STRUCTURAL FEATURES AND CLASSIFICATION OF VIRUSES

MORPHOLOGY AND PROPERTIES OF BACTERIOPHAGES

1. Bacteriophages ("bacteria eaters") are viruses that parasitize in bacterial cells. Their size ranges from 20 to 200 nm, i.e. in the same range as the size of the viruses. Like viruses, bacteriophages pass through bacterial fine-pore filters (that is, they are filtering agents) and are able to reproduce only in living young bacterial cells.

2. Bacteriophages are widespread in nature and are found everywhere where bacteria are found. They can be isolated from water, soil, milk; they can be found in the body of humans and animals. With intestinal diseases, the bacteriophage can be isolated from the patients' feces; at other infections it is allocated from a corresponding material (from a pus, a sputum, etc.). Especially easy it stands out in the period of recovery.

In addition to bacteriophages, viruses of fungi and actinomycetes have been detected; they received a common name - phages.

3. Using an electron microscope, it is shown that most bacteriophages have the form of a tadpole or a sperm from 20 to 800 nm. They consist of a head (65-100 nm) and a caudal process (100-200 nm). The appendage is a rod, clad in a sheath, capable of contraction, and ends with a hexagonal plate with short spines, from which fibrils departing, which determine the specificity of phage adsorption on the bacterial cell. Inside the head is a double-stranded DNA, packed in the form of a spiral. The capsid, surrounding the DNA and tail process, consists of protein subunits stacked in a cubic or spiral type. Protein proteins differ from the head proteins. In addition to structural proteins, enzyme proteins, lysozyme and ATPase, are involved in the phage process, which participate in interaction with the cell (dissolution of the host cell wall and shrinkage of the cover). In the head there are internal genomic proteins (they include polyamines - spermatozoa and putrescine) involved in stabilizing the structure of DNA.

Some phages instead of DNA contain RNA; for some - the cover does not shrink. There are phages with short processes (round, cubic, rod-shaped or filiform) and without them.

5. Phages have well-pronounced antigenic properties, in the body antibodies are formed on them, neutralizing the phage's lytic activity. The action of antifog serums is strictly specific, therefore, phages differ from each other in antigenic properties. The phages display group antigens (common to related phages) and type-specific antigens (strictly specific). According to antigenic properties, some phages are divided into serological groups (for example, the staphylococcus phage has 6 serogroups).

6. Bacteriophages are more resistant to physical and chemical factors than bacteria. Well-tolerated freezing (-185С), drying, heating up to + 70C, stored for a long time at low temperatures, stored in sealed tubes for years, and when air is destroyed, after 6-8 weeks. In alcohol, ether, chloroform, the phage does not change noticeably; 0.5% solution of the mercury and 1% solution of phenol have no effect on it. 1% formalin solution inactivates the phage after a few minutes; The harmful effect of glycerol in high concentrations, UV rays, ionizing radiation. Bacteriophages have a high sensitivity to acids. This should be considered for enteral administration of phage with therapeutic or prophylactic purposes (soda is used to neutralize the HCl of the stomach).

Phages have the ability to adapt to environmental conditions (temperature, etc.) and adapt to other types of bacteria, changing their specificity. For example, the typhoid phage can be made active in relation to the latter by successive re-crossings on the dysentery culture. It is possible to obtain a phage that is resistant to high concentrations of glycerol. Variants of phages resistant to antifog sera are known.

 

Types of cell culture

1. Primary cell culture:

· These are normal cells derived from animal or human cells.

· They are able to grow only for limited time and cannot be maintained in serial culture.

· They are used for the primary isolation of viruses and production of vaccine.

· Examples: Monkey kidney cell culture, Human amnion cell culture

2. Diploid cell culture (Semi-continuous cell lines):

· They are diploid and contain the same number of chromosomes as the parent cells.

· They can be sub-cultured up to 50 times by serial transfer following senescence and the cell strain is lost.

· They are used for the isolation of some fastidious viruses and production of viral vaccines.

· Examples: Human embryonic lung strain, Rhesus embryo cell strain

3. Heteroploid cultures (Continuous cell lines):

· They are derived from cancer cells.

· They can be serially cultured indefinitely so named as continuous cell lines

· They can be maintained either by serial subculture or by storing in deep freeze at -70°c.

· Due to derivation from cancer cells they are not useful for vaccine production.

· Examples: HeLa (Human Carcinoma of cervix cell line), HEP-2 (Humman Epithelioma of larynx cell line), Vero (Vervet monkey) kidney cell lines, BHK-21 (Baby Hamster Kidney cell line).

Advantages of cell culture

1. Relative ease, broad spectrum, cheaper and sensitivity

STRUCTURAL FEATURES AND CLASSIFICATION OF VIRUSES

1. Viruses are the smallest microorganisms that do not have a cellular structure (non-cellular forms of life). They are united in a separate kingdom of Vira. The sizes of viruses are expressed in nanometers (nm); 1 nm = 10^-3 μm. Because of the small size, the viruses are not visible in the light microscope and are able to pass through bacterial filters.

2. The viruses were discovered in 1892. Ivanovsky. He found a filter agent filtered through bacterial filters, causing a mosaic disease of tobacco leaves. He first experimentally reproduced the infectious disease of the "tobacco mosaic", lubricating the leaves of healthy plants with a gruel made from the tissues of sick plants, and he managed to infect healthy plants with a filtrate from the tissues of diseased plants. The presence of a contagious start in the filtration of plant tissues was proved. At the same time, no microorganisms were detected microscopically in the filtrate. Because the contagious beginning possessed the ability to pass through bacterial filters (ie, filter), Ivanovsky gave it the name of a "filtering virus" (virus).

