Viruses- the smallest pathogens of infectious diseases. Virus(lat. virus- “poison”) is a non-cellular infectious agent that can reproduce only inside living cells. Until the end of the 19th century. the term "virus" was used in medicine to refer to any infectious agent that causes disease. This word acquired its modern meaning after 1892, when the Russian botanist D.I. Ivanovsky established the “filterability” of the causative agent of tobacco mosaic disease (tobacco mosaic). He showed that cell sap from plants infected with this disease, passed through special filters that retain bacteria, retains the ability to cause the same disease in healthy plants. Five years later, another filterable agent - the causative agent of foot-and-mouth disease in cattle - was discovered by the German bacteriologist F. Loeffler. In 1898, the Dutch botanist M. Beijerinck repeated these experiments in an expanded version and confirmed Ivanovsky’s conclusions. He called the “filterable poisonous principle” that causes tobacco mosaic a “filterable virus.” This term has been used for many years and has gradually been shortened to one word - "virus".
In 1901, American military surgeon W. Reed and his colleagues found that the causative agent of yellow fever is also a filterable virus. Yellow fever was the first human disease identified as viral, but it took another 26 years for its viral origin to be definitively proven.
-Viruses- the simplest form of life, a microscopic particle consisting of nucleic acid molecules (DNA or RNA) enclosed in a protein shell (capsid) and capable of infecting living organisms. Their sizes range from 20 to 300 nm. The genetic material is represented by one nucleic acid molecule (DNA or RNA), not associated with proteins. The molecular weight of DNA (RNA) varies from 3·106 to 5·106. The nucleic acid of viruses can be single- or double-stranded (circular or linear). After entering the host cell, the nucleic acid of the virus, using the cell’s enzyme system, begins to replicate, synthesizing specific proteins and new viral particles.
-From others Infectious agents viruses are distinguished by a capsid. Viruses, with rare exceptions, contain only one type of nucleic acid: either DNA or RNA.
Sizes – from 15 to 2000 nm (some plant viruses). The largest among animal and human viruses is the causative agent of smallpox - up to 450 nm.
Simple viruses have an envelope - capsid, which consists only of protein subunits ( capsomeres). The capsomeres of most viruses have helical or cubic symmetry. Virions with helical symmetry are rod-shaped. Most viruses that infect plants are built according to the spiral type of symmetry. Most viruses that infect human and animal cells have a cubic type of symmetry.
Complex viruses
Complex viruses can be additionally covered with a lipoprotein surface membrane with glycoproteins that are part of the plasma membrane of the host cell (for example, smallpox viruses, hepatitis B), that is, they have supercapsid. With the help of glycoproteins, specific receptors are recognized on the surface of the host cell membrane and the viral particle attaches to it. The carbohydrate regions of glycoproteins protrude above the surface of the virus in the form of pointed rods. The additional envelope can merge with the plasma membrane of the host cell and facilitate the penetration of the contents of the viral particle deep into the cell. Additional shells may include enzymes that ensure the synthesis of viral nucleic acids in the host cell and some other reactions.
Bacteriophages have a rather complex structure. They are classified as complex viruses. For example, bacteriophage T4 consists of an expanded part - a head, a process and tail filaments. The head consists of a capsid that contains nucleic acid. The process includes a collar, a hollow shaft surrounded by a contractile sheath resembling an extended spring, and a basal plate with caudal spines and filaments.
Classification of viruses
The classification of viruses is based on the symmetry of the viruses and the presence or absence of an outer shell.
| Deoxyviruses | Riboviruses | ||
| DNA double-stranded | DNA single-stranded | RNA double-stranded | RNA single-stranded |
| Cubic symmetry type: – without outer shells (adenoviruses); – with external membranes (herpes) | Cubic symmetry type: – without outer membranes (some phages) | Cubic symmetry type: – without outer shells (retroviruses, plant wound tumor viruses) | Cubic symmetry type: – without outer shells (enteroviruses, poliovirus) Spiral symmetry type: – without outer shells (tobacco mosaic virus); – with outer membranes (influenza, rabies, oncogenic RNA-containing viruses) |
| Mixed type of symmetry (T-paired bacteriophages) | |||
| Without a certain type of symmetry (pox) | |||
Viruses exhibit vital activity only in the cells of living organisms. Their nucleic acid is capable of causing the synthesis of viral particles in the host cell. Outside the cell, viruses do not show signs of life and are called virions.
