General Properties of Viruses Dr Yagoub Hmadt Allah

General Properties of Viruses Dr. Yagoub Hmadt Allah Ass. Prof. medical microbiology

General Properties of virus It is very smallest infectious agent (20 – 350 nm) u Obligate intracellular parasites u Contain only one type of nucleic acid, either DNA or RNA u Do not possess cellular organization u Lacks enzymes necessary for protein & NA synthesis u Depends on host cell machinery for replication u Causes a large no. of human diseases ranging from minor ailments like common cold to terrifying diseases such as rabies, HIV etc. 1/1/2013 Dr. yagoub, Microbiology 2 u

General Properties u Morphology – size, structure, shape, chemical properties, resistance u Replication u Hemagglutination u Cultivation u Viral assay u Viral infections: virus-host interactions 1/1/2013 3 Dr. yagoub, Microbiology

Morphology - Size u Much smaller than bacteria u “Filterable agents” – can pass through filters that can hold back bacteria u Vary widely in size: n n u Largest – poxvirus (300 nm) Smallest – parvovirus (20 nm) Virion – extracellular infectious virus particle 1/1/2013 4 Dr yagoub, Microbiology

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Morphology - Structure of Virus E L E C T R O N Nucleic acid Capsid Envelope Peplomer Size in nanometers 1/1/2013 6 Dr yagoub, Microbiology M I C R O S C O P E

Morphology – Structure & Shape of a virus u Nucleic acid & capsid with or without envelope. u Capsid – the protein coat surrounding the nucleic acid core. It n n protects nucleic acid from inactivation helps to introduce viral genome into host cell u Capsomers - the repeating protein subunits that make up the capsid u Protomers – the polypeptide chains which make up the capsomers 1/1/2013 7 Dr yagoub Microbiology

4. Mature Capsid 1. Protomers 2. Capsomers 1/1/2013 8 3. Pro. Capsid Dr yagoub, Microbiology

Morphology – Structure & Shape of a virus: Capsid u Capsomers – symmetrically arranged to form an impenetrable shell (capsid) around the nucleic acid core. u This n n 1/1/2013 symmetry is of two types: Icosahedral (cubical) Helical 9 Dr yagoub, Microbiology

Morphology – Structure & Shape of a virus: Capsid u Icosahedron – a polygon with 12 corners (vertices) & 20 sides (facets) n n Side – equilateral triangle Two types of capsomers form the capsid Pentagonal capsomers form the vertices Hexagonal capsomers form the sides. u Helical – the capsomers & nucleic acid are wound together to form a helical or spiral tube. u The overall shape of virus is quite variable, but mostly they are spherical. 1/1/2013 10 Dr yagoub, Microbiology

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Morphology – Structure & Shape of a virus: Envelope May or may not be present u Derived from the host cell membrane u Lipoprotein in nature – lipid is of host cell origin while protein is from virus. u Protein subunits seen as projecting spikes on the surface of envelope – called Peplomer. u A virus may have more than one type of peplomer e. g. influenza virus. u Confers chemical, antigenic & biological properties. u Susceptible to lipid solvents u 1/1/2013 12 Dr yagoub, Microbiology

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Resistance u Very heat labile but stable at low temperatures n Inactivated within seconds at 56 C. n Can be kept frozen at -70 C for long term storage. u Inactivated by sunlight, UV rays & ionising radiations. u More resistant than bacteria to chemical disinfectants. u Most active antiviral agents (virucidal) – oxidising agents like hydrogen peroxide, potassium permangnate, hypochlorites 1/1/2013 14 Dr yagoub, Microbiology

Viral replication u Viruses have no metabolic activity of their own. Therefore, they depend on living cells for providing energy and synthetic machinery for synthesis of : u 1 - viral nucleic acid , 2 - viral protein. u The virus genome provides the host cell with the genetic information needed for its replication. 11/23/2020 15 Dr y. h, Microbiology

The sequence of events for virus replication Stages in virus replication begin when virions infect cells Attachment/ Adsorption Penetration Uncoating Biosynthesis Maturation & Assembly Release 1/1/2013 16 Dr yagoub, Microbiology