Later (1898, 1901), the same filtering agents were discovered which caused foot and mouth disease in animals, yellow fever in humans. So, during the decade, unusual pathogens of infectious diseases of plants, animals and humans were discovered.

At present, more than 600 pathogens of various viral infections in humans are known: smallpox, rabies, influenza, mumps, measles, poliomyelitis, tick-borne encephalitis, hemorrhagic fever, AIDS, etc. A certain role of viruses in the development of malignant tumors is shown.

3. Viruses are obligate intracellular parasites. For the manifestation of vital activity they need a full-fledged living cell. They are able to penetrate the cell, and only in it viruses can multiply, using the ready metabolism. This is connected with the impossibility of cultivating viruses on artificial nutrient media

4. Because viruses are not visible in a light microscope and do not grow on artificial nutrient media, their study is difficult. Elucidation of the structure and properties of viruses became possible after the invention of an electron microscope and the use of new research methods, such as gradient centrifugation, chromatography, immunoelectrophoresis.

5. The viruses seem to stand between animate and inanimate nature. Before entering the cell, they behave like molecules of giant size and look like crystals of different shapes. In a cell, viruses become living beings that can reproduce and transmit their properties by inheritance. In connection with this, two forms of the existence of viruses are distinguished: a) extracellular form - virion; it is already a viral particle formed; b) intracellular form or autonomic reproductive form.

6. Virions have sizes from 15-18 nm (polio virus) to 300-400 nm (variola virus). The dimensions of the virions are determined by electron microscopy, by ultrafiltration through filters with a known pore diameter and by ultracentrifugation. The structure and chemical composition were studied by electron microscopy.

The form of the virions can be different: rod-shaped (tobacco mosaic virus), bullet (rabies virus), spherical (polio virus), cuboidal (variola virus), in the form of a sperm (many bacteriophages).

Virions consist of a single type of nucleic acid (DNA or RNA) and proteins, i.e. is a complex of these two types of organic compounds (nucleoproteins). In the center is DNA or RNA, which is surrounded by one or two shells.

7. Simple viruses have one protein shell - a capsid, which is built from repeated morphological subunits - capsomers, packed in a strictly ordered structure in the form of a spiral, cubic or mixed symmetry. Nucleic acid and capsid together are called nucleocapsid.

8. Complex viruses have two shells: capsid and supercapsid, which besides proteins also contain lipids and carbohydrates and is formed during the virus release from the host cell, i.e. Supercapsid is a derivative of the membrane structures of the host cell.

9. DNA or RNA involved in the storage and transmission of hereditary information, determine the infectious properties of viruses.

Virus proteins have specific specificity, determine their antigenic and immunogenic properties. Perform the following functions:

a) the capsid proteins protect the nucleic acid, participate in the adsorption of the virus on the surface of the host cell; resistant to the action of proteolytic enzymes (enzymes that break down proteins); have the ability to self-assembly;

b) internal histone-like proteins are in complex with the nucleic acid and stabilize its structure;

c) virion enzymes participate in the penetration of viral NK into the cell (lysozyme, ATPase, etc.), in replication and transcription (DNA and RNA polymerase).

In addition to the virus's own enzymes, a group of virus-induced enzymes (RNA-dependent DNA polymerases) is isolated. Information on their structure is stored in the genome of the virus, and the synthesis proceeds on the ribosomes of the host cell.

In the process of interaction with the virus, a group of enzymes of the host cell (cellular enzymes) that participate in the synthesis of viral components are modified.

Lipids and carbohydrates of complex viruses form complexes with proteins - lipoproteins and glycoproteins. As part of the super-capsid, they protect the virus from adverse environmental effects. Glycoproteins - viral antigens - hemagglutinins, which cause the reaction of erythrocyte agglutination.

10. Viruses have a unique gene, because contain either DNA or RNA. They are usually haploid, i.e. have one set of genes. Nucleic acids can be single- or double-stranded, linear, ring, fragmented. Among the RNA-containing viruses, viruses with a positive genome are distinguished (they have a plus-strand RNA). RNA of these viruses simultaneously performs the hereditary function and function of mRNA. There are also viruses with a negative genome (have minus strand RNA). RNA of these viruses performs only a hereditary function.

The genome of viruses can be built into the genome of the host cell (in its DNA) in the form of provirus, being at the same time a genetic parasite of the cell. Nucleic acids of some viruses (eg, the herpes virus) can be located in the cytoplasm of the host cell, resembling plasmids.

11. The classification of viruses is based on the following principles: the type of nucleic acid (DNA or RNA), its structure, the number of threads (one or two), the characteristics of the reproduction of the viral genome; the size and morphology of virions, the number of capsomers and the type of symmetry, the presence of a supercapsid; sensitivity to ether, deoxycholate; place of reproduction in a cage; antigenic properties.

In virology, the following taxonomic categories are used: the family (the name ends in viridae), the subfamily (the name ends in virinae), the genus (the name ends in a virus). By the type of nucleic acid, viruses are divided into 2 groups: DNA viruses and RNA viruses. Names of genera and subfamilies are not formulated for all viruses. The species of the virus did not receive a binary name as in bacteria. For example, rabies virus - RNA virus, belongs to the family Rhabdoviridae, the genus Lyssavirus.

12. In addition to conventional viruses, prion-protein particles are known to have infectivity (they have the form of fibrils, 10-20  100-200 nm) and viroids are small molecules of ring, supercoiled RNA that do not contain protein, causing diseases in plants.

 

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