The life cycle of the virus consists of two phases: extracellular(virion), in which it does not show signs of vital activity, and intracellular. Viral particles outside the host’s body do not lose their ability to infect for some time. For example, the polio virus can remain infectious for several days, and smallpox for months. The hepatitis B virus retains it even after short-term boiling.
The active processes of some viruses occur in the nucleus, others in the cytoplasm, and in some, both in the nucleus and in the cytoplasm.
Types of interaction between cells and viruses
There are several types of interactions between cells and viruses:
- Productive – the nucleic acid of the virus induces the synthesis of its own substances in the host cell with the formation of a new generation.
- Abortive – reproduction is interrupted at some stage, and a new generation is not formed.
- Virogenic – the nucleic acid of the virus is integrated into the genome of the host cell and is not capable of reproduction.
Representatives of non-cellular life forms are viruses - tiny particles that penetrate inside the cell. The branch of microbiology that studies viruses is called virology.
general description
Viruses are found in the atmosphere, soil, and water. There are viruses of plants, animals, fungi, and bacteria. Viruses that infect bacteria are called bacteriophages. There are satellites that enter the cell only if there is an additional virus in it.
Rice. 1. Bacteriophage.
Most viruses cause infections; some types have no visible effect. One of the interesting facts is the presence of viral residues in human DNA.
Viruses have a variety of shapes (balls, spirals, rods) and smallest sizes - 20-300 nm (1 million nm in 1 mm). The largest viruses are mimiviruses, having a diameter of 500 nm. They imitate the structure and activity of bacteria, and some scientists consider mimiviruses to be a transitional form from viruses to bacteria.
Rice. 2. Mimiviruses.
A brief description of viruses and their differences from living and nonliving matter is presented in the table.
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Viruses are classified into a separate kingdom and classified into five taxa. Most viruses have not yet been studied and classified.
Modern classification includes:
- 9 squads;
- 127 families;
- 44 subfamilies;
- 782 genera;
- 4686 species.
Biologist David Baltimore in 1971 developed an alternative classification of viruses based on the characteristics of genetic information. Baltimore distinguished what types of viruses there are based on RNA or DNA content.
Its classification can be combined into three large groups:
- DNA viruses;
- RNA viruses;
- Viruses that convert RNA into DNA.
The main types of viruses in biology according to Baltimore are presented in the table.
|
Name |
Baltimore class |
Peculiarities |
Examples |
|
DNA viruses |
Double-stranded DNA. Reproduction in the cell nucleus |
Smallpox, herpes, papillomas viruses |
|
|
Single-stranded DNA. Reproduction in the nucleus |
Parvoviruses |
||
|
DNA is both double-stranded and single-stranded |
Hepatitis B virus |
||
|
RNA viruses |
Double-stranded RNA. Reproduction in the cytoplasm |
Reoviruses, rotaviruses |
|
|
Single-stranded messenger RNA (plus strand) |
Picornaviruses, flaviviruses |
||
|
Single-stranded RNA that carries no information (minus strand) |
Orthomyxoviruses, filoviruses |
||
|
RNA and DNA |
Single-stranded RNA (plus strand) turns into DNA |
Retroviruses (HIV) |
Viruses are structures that change the DNA of a cell, causing the cell to produce new viruses. When there are too many viruses, they rupture the cell membrane, come out and infect new cells. Sometimes they do not kill the cell, but bud from it.
Rice. 3. A virus that invades a cell.
What have we learned?
