Viruses - Exam points Medical Microbiology

Virus means poison that are associated with death and disease.
They are obligate intracellular parasites. i.e. they can reproduce or replicate only inside a host cell.
They contain only one type of nucleic acid, either DNA or RNA.
They lack enzymes required for protein and nucleic acid synthesis.
They multiply by a complex process and not by binary fission.
They are unaffected by antibacterial antibiotics.
Virus can cause cancer in animals and birds, as well as in humans.
Capsid
Virus is surrounded by a protein coat called capsid.
The capsid with the enclosed nucleic acid is known as nucleocapsid.
The function of the capsid is to protect the nucleic acid from inactivation by nucleases and other deleterious agents in the environment.
They also concern with introducing viral genome into host cells by adsorbing readily to cell surfaces.
The capsid is composed of individual protein subunits are called capsomeres.
The number of capsomeres are characteristics for a particular virus.
Mainly there are 2 types of capsid symmetry – Icosahedron and Helical symmetry.

1. Presented By Sijo A Ph.D. Research Scholar (Microbiology) School of Biosciences, MACFAST College Tiruvalla, Kerala, India VIRUS – MORPHOLOGY AND CHARACTERISTICS

2. Virus means poison that are associated with death and disease. They are obligate intracellular parasites. i.e. they can reproduce or replicate only inside a host cell. They contain only one type of nucleic acid, either DNA or RNA. They lack enzymes required for protein and nucleic acid synthesis. They multiply by a complex process and not by binary fission. They are unaffected by antibacterial antibiotics. Virus can cause cancer in animals and birds, as well as in humans. VIRUS

3. MORPHOLOGY AND STRUCTURE OF VIRUS

4. MORPHOLOGY AND STRUCTURE OF VIRUS

5. Virion size The extracellular infectious virus particle is called virion. Viruses are much smaller than bacteria. The size of the virion ranges from 10-300 or 400 nm in diameter. The largest virus – Pox virus Smallest virus – Parvovirus Now electron microscope is used to estimate the size of virus particle MORPHOLOGY AND STRUCTURE OF VIRUS

6. POX VIRUS AND PARVOVIRUS Small pox Monkey pox (Mpox) Chicken pox Parvovirus B19 (slapped cheek disease)

7. CANINE PARVOVIRUS

8. Capsid Virus is surrounded by a protein coat called capsid. The capsid with the enclosed nucleic acid is known as nucleocapsid. The function of the capsid is to protect the nucleic acid from inactivation by nucleases and other deleterious agents in the environment. They also concern with introducing viral genome into host cells by adsorbing readily to cell surfaces. The capsid is composed of individual protein subunits are called capsomeres . The number of capsomeres are characteristics for a particular virus. Mainly there are 2 types of capsid symmetry – Icosahedron and Helical symmetry. STRUCTURE OF VIRUS

9. a) Icosahedron Symmetry It’s a polyhedron with 20 triangular faces and 12 corners. Its one of the natures favorite shape. Capside composed of many copies of protein subunits are called Protomers . Protomers may be pentamers (5 subunit) or hexamers (6 subunit) Eg :- Herpes simplex virus, Polio virus VIRUS CAPSID TYPES b) Helical symmetry It’s a hollow tubes with protein walls. 15-18 nm in diameter, 300 nm long. The size of the helical depends on both protomers and nucleic acid enclosed within the capsule. Eg :- Tubulovirus , Tobacco mosaic virus (TMV)

10. 2.Envelope Many viruses are surrounded by a flexible membrane is known as envelope. The envelope is composed of lipids and proteins and is similar to the host cell membrane. The envelope contains functional projections are called spikes or peplomers . These are concern with attachment of virus with the host cell. In some viruses, the envelope is disrupted by solvents like ether. It blocks lipid mediated activities and blocks envelope proteins. Such virus is called “ ether sensitive”. STRUCTURE OF VIRUS

11. Example: Envelope of Influenza Virus Family: Orthomyxoviridae It contain RNA as the genetic material. 2 Spike proteins: Hemagglutinin (H), Neuraminidase (N) Hemagglutinin (H) It helps the virus to bind red blood cell membranes and cause hemagglutination. Neuraminidase (N) It helps the virus to penetrate the mucous layers of respiratory epithelium to reach host cells. STRUCTURE OF VIRUS Types: A, B, C, D A – affect variety of animal species (birds) and humans. B – localized outbreak in humans C – infects humans and pigs D – infects cattle

12. PANDEMIC FLU For a pandemic influenza to occur, three key conditions must be met : A novel virus to which humans have no immunity emerges . The virus causes severe illness or death in humans . The virus spreads easily from person to person across the globe.

13. Nucleic acids Viruses can store their genetic information in 4 different types of nucleic acid. i ) SS DNA ii) dsDNA iii) ssRNA iv) dsRNA These are found in animal viruses. Plant virus contain ssRNA as a genome. Phages contain SS DNA or SS RNA. Some viruses contain unusual bases. For example T-even phages of E.coli contain 5-hydroxymethylcytosine instead of cytosine. Glucose is attached to the hydroxyl group. RNA virus contain 2 strands: positive and negative Positive: it acts like mRNA & translated into proteins by host cells ribosome. Negative: it does not make sense to the host cell ribosome. RNA genome is a segmented genome . They are divided into separate parts. Each part codes one protein. STRUCTURE OF VIRUS

14. A virus must hijack a living host cell to replicate . The host cell synthesizes all the necessary viral components . The synthesized components are assembled into new virions . New virions are released from the host cell . Viral Replication Cycle : Divided into five distinct steps . Penetration Attachment Uncoating Biosynthesis Assembly Release GENERAL FEATURES OF VIRUS REPLICATION

