MICROBIOLOGY: STUDY OF VIRUSES

2a) Viruses: Introduction

1. Study of viruses is called virology and Beijerinck is recognized as 'father of virology'. He reported enormous power of multiplication of viruses inside host cell. He also studied mechanism of nitrogen fixation by microorganisms like bacteria.
2. The term 'virus' has been derived from Greek word 'virion' which means 'poison'. Viruses are defined as hyperparasites and 'filterable entities' not having any cellular organization and defined as hyperparasites and 'filterable entities' not having any cellular organization and because of this cell theory is not applicable on them. They are thus exceptions to cell theory. They form crystals when taken out of the host cell. This shows non-living nature of viruses, which was first revealed by Stanley while working on 'Tobacco Mosaic Virus'. Nucleoprotein structure of viruses was first studied by Bawden & Pierre. In nucleoprotein structure of viruses, core of nucleic acid is surrounded by coat of protein. Viruses are unique as they contain only one type of nucleic acid. Mutation in viruses was first reported by Delbruck. Mutation and enormous power of reproduction confirm living nature of viruses. Viruses are thus the only entities, which have characters of non-living and living entities both. They are thus 'connecting links' between non-living entities and lining entities.

2b) Viruses: Historical Studies

Studies in viruses started with independent works of Pasteur and Adolf Mayer. Pasteur prepared vaccines of rabies virus and used the term 'virus' for first time. Mayer reported mosaic like spots on leaves of tobacco plant infected with virus. He described the virus as Tobacco Mosaic Virus [TMV]. Ivanowski passed the cell sap of infected tobacco leaves through bacteria proof bacteria proof filter and sprayed the filtrate on healthy leaves. Mosaic spots appeared again. Ivanowski through his experiment described viruses as filterable entities. Famous conclusion of Beijerinck about viruses i.e. 'contagium vivum fluidum' means sap retains the infection of viruses (living fluid infectant). Takami of Holland studied transmission of viruses through various agencies. He observed that insects are important agencies in transmission of viruses. Insects that transmit viruses are called 'vertors'. Viruses-vector relationship is highly specific.

Culture of viruses was prepared for the first time by Ender. It was poliomyelitis virus. Hershey and Chase observed that nucleic acid is the infective part of virus. They selected Bacteriophages for their experiment. Bacteriophages are DNA containing viruses which attack bacteria. They were discovered by Twort; further studies were made by DeHerelle. Experiment of Hershey & Chase is another example which confirms status of DNA as genetic material. Safferman & Morris discovered 'Cyanophages' i.e., viruses which attack blue green algae like Lyngbya, Phormidium & Plectonema. They are also called LPP-1 on the basis of hosts. Mycophages were discovered by Sinoten. They contain ds RNA.

Issac & Lindeman discovered a protein called Interferon secreted by host cells against infection of viruses. They selected cold virus for their experiment. Gibbs described viruses in coded form which is called 'Cryptogram of viruses' Cryptogram of viruses is based on 4 pairs of characters:

First pair : Type of nucleic acid in virus i.e. R or D

Number of thread in nucleic acid 1 or 2

Second pair : Molecular weight of viruses in millions i.e. 2

Percentage of nucleic acid of the viruses 5

Third pair : Shape of the virus in total i.e. E (elongeted) or S (spiral) or

X (unknown)

Shape of the protein coat of the virus E or S or X

(E' means elongated structure or rod shaped structure with blunt ends)

Fourth pair : Host/vector i.e. S (seed plant), I(invertebrate), V(vertebrate),

B(bacterium), Fu(fungus), Di(diptra), Af(aphid), Zero or 0 (novector),

X(unknown vector) Vector of the virus X

Thus, for example R/1; 2/5; E/E; S/X is cryptogram of TMV.

For Polia Virus (Poliovirus Primus) cryptogram is R/1; 5/30; S/S; V/O.

For Influenza Virus (mixovirus influenzae) R/1; 2-3/10; S/E; V/O.