From the report of grades 5-6 we learned about the structure, features, and classification of viruses. They cannot be classified as either living nature or inanimate matter. In structure, viruses are proteins that carry hereditary information that is integrated into a living cell. Biologist Baltimore identified seven classes of viruses depending on the structural features of the genetic material.
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Lecture No. 5. General virology. Classification, structure and features of the biology of viruses. Bacteriophages.
Discovery of viruses by D.I. Ivanovsky in 1892 laid the foundation for the development of the science of virology. Its faster development was facilitated by the invention of the electron microscope and the development of a method for cultivating microorganisms in cell cultures.
Currently, virology is a rapidly developing science, which is due to a number of reasons:
The leading role of viruses in human infectious pathology (examples are influenza virus, HIV human immunodeficiency virus, cytomegalovirus and other herpes viruses) against the background of the almost complete absence of specific chemotherapy;
The use of viruses to solve many fundamental questions in biology and genetics.
Basic properties of viruses (and plasmids), in which they differ from the rest of the living world.
1.Ultramicroscopic dimensions (measured in nanometers). Large viruses (smallpox virus) can reach sizes of 300 nm, small ones - from 20 to 40 nm. 1mm=1000µm, 1µm=1000nm.
3.Viruses are not capable of growth and binary fission.
4.Viruses reproduce by reproducing themselves in an infected host cell using their own genomic nucleic acid.
6. The habitat of viruses is living cells - bacteria (these are bacterial viruses or bacteriophages), plant, animal and human cells.
All viruses exist in two qualitatively different forms: extracellular- virion and intracellular virus. The taxonomy of these representatives of the microcosm is based on the characteristics of virions, the final phase of virus development.
Structure (morphology) of viruses.
1.Virus genome form nucleic acids, represented by single-stranded RNA molecules (in most RNA viruses) or double-stranded DNA molecules (in most DNA viruses).
2.Capsid- a protein shell in which the genomic nucleic acid is packaged. The capsid consists of identical protein subunits - capsomers. There are two ways to package capsomers into a capsid—helical (helical viruses) and cubic (spherical viruses).
With spiral symmetry protein subunits are arranged in a spiral, and between them, also in a spiral, the genomic nucleic acid (filamentous viruses) is laid out. With cubic type of symmetry virions can be in the form of polyhedra, most often twenty-hedra - Icosahedrons.
3.Simply designed viruses only have nucleocapsid, i.e. the complex of the genome with the capsid is called “naked”.
4. Other viruses have an additional membrane-like shell on top of the capsid, acquired by the virus at the time of exit from the host cell - supercapsid. Such viruses are called “dressed”.
In addition to viruses, there are even more simply organized forms of agents capable of being transmitted - plasmids, viroids and prions.
The main stages of interaction between the virus and the host cell.
1.Adsorption is a trigger mechanism associated with interaction specific receptors of the virus and the host (in the influenza virus - hemagglutinin, in the human immunodeficiency virus - glycoprotein gp 120).
2. Penetration - by fusion of the supercapsid with the cell membrane or by endocytosis (pinocytosis).
3.Release of nucleic acids - “undressing” of the nucleocapsid and activation of the nucleic acid.
4. Synthesis of nucleic acids and viral proteins, i.e. subordination of the host cell systems and their work for the reproduction of the virus.
5. Virion assembly - association of replicated copies of viral nucleic acid with capsid protein.
6. Exit of viral particles from the cell, acquisition of supercapsid by enveloped viruses.
Outcomes of the interaction of viruses with the host cell.
1.Abortion process- when cells are freed from the virus:
When infected defective a virus whose replication requires a helper virus, independent replication of these viruses is impossible (so-called virusoids). For example, hepatitis delta (D) virus can replicate only in the presence of hepatitis B virus, its Hbs - antigen, adeno-associated virus - in the presence of adenovirus);
When a virus infects cells that are genetically insensitive to it;
When sensitive cells are infected with a virus under non-permissive conditions.
