subfamily Paramyxovirinae, genus Pneumovirus,species
Human respiratory syncytial virus.
 Classification & Nomenclature of Viruses
 Order – suffix -virales
 Family- suffix -viridae
 Subfamily-suffix- virinae
 Genera-suffix - virus
 species, types, sub-types, strains and variants
 Egs: Parvoviridae (family), Parvovirinae (subfamily),
Parvovirus(genus)

NB: parvus = small

 Astroviridae (family), Astrovirinae (subfamily) , Astrovirus (genus)
NB: Astron= star
 Ebola:
 Baltimore Classification
• The Baltimore Classification is a virus classification system that
groups viruses into families, depending on their type of genome
(DNA or RNA), strandedness (single-stranded , double-stranded )
and their method of replication (based on the method of viral
mRNA synthesis).
Central dogma of molecular : DNA >VIA TRANSCRIPTION > RNA VIA
TRANSLATION > PROTEIN AS mRNA
Classification of viruses by mRNA synthesis:
• I: dsDNA viruses: DNA –dependent RNA Polymerase(either virus
encoded or hijacked from host cell) copying mechanism achieves
converting the ds DNA into mRNA ;
e.g. Herpesviruses, Adenoviruses, Papillomaviruses,Poxviruses

• II: ssDNA viruses (+ strand or "sense") DNA: first duplicates the

single DNA into double strand then transcribed into mRNA e.g.
Parvoviruses
 Baltimore Classification
• III: dsRNA viruses : has copying mechanism of RNA-dependent
RNA polymerase to transcribe into mRNA ;e.g. Reoviruses
• IV: (+)ssRNA viruses: (+ strand or + sense) RNA: converted into
minus strand then used as template to make more mRNA; e.g.
Picornaviruses, Togaviruses
• V: (−)ssRNA viruses (− strand or antisense) RNA: transcribed
directly into the mRNA ,though they are duplicated into the +
strand first and so copies back the – strand e.g. Orthomyxoviruses,
Rhabdoviruses
• VI: ssRNA-RT viruses (+ strand or +sense) RNA with DNA
intermediate in life-cycle: first converts the RNA into a single DNA
strand then two strands then transcribed into mRNA e.g.
Retroviruses
• VII: dsDNA-RT viruses: the ds DNA is reverse transcribed into RNA
and then into mRNA e.g. Hepadnaviruses
 Assignments
1. Categorize viruses by the genome type, presence or absence of
envelope, polarity and stranded nature and by RNA synthesis. TO BE
SUBMITTED
2. Tabulate the family, subfamily and genera names of the DNA and
RNA viruses, giving the meaning to the root name and giving specific
names of each. TO BE SUBMITTED
 REFERENCE BOOKS
• Medical Microbiology by Geo F. Brooks, Janet S. Butel
and Stephen A. Morse.
• Microbiology & Immunology by William W. Yotis.
• Bacteriology, Virology and Immunology for Students
of Medicine by FS Stewart and TSL Beswick.
• Medical Virology by David O. White and Frank J.
Fenner.
• Lecture Notes on Medical Virology,D.J.Jeffries