• 1 - Attachment: virus and cell are brought into contact by random collision, but attachment occurs if the cell membrane contains specific receptors for the virus, e. g HIV binds to CD 4 receptors on helper T cells. • 2 - penetration and uncoating: non envoloped virions are taken into animal cells by endocytosis where they are uncoated by lysosomal enzymes. Enveloped viruses penetrate the membrane by fusion between the virus envelope and the cell membrane releasing the nucleocapsid into the cell; uncoating may occur at the cell surface. Uncoating renders viral nucleic acid accessible for transcription and replication. 11/23/2020 17 Dr yagoub. h, Microbiology

Pathways for Viral Entry into Host Cell Surface Fusion Receptor-Mediated Endocytosis Fusion in Endosome Lysis of Endosome 1/1/2013 18 Dr yagoub, Microbiology

Viral Replication 3 - Eclipse phase : it is the period after penetration during which no infectious virus componant can be detected inside the host cell. During this phase the cell rediracted toward synthesizing early proteins (enzymes) which are essential for viral replication. {from the stage of penetration till the appearance of mature daughter virions, the virions cannot be detected inside the host cell}. 1/1/2013 19 Dr yagoub, Microbiology

4 - Intracellular viral synthesis: it includes synthesis of both viral nucleic acid and proteins. The viral nucleic acid (genome) replicates by using strand of the parental nucleic acid as a template for the production of progeny DNA or RNA molecules. The essential step in protein synthesis is transcription of m. RNA from viral nucleic acid. 11/23/2020 20 Dr. Yagoub. H, Microbiology

The m. RNA is translated to viral proteins (capsid and enzymes) using host cell ribosomes. The m. RNA transcription varies depending on the nucleic acid type: a. In double stranded (ds) DNA viruses, m. RNA is transcribed from DNA by DNA dependent RNA polymerase ( transcriptase). +ss DNA ds DNA-ss DNA RNA polymerase 11/23/2020 21 m. RNA

b. In single stranded RNA viruses of positive polarity (+ sense) the ss. RNA itself acts as m. RNA for translation into proteins. + ss. RNA = m. RNA c. The ss. RNA viruses of negative polarity ( - sense) must be transcribed by RNA dependent RNA polymerase – which is present in the virus into complementary (+ sense) m RNA. – ss. RNA polymerase + ss. RNA = m. RNA 11/23/2020 22 Y. H, Microbiology

d. The ss. RNA of retroviruses (+ sense) is transcribed by a unique virion associated reverse transcriptase into complementary ss. DNA, which is converted into ds. DNA, which becomes integrated into the cellular genome causing malignant transformation of cells in vivo and in vitro. It may be transcribed into m. RNA. +ss. RNA Reverse transcriptase ss. DNA m. RNA 11/23/2020 23 Dr y. h Microbiology ds. DNA Integrated Transcribed to

e. Assembly of viral nucleic acid and protein coats to form mature virus particles occurs in the cytoplasm ( e. g. poliovirus) or in the nucleus e. g herpes viruses. f. Release: mature virus particles will accumulate in the cell in enonmous number and are liberated by rupturing the cell i. e cytolysis. The viruses may release by a slow process of leaking or budding through the cell membrane. Enveloped viruses will acquired lipoprotein envelope during budding. 11/23/2020 24

Classification of medical important viruses: u The classification of viruses depends on their structure, antigenic composition and other properties. Viral famillies and genera have been designated, though differentiation into species is still incomplete. u Viruses are classified into two major divisions depending on the type of nucleic acid. 11/23/2020 25 Dr y. h, Microbiology

u 1 - deoxyriboviruses, which contain DNA. u 2 - riboviruses, which contain RNA. Both of these are further subdivided mainly on the basis of size and shape of the virion, symmetry of the nucleocapsid and strandness of the nucleic acid. 11/23/2020 26 Dr yagoub hamadta Allah Microbiology

Major families of viruses are briefly are: A- DNA viruses: 1. parvoviridae: - Size : 18 – 26 nm - Symmetry : icosahedral - Envelope : absent - DNA : single stranded - Example : parvovirus which cause gastroenteritis and haemolytic disease 11/23/2020 27 Dr. yagoub hamadt allah, Microbiology

u 2 - - papovaviridae: Size : 40 – 55 nm Symmetry : icosahedral Envelope : absent DNA : double stranded Example : i. papilloma virus which causes cutaneous, genital and laryngeal warts. ii. Polyomavirus which cause neurological diseases 11/23/2020 28 Dr yagoub hamadt allah, Microbiology