2.Productive process- replication (production) of viruses:
- death (lysis) of cells(cytopathic effect) - the result of intensive reproduction and the formation of a large number of viral particles - a characteristic result of a productive process caused by viruses with high cytopathogenicity. The cytopathic effect on cell cultures for many viruses is of a fairly recognizable specific nature;
- stable interaction, which does not lead to cell death (persistent and latent infections) - the so-called viral transformation of a cell.
3.Integrative process- integration of the viral genome with the genome of the host cell. This is a special version of a productive process similar to stable interaction. The virus replicates along with the genome of the host cell and can remain latent for a long time. Only DNA viruses can integrate into the host DNA genome (the “DNA-in-DNA” principle). The only RNA viruses capable of integrating into the genome of a host cell, retroviruses, have a special mechanism for this. The peculiarity of their reproduction is the synthesis of proviral DNA based on genomic RNA using the reverse transcriptase enzyme, followed by the integration of DNA into the host genome.
Basic methods of cultivating viruses.
1.In the body of laboratory animals.
2. In chicken embryos.
3. In cell cultures - the main method.
Types of cell cultures.
1.Primary (trypsinized) cultures- chicken embryo fibroblasts (CHF), human fibroblasts (CHF), kidney cells of various animals, etc. Primary cultures are obtained from cells of various tissues most often by crushing and trypsinization and are used once, i.e. It is always necessary to have appropriate organs or tissues.
2.Diploid cell lines suitable for repeated dispersion and growth, usually no more than 20 passages (lose their original properties).
3.Interconnected lines(heteroploid crops), are capable of repeated dispersion and grafting, i.e. to multiple passages, most convenient in virological work - for example, tumor cell lines Hela, Hep, etc.
Special nutrient media for cell cultures.
A variety of synthetic virological nutrient media of complex composition are used, including a large set of different growth factors - medium 199, Needle, Hanks solution, lactalbumin hydrolyzate. pH stabilizers (Hepes), blood serum of various species (fetal calf serum is considered the most effective), L-cysteine and L-glutamine are added to the media.
Depending on the functional use of the environment, there may be height(with a high content of blood serum) - they are used for growing cell cultures before adding viral samples, and supportive(with less serum content or no serum) - for maintaining virus-infected cell cultures.
Detectable manifestations of viral infection of cell cultures.
1. Cytopathic effect.
2. Identification of inclusion bodies.
3. Detection of viruses by fluorescent antibodies (MFA), electron microscopy, autoradiography.
4. Color test. The usual color of culture media used, containing phenol red as a pH indicator, under optimal cell culture conditions (pH about 7.2) is red. Cell reproduction changes the pH and, accordingly, the color of the medium from red to yellow due to a shift in pH to the acidic side. When viruses multiply in cell cultures, cell lysis occurs, and the pH and color of the medium do not change.
5. Detection of viral hemagglutinin - hemadsorption, hemagglutination.
6.Method of plaques (plaque formation). As a result of the cytolytic effect of many viruses on cell cultures, zones of mass cell death are formed. Plaques are detected - viral “cell-negative” colonies.
Nomenclature of viruses.
The name of the family of viruses ends in “viridae”, genus - “virus”, special names are usually used for the species, for example, rubella virus, human immunodeficiency virus - HIV, human parainfluenza virus type 1, etc.
Bacterial viruses (bacteriophages).
The natural habitat of phages is a bacterial cell, so phages are distributed everywhere (for example, in wastewater). Phages have biological characteristics that are also characteristic of other viruses.
The most morphologically common type of phages is characterized by the presence of an icosahedral head, a process (tail) with spiral symmetry (often has a hollow rod and a contractile sheath), spines and processes (filaments), i.e. outwardly they somewhat resemble a spermatozoon.
The interaction of phages with a cell (bacterium) is strictly specific, i.e. bacteriophages are able to infect only certain species and phagotypes bacteria.
The main stages of interaction between phages and bacteria.
1.Adsorption (interaction of specific receptors).
2. The introduction of viral DNA (phage injection) is carried out by lysing a section of the cell wall with substances such as lysozyme, contracting the sheath, pushing the tail rod through the cytoplasmic membrane into the cell, and injecting DNA into the cytoplasm.
