Microbiology: Virus: An Obligate Intracellular Parasite

Virus

1. Introduction of Virus

2.History of Virus

3.Characteristics of Virus

4.Size of Virus

Virus- An infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host.

History of Virus

•Beijerinck (1897) coined the Latin name “virus” meaning poison

•He studied filtered plant juices & found they caused healthy plants to become sick.

•In 1886, the Dutch chemist Adolf Mayer showed that tobacco mosaic disease (TMD) was transmissible from a diseased plant to a healthy plant.

•In 1892, in an attempt to isolate the cause of TMD , the Russian bacteriologist Dimitri Iwanowski filtered the sap of diseased plants through a porcelain filter that was designed to retain bacteria. He expected to find the microbe trapped in the filter: instead, he found that the infectious agent had passed through the minute pores of the filter. When he infected healthy plants with the filtered fluid, they contracted TMD

•Wendell Stanley (1935) studies crystallized sap from sick tobacco plants. He discovered viruses were made of nucleic acid and protein.

Characteristics of Virus

Size of Virus

Classification of Virus

A. Based on the type of host needed , virus maybe classified as:

1.Bacteria viruses (Phaginae): they have DNA and are called bacteriophages or simply phages

2.Plant viruses (Phytophagniae): they have RNA and infect potato, sugarcane, tobacco, etc.

3.Animal viruses (Zoophaginae): they usually have DNA but may also have RNA & infect man, pigeon, parrot, dog, cow, arthropods,etc.

B. Based on physicochemical properties :

a. Primary characteristics :

1. Chemical nature of nucleic acid: RNA/DNA, single stranded double stranded, molecular weight

2. Structure of virion: helical, icosahedral, complex, naked /enveloped, no. of capsomers for icosahedral, diameter of nucleocapsule for helical viruses

b. Secondary characteristics :

1. Host range: Host species , specific host tissue / cell type

2. Specific surface structure: e.g. antigenic properties

3. Mode of transmission: e.g. faeces

●C. According to Nucleic acid

• A nucleic acid that encodes information for making proteins needed by virus are called positive sense nucleic acid

•A nucleic acid made up of bases complimentary for those of a positive sense nucleic acid are called as negative sense nucleic acid

a.DNA Virus

b.RNA Virus

Structure of Virus

•A virion is a complete fully developed viral particle composed of nucleic acid, surrounded by a coat that protects it from the environment & serves as a vehicle of transmission from one host cell to another.

•Viruses are not cellular, so they do not have a nucleus, cytoplasm & cell membrane

NUCLEIC ACID

•They contain either DNA or RNA , which is the genetic material

•Nucleic acid may be single stranded or double stranded

•It can be linear or circular

•In some viruses , they are present as separate segments ( e.g. influenza)

CAPSID OR ENVELOPE

•The nuclei acid of viruses is surrounded by a protein coat called the capsid

•Each capsid is composed of protein subunits called the capsomers.

•In some viruses , capsid is covered by envelop which usually consists of some combination of lipids , proteins & carbohydrates

•Some animal viruses when released from the host cell are coated with the layer of host cell plasm membrane which becomes the viral envelope

•Depending on the virus , the envelope may or may not be covered with spikes, which are carbohydrate protein complexes that project from the surface of the envelop

•some virus attach to host cell with help of spikes; can be used for identification of viruses

•Viruses whose capsid are not enveloped – naked viruses

•The capsid of a naked virus protects the nucleic acid from nuclear enzymes in biological fluids & promotes attachment of virus to susceptible host cells

•It also serves as a vehicle for transmission from one host cell to another

MORPHOLOGY OF VIRUS

1. Helical Virus

2. Polyhgral Virus

3. Enveloped Virus

4. Complex Virus

Effect of physical and chemical agents on Virus

•Human pathogenic viruses are inactivated after exposure of 60°C for 30 min

•They are stable at low temperatures & routinely stored at -40°C to -70°C

•Viruses are inactivated by heat & UV rays

•They are inactivated by organic solvents like chloroform & ether

•They are resistant to disinfectants like phenol , alcohols & quaternary ammonium compounds

•High concentrations are effective

•IPA - not effective against non –enveloped viruses

•Chlorine , hypochlorites , iodine , aldehydes , ethylene oxide, ascorbic acid & ozone are active against viruses

•They are not affected by antibiotics such as penicillins , streptomycins , tetracyclines , sulphonamides ,etc.