15. Bacteriophages are viruses that infect bacteria. Also known as phages (coming from the root word ‘ phagein ’ meaning “to eat ”). Bacteriophages were discovered independently by Frederick W. Twort in the U.K and Félix d’Hérelle in France . These are found throughout the world in different environments (soil, water) and are even recognized as one of the most abundant biological agents on earth . It consist of a nucleic acid genome which is enclosed inside a shell of phage-encoded capsid proteins . The mechanism of infection of bacteriophages remains almost the same where they first attach to the host cell and enter their genome into the host cell to suspend the host cellular machinery ( Lambda phage and T2 phage – that only infect E. coli ) The ability of phages to infect and possibly kill infectious bacterial agents puts forward their potential as a possible supplement or replacement for antibiotic agents . BACTERIOPHAGE

16. TYPES OF PHAGES Life cycles of Bacteriophages Bacteriophages, like all other viruses, follow a similar trajectory where the virus enters the bacterial host cell in order to replicate. 1. Lytic cycle - DNA replicates separately from the host DNA . 2. Lysogenic cycle - the viral DNA is incorporated into the host DNA

17. LYTIC CYCLE (Infective or Virulent Cycle) The lytic cycle is a virulent infection caused by virulent phage and it results in the destruction of a cell .

18. LYSOGENIC CYCLE (Temperate or Non-virulent cycle) I ncorporation of the bacteriophage genome into the host genome – Prophage DNA It’s a non-virulent infection as the bacteriophage doesn’t kill the host cell.

19. MODE OF ACTION OF ANTIVIRAL DRUGS Viruses rely on their eukaryotic hosts for reproduction and metabolic functions . Most antiviral drugs also affect host structures, leading to host toxicity . However , some compounds are more toxic to viruses than to the host . A few antiviral agents specifically target viruses . Significant progress in antiviral drug development is largely due to efforts aimed at controlling infections caused by the human immunodeficiency virus (HIV), the virus responsible for AIDS . These achievements have led to advancements in the development and use of antiviral agents.

20. ANTIVIRAL DRUGS - EXAMPLES Nucleoside analogs or nucleoside reverse transcriptase inhibitors (NRTI) The first compound to gain universal acceptance in this category was zidovudine , or azidothymidine (AZT). AZT inhibits retroviruses such as HIV. Azidothymidine is chemically related to thymidine but is a dideoxy derivative, lacking the 3-hydroxyl group . AZT inhibits multiplication of retroviruses by blocking reverse transcription and production of the virally encoded DNA intermediate. This inhibits multiplication of HIV . Many NRTIs also lose their antiviral potency with time due to the emergence of drug-resistant viruses. 2. Non-nucleoside reverse transcriptase inhibitor (NNRTI) Nevirapine binds directly to reverse transcriptase and inhibits reverse transcription. NNRTIs generally induce some level of host toxicity because their action also affects normal host cell nucleic acid synthesis.

21. ANTIVIRAL DRUGS - EXAMPLES 3. Protease inhibitors Protease inhibitors are a class of antiviral drugs used for HIV treatment . These drugs prevent viral replication by binding to the active site of HIV protease . Inhibition of this enzyme blocks the processing of large viral proteins into individual components . This action prevents the maturation of the virus. 4. Influenza antiviral agents Two categories of drugs effectively limit influenza infection : adamantanes and neuraminidase inhibitors . Adamantanes ( amantadine and rimantadine ) are synthetic amines that interfere with an influenza A ion transport protein . Neuraminidase inhibitors ( oseltamivir (brand name Tamiflu ), and zanamivir ( Relenza )) block the active site of neuraminidase in influenza A and B viruses . This inhibition prevents virus release from infected cells.

22. ANTIVIRAL DRUGS - EXAMPLES 5 . Interferons Virus infected cells secrets certain proteins which protects the uninfected cells from further viral replication is called Interferons. High production of interferons occurs with low-virulence viruses; little is produced for highly virulent viruses . Three molecular forms of interferons : IFN- α : produced by leukocytes IFN- β : produced by fibroblasts IFN- γ : produced by immune lymphocytes Interferon activity is host-specific, not virus-specific . They have potential as antiviral and anticancer agents, with approved recombinant forms available . Clinical use is limited due to the need for local delivery of high concentrations.

23. VIROIDS Viroids : Viroids are infectious RNA molecules without a capsid. Size: 246 to 399 nucleotides . It contains circular, single stranded RNA molecules Cause numerous plant diseases. Examples of viroids : Coconut cadang-cadang viroid (246 nucleotides) Citrus exocortis viroid (375 nucleotides) Potato spindle tuber viroid ( 359 nucleotides )

24. PRIONS Prions are infectious proteins without genetic material. They cause animal diseases like : Scrapie in sheep Bovine spongiform encephalopathy (BSE or "mad cow disease") in cattle Chronic wasting disease in deer and elk Kuru and Creutzfeldt– Jakob disease (CJD) in humans No prion diseases are known in plants, but prions have been found in yeast . Animal prion diseases are called transmissible spongiform encephalopathies (TSEs ). Stanley B. Prusiner won the Nobel Prize in 1997 for his work on prions . In 1996, BSE in cattle was linked to a new form of CJD in humans, called variant CJD ( vCJD ). vCJD spread through contaminated beef, raising global health concerns and affecting the animal husbandry industry . Most BSE cases were in the UK and EU, linked to improper feeding practices (feeding rendered cattle/sheep tissue to uninfected animals ). The practice was banned in 1994 in the EU, leading to a significant drop in BSE cases.No TSE transmission has been found in swine, chicken, or fish. Stanley B. Prusiner Prions