Types of interaction between virus and cell. Productive type of virus-cell interaction

The interaction of the virion with a living cell occurs in several stages.

IN initial (preparatory) period The virion attaches to the cell, penetrates into it, after which the protein shell of the virion is destroyed, releasing the nucleic acid.

Coming hidden (latent) period viral infection, during which the presence of viral particles in the infected cell cannot be detected by any methods - the parent virion seems to disappear. During this period, the viral nucleic acid that has entered the cell organizes the synthesis of the viral components of the offspring, using the host’s enzymatic system for this purpose. The reproduction cycle ends with the formation of daughter virions and their release from the cell ( final period ).

Simpler bacteria are not capable of capturing particles from the environment themselves. Therefore, bacteriophages have special devices for overcoming a dense bacterial wall. The end of the tail contains a special enzyme that dissolves the bacterial membrane. Then the microscopic “muscles” of the tail contract and the phage nucleic acid is “injected” into the cell, as if injected with a syringe.

As a result, the protein coat of the phage remains on the surface of the cell, and only the nucleic acid enters the cell.

The nucleic acids of viruses carry out a program to create new viral offspring in the cell. This has been proven by original experiments. It was possible to separate viruses into their constituent components - proteins and nucleic acids. It turned out that cell infection and virus multiplication occurred only after viral nucleic acid was added to the cells. In other words, the nucleic acids of viruses themselves can cause the reproduction of viruses, i.e., they have infectious properties. In another experiment, two viruses were separated into their constituent components and then “dressed up”: the nucleic acid of one virus was “dressed” in the shell of the other. The resulting hybrids infected sensitive cells. It was discovered that both “disguised” viruses are capable of multiplying, and the resulting offspring are always similar to the virus whose nucleic acid the hybrid contained.

The viral nucleic acid that has entered the cell controls all processes of virus reproduction. First, it forces the cell to synthesize so-called early proteins, which suppress the cell’s own metabolism and ensure the synthesis of nucleic acids of daughter particles. Their formation occurs as a result of self-copying of the parent nucleic acid. The genetic information contained in the nucleic acid of the virus determines the composition of the proteins from which the daughter particles of the so-called late proteins are built. In DNA-containing viruses, this information is realized in the usual way for a cell: informational RNA (transcription) is synthesized on DNA, which controls the subsequent biosynthesis of proteins (translation). The nucleic acid of many RNA-containing viruses combines both genetic and informational functions: RNA is involved in both replication and translation (in the reproduction of nucleic acids and virus protein). In many viruses, the construction of protein shells and internal contents occurs separately. The cell “accumulates” individual parts, which are then combined to form viral particles. When a sufficient number of “blanks” for future viral particles have accumulated in an infected cell, a kind of assembly of parts begins (composition). This process usually occurs near the cell membrane, which takes part in it. The viral particle often contains substances that

characteristic of the cell in which the virus multiplies. For example, for the influenza virus, the final stage of the formation of a viral particle is a kind of enveloping it with a layer of cell membrane. That is, the cell not only “feeds” and “waters” the virus, but also “dresses” them goodbye. The last stage of interaction between the virus and the cell is usually short-lived. The resulting full-fledged viral particles quickly exit into the external environment. The production of offspring in bacteriophages occurs in a very unique way. It is usually accompanied by the dissolution (lysis) of bacterial cells under the action of a special enzyme, which accumulates in the cell parallel to the reproduction of the phage and leads to its destruction and death. Under a microscope you can clearly see how this happens. Sometimes the bacteria seem to explode, in other cases a hole forms in the bacterium (in the middle or at one of the ends) through which its contents flow out. From one dead bacterium, up to several hundred new phage particles can be released. The process of phage reproduction continues until all bacteria sensitive to this phage are destroyed. Smallpox, polio, and encephalitis viruses are also characterized by the rapid release of hundreds, and sometimes thousands of daughter virions into the environment. Other human and animal viruses (herpes virus, mumps virus, reovirus) leave cells as they mature. These viruses manage to complete several cycles of reproduction before the cells die, gradually depleting the synthetic resources of the cells and causing their destruction. In some cases, V. can accumulate inside cells, forming crystal-like clusters (V. rabies, adenoviruses, etc.), which are called inclusion bodies.