For T4 phage D/2; 130/40; X/X; B/O.

2c) Viruses: Some Terms

1. A complete virus having a core of nucleic acid and coat of protein is called virion. Protein coat of the virus is termed as capsid; units of capsid are capsomeres. A virus drops its infective part i.e., nucleic acid alone inside host cell; empty protein coat remains outside. This empty protein coat, which remains outside the host called ghost protein, disappears later on. Raw material for new protein coat of the progeny viruses is contributed by the host.

Þ A few proteins in some viruses are enzymatic in functions e.g. lysozyme in bacteriophages, reverse transcriptase(RNA dependent DNA polymerase) in retrovituses, neuraminidase in influenza virus etc.

2. Viriod: Subviral agent having infectious nucleic acid alone is called viroid. Viroids are RNA & particles lacks protein coat. Spindle disease in photo tuber is caused by viroid, which is PSTV. Viroids were discovered by Diener & Rayner.
3. Prion or Slow Virus: Protein macromolecules causing infections are called prions. They lack ucleic acids. Scrappes & Kuru diseases (Severe brain infection) is sheep are caused by prions. They were first reported by Priusner who was awarded Nobel Prize for reporting Mad Cow disease caused by PrP (prion related protein). Bovine spongiform encephalopathy or Creulzfeldt Jacob disease (a type of insanity in young people), Kuru disease (the laughing death), Alzheimer's disease are caused by prions. In 1976 D.C.Gujdusek was awarded Nobel Prize for his work on prion based diseases.
4. Viruses transmitted by insects are called Arboviruses. Viruses causing Dengue, encephalitis etc are arboviruses.
5. Viruses which contain ss-DNA are called 'Gemini viruses'. ÆX174 phages are geminiviruses. They were reported by Sinsheimer.
6. Viruses which enter inside host cell as intact structures, are called Adeno viruses. Well defined protein coat is absent in Adenoviruses.
7. Virusoids or Satellilte RNAs (Rundle et. Al 1981). These are small circular RNAs similar to viroids, but are located inside the protein coat of a true virus. Virusoids required the assistance of virus for their replication.

2d) Viruses: Classification and Nomenclature

Earlier it was believed that plant viruses i.e. Phytophagineae contain RNA whereas animal viruses i.e. Zoophagineae contain DNA. This is, however, no more correct now. The only thing which is true about viruses is presence of only one type of nucleic acid. Cold virus i.e. Rhinovirus and poliovirus i.e. Enterovirus is together placed under Picorna virus.

2e) Viruses: Disease on Plants – Symptoms

1. Mosaic like spots may develop on leaves.
2. Leaves may show rolling or curling.
3. Leaves may show loss of chlorophyll (Chlorosis) or death of tissues (necrosis).
4. Plants may show abnormal or stunted growth.
5. Plants may show uncontrolled cell division i.e. hyperplasia(cell number increase) and unwanted swelling i.e. Hypertrophy in host.
6. Plants may show loss of entire inflorescence or flowers may become green i.e. virescence in flowers.

2f) Viruses: Some Common Viral Diseases

1. Plant Viral Diseases. Tobacco Mosaic, Potato Mosaic Pumpkin Mosaic, Apple Mosaic, Tulip Mosaic (Broken Tulip), Wheat Mosaic Streak, Bhindi Yellow Vein Mosaic, Potato Leaf Roll, Tobacco Leaf Curl, Papaya Leaf Curl. Banana Bunchy Top, Tomato Bunchy Top, Rice Tungro etc.

2. Animal Viral Diseases. Foot and Mouthm, Enquine encephalitis, Rinderpest, Kysanur Forest Disease (KFD) of monkeys (also humans), New Castle or Ranikhet of Fow].

3. Human Viral Diseases. Poliomyelitis, Rabies, Mumps, Measles, German Measles, Chickenpox, Smallpox (eradicated), Dengue, Encephalitis, Common cold, Flu, some gastrointestinal diseases, Yellow Fever, Herpes, Hepatitis, SARS, AIDS.