3 - Adenoviridae: - Size : 70 – 90 nm - Symmetry : icosahedral - Envelope : absent - DNA : double stranded - Example : adenovirus. some can cause respiratory disease and conjunctivitis. 11/23/2020 29 Dr yagoub hamadtallah, Microbiology

4 - herpesviridae: - Size : 100 – 200 nm - Symmetry : icosahedral - Envelope : present - DNA : double stranded - Example : i. herpes simplex virus - ii. Varicella /zoster viruses. 11/23/2020 30 Dr yagoub hamadtallah, Microbiology

5 - poxviridae: - Size : 300 – 450 nm. X 170 -260 nm(brick shaped) - Symmetry : unknown - Envelope : present - DNA : double stranded - Example : poxviruses –Variola, cowpox, monkey pox…. . 11/23/2020 31 Dr yagoub hamadt allah, Microbiology

6 - hepadnairidae: - Size : 42 nm - Symmetry : unknown - Envelope : present - DNA : partially double stranded - Example : hepatitis – B virus 11/23/2020 32 Dr yagoub hamadt allah, Microbiology

Riboviruses(RNA viruses): RNA viruses included: 1 - Picornaviridae. 2 - Reoviridae (double stranded). 3 - Orthomyxoviridae. 4 - Paramyxoviridae. 5 - Rhabdoviridae. 6 - Bunyaviridae. 11/23/2020 33 Dr y. h, Microbiology

7 - Coronaviridae. 8 - Togaviridae. 9 Arenaviridae. 10 - Retroviridae. 11/23/2020 34 Dr Ekta, Microbiology

Abnormal Replicative Cycles u Incomplete viruses - A proportion of daughter virions are not infective, due to defective assembly. u Defective viruses – genetically defective, unable to give rise to fully formed progeny. u Abortive infection – defect in the type of cell (non permissive cell), not in the parental viruses. 1/1/2013 35 Dr yagoub, Microbiology

Viral Hemagglutination u Hemagglutination n u Originally seen with the Influenza virus by Hirst in 1941. A convenient method of detection & assay of Influenza virus. Due to the presence of Hemagglutinin spikes on the surface. Reversal of hemagglutination – Elution n Due to the presence of Neuraminidase enzyme, Receptor Destroying Enzyme (RDE) Destruction of receptor – reversal of hemagglutination – release of virus from the red cell surface Found only in Myxoviuses. 1/1/2013 36 Dr yagoub, Microbiology

Virus Culture Embryonated Egg Chorioallantioc membrane (CAM) Allantoic cavity Amniotic cavity Yolk Sac Cell Lines/ Tissue cultures Primary Diploid/ Secondary Continuous Animal inoculation 1/1/2013 37 Suckling mice Dr yagoub, Microbiology

Embryonated Hen’s Egg u Chorioallantoic membrane (CAM) – visible lesions called pocks. Each infectious virus particle forms one pock. e. g. Variola, Vaccinia virus u Allantoic cavity – Influenza virus (vaccine production) & paramyxoviruses u Amniotic cavity – primary isolation of Influenza virus u Yolk sac – Chlmyadia, Rickettsiae & some viruses 1/1/2013 38 Dr yagoub, Microbiology

Embryonated Hen’s Egg 1/1/2013 39 Dr yagoub, Microbiology

Viral Assay u Viral content of a specimen: Total no. of 1. 2. u Virus particles – EM, HA Infectious virions only Assay of Infectivity: two types 1. 2. Quantitative assays – actual no. of infectious particle in an inoculum Quantal assays – indicate the presence or absence of infectious viruses, carried out in animals, eggs or tissue cultures 1/1/2013 40 Dr yagoub, Microbiology

Viral Assay of Infectivity: Quantitative assays u n n Plaque assay in monolayer cell cultures Pock assay on CAM *Each plaque/ pock represents one infectious virus. n Plaques are clear zones that develop on lawns of host cells. n The virus plaque is analogous to the bacterial colony. 1/1/2013 41 Dr yagoub, Microbiology