With influenza, rabies, psittacosis, smallpox, such bodies are found in the cytoplasm of cells, with spring-summer encephalitis, yellow fever, herpes and poliomyelitis - in the nucleus; In some infections, inclusion bodies were found both in the nucleus and in the cytoplasm. Research in recent years has shown that in the vast majority of cases these inclusions are colonies of the virus, and their formation is natural at a certain stage of the reproduction of infectious agents. The high specificity of intracellular inclusions in viral diseases makes it possible to use this sign for diagnosis. For example, cytoplasmic inclusions (the so-called Negri bodies) found in nerve cells of the brain are the main evidence of rabies, and specific round or oval formations (the so-called Guarnieri bodies) found in epithelial cells indicate smallpox. Inclusions have also been described in encephalitis, spinal paralysis, foot-and-mouth disease and other diseases. Plant viruses form very peculiar inclusions that have a crystalline form. That is, the reproduction of viruses occurs in a special, incomparable way. First, viral particles penetrate into the cells and viral nucleic acids are released. Then the details of future viral particles are prepared. Reproduction ends with the assembly of new viral particles and their release into the environment. The loss of any of these stages leads to a disruption of the normal cycle and entails either complete suppression of V. reproduction or the appearance of inferior offspring.

The main stages of interaction between the virus and the host cell.

1. Adsorption - a trigger mechanism associated with the interaction of 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. Nucleic acid release - ―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.

Forms of viral infection.

At the level of the macroorganism, the main forms of viral lesions are not fundamentally different from those observed when individual cells are infected by viruses.

Productive viral infection with the formation of daughter populations and characteristic clinical manifestations is possible only if there are sensitive cells in the infected body in which the reproductive cycle of the pathogen is carried out. For example, the polio pathogen can replicate only in the cells of the gastrointestinal tract and central nervous system of primates and humans.

Abortion infection develops when the pathogen penetrates into insensitive cells (for example, when the bovine leukemia virus enters the human body) or into cells that are not capable of providing a full reproductive cycle (for example, those at the G0 stage of the cell cycle). The ability of cells to maintain virus-specific reproductive processes also suppresses IFN, the antiviral effect of which is directed against a wide variety of viruses.

Persistent viral infection occurs during such an interaction between the virus and the infected cell, when the latter continues to perform its own cellular functions. If infected cells divide, an infected clone is formed. Thus, an increase in the number of infected cells contributes to an increase in the overall population of the pathogen in the body. However, persistent viral infections usually impair cellular functions, eventually leading to clinical manifestations. In humans, the development of persistent infections depends to a certain extent on age. For example, intrauterine infection with rubella measles virus or cytomegalovirus (CMV) leads to time-limited persistence of the pathogen. The appearance of symptoms is associated with the ability of the fetus to develop immune responses to the infectious agent.

Latent (hidden) viral infection . While persistent infections are accompanied by the constant release of daughter viral populations, in latent lesions they are formed sporadically. The reproductive cycle of such pathogens slows down sharply in the later stages and is activated under the influence of various factors.

Latent infections are characteristic of most herpesviruses, causing recurrent and usually non-progressive diseases.

Inapparate infections *from lat. in-, denial, + appareo, be+ are accompanied by asymptomatic circulation of small amounts of the pathogen in individual organs. In this case, the pathogen can only be identified using special methods. What distinguishes such lesions from asymptomatic carriage is the high likelihood of clinical manifestations. This term is used for a number of infections in which there are no obvious signs of disease. In the practice of viral infections in humans, the alternative term “subclinical infection” is often used. Actually, latent infections can be regarded as chronically occurring in-device infections, in which a balance is established between the body and the pathogen.