3. Phage reproduction.
4. Output of daughter populations.
Basic properties of phages.
Distinguish virulent phages, capable of causing a productive form of the process, and temperate phages, causing reductive phage infection (phage reduction). In the latter case, the phage genome in the cell is not replicated, but is introduced (integrated) into the chromosome of the host cell (DNA in DNA), the phage turns into prophage This process is called lysogeny. If, as a result of the introduction of a phage into the chromosome of a bacterial cell, it acquires new heritable characteristics, this form of bacterial variability is called lysogenic (phage) conversion. A bacterial cell carrying a prophage in its genome is called lysogenic, since the prophage, if the synthesis of a special repressor protein is disrupted, can enter the lytic development cycle and cause a productive infection with lysis of the bacterium.
Temperate phages are important in the exchange of genetic material between bacteria - in transduction(one of the forms of genetic exchange). For example, only the causative agent of diphtheria has the ability to produce exotoxin, into whose chromosome is integrated a moderate prophage carrying operon tox, responsible for the synthesis of diphtheria exotoxin. Temperate phagetoxcauses lysogenic conversion of nontoxigenic diphtheria bacillus into toxigenic one.
According to the spectrum of action In bacteria, phages are divided into:
Polyvalent (lyse closely related bacteria, such as salmonella);
Monovalent (lyse bacteria of one species);
Type-specific (lyse only certain phage products of the pathogen).
On solid media, phages are more often detected using a spot test (formation of a negative spot during colony growth) or the agar layer method (Gracia titration).
Practical use of bacteriophages.
1.For identification (determination of phagotype).
2.For phage prophylaxis (stopping outbreaks).
3.For phage therapy (treatment of dysbacteriosis).
4.To assess the sanitary state of the environment and epidemiological analysis.
Structure
Examples of icosahedral virion structures.
A. A virus that does not have a lipid envelope (for example, picornavirus).
B. Enveloped virus (eg, herpesvirus).
The numbers indicate: (1) capsid, (2) genomic nucleic acid, (3) capsomere, (4) nucleocapsid, (5) virion, (6) lipid envelope, (7) membrane envelope proteins.
Classification
Squad ( -virales) Family ( -viridae) Subfamily ( -virinae) Genus ( -virus) View ( -virus)Baltimore classification
Nobel laureate, biologist David Baltimore, proposed his own classification scheme for viruses based on differences in the mechanism of mRNA production. This system includes seven main groups:
- (I) Viruses that contain double-stranded DNA and do not have an RNA stage (for example, herpesviruses, poxviruses, papovaviruses, mimivirus).
- (II) Double-stranded RNA viruses (eg rotaviruses).
- (III) Viruses containing a single-stranded DNA molecule (eg, parvoviruses).
- (IV) Viruses containing a single-stranded RNA molecule of positive polarity (for example, picornaviruses, flaviviruses).
- (V) Viruses containing a single-stranded RNA molecule of negative or double polarity (for example, orthomyxoviruses, filoviruses).
- (VI) Viruses containing a single-stranded RNA molecule and having in their life cycle the stage of DNA synthesis on an RNA template, retroviruses (for example, HIV).
- (VII) Viruses containing double-stranded DNA and having in their life cycle the stage of DNA synthesis on an RNA template, retroid viruses (for example, hepatitis B virus).
Currently, both systems are used simultaneously to classify viruses, as complementary to each other.
Further division is made on the basis of such characteristics as genome structure (presence of segments, circular or linear molecule), genetic similarity with other viruses, the presence of a lipid membrane, taxonomic affiliation of the host organism, and so on.
Story
Application of viruses
Links
- “The Nobel Committee was hit by viruses” Article. Newspaper "Kommersant" No. 181 (3998) dated 10/07/2008.
Literature
- Mayo M.A., Pringle C.R. Virus taxonomy - 1997 // Journal of General Virology. - 1998. - No. 79. - P. 649-657.