Dormant (cryptogenic) viral infection - a form of manifestation of a viral infection in which the pathogen is in an inactive state in separate foci (for example, in the nerve ganglia). Clinically, the infection manifests itself only when the body’s defenses are sharply weakened. For example, type 3 herpes virus, which causes chickenpox during initial infection, persists in the body for life. Recurrence of the disease in the form of herpes zoster is possible only with impaired immune status (most often in old age).

Slow viral infections characterized by a long incubation period (months and years), during which the pathogen multiplies, causing increasingly obvious tissue damage. Initially, the pathogen multiplies in a limited group of cells, but gradually infects an increasing number of them. The diseases end with the development of severe lesions and death of the patient. Slow viral infections include subacute sclerosing panencephalitis, HIV infection, etc.

№ 19 Types of virus-cell interaction. Stages of viral reproduction.
Types of virus-cell interaction. There are three types of interaction between the virus and the cell: productive, abortive and integrative.
Productive type- ends with the formation of a new generation of virions and the death (lysis) of infected cells (cytolytic form). Some viruses leave cells without destroying them (non-cytolytic form).
Abortive type- does not end with the formation of new virions, since the infectious process in the cell is interrupted at one of the stages.
Integrative type, or virogeny- characterized by the incorporation (integration) of viral DNA in the form of a provirus into the cell chromosome and their joint coexistence (joint replication).
Reproduction of virusescarried out in several stages, successively replacing each other: adsorption of the virus on the cell; penetration of the virus into the cell; “undressing” the virus; biosynthesis of viral components in the cell; formation of viruses; release of viruses from the cell.
Adsorption.The interaction of a virus with a cell begins with the process of adsorption, i.e., the attachment of viruses to the cell surface. This is a highly specific process. The virus is adsorbed on certain areas of the cell membrane - the so-called receptors. Cellular receptors can have a different chemical nature, representing proteins, carbohydrate components of proteins and lipids, lipids. The number of specific receptors on the surface of one cell ranges from 10 4 to 10 5. Consequently, tens and even hundreds of viral particles can be adsorbed on the cell.
Penetration into the cell. There are two ways for animal viruses to enter a cell: viropexys and fusion of the viral envelope with the cell membrane. With viropexis, after the adsorption of viruses, invagination (invagination) of a section of the cell membrane and the formation of an intracellular vacuole, which contains a viral particle, occur. The vacuole with the virus can be transported in any direction to different parts of the cytoplasm or the cell nucleus. The fusion process is carried out by one of the surface viral proteins of the capsid or supercapsid shell. Apparently, both mechanisms of virus penetration into the cell do not exclude, but complement each other.
"Strip".The process of “undressing” involves removing the protective viral shells and releasing the internal component of the virus, which can cause an infectious process. “Undressing” of viruses occurs gradually, in several stages, in certain areas of the cytoplasm or nucleus of the cell, for which the cell uses a set of special enzymes. In the case of virus penetration by fusion of the viral envelope with the cell membrane, the process of virus penetration into the cell is combined with the first stage of its “undressing”. The end products of "undressing" are the core, nucleocapsid or nucleic acid of the virus.
Biosynthesis of virus components. The viral nucleic acid that has entered the cell carries genetic information that successfully competes with the genetic information of the cell. It disorganizes the functioning of cellular systems, suppresses the cell’s own metabolism and forces it to synthesize new viral proteins and nucleic acids that are used to build viral offspring.
The implementation of the genetic information of the virus is carried out in accordance with the processes of transcription, translation and replication.
Formation (assembly) of viruses. Synthesized viral nucleic acids and proteins have the ability to specifically “recognize” each other and, if their concentration is sufficient, they spontaneously combine as a result of hydrophobic, salt and hydrogen bonds.
There are the following general principles for assembling viruses with different structures:
1. The formation of viruses is a multi-stage process with the formation of intermediate forms;
2. The assembly of simply arranged viruses involves the interaction of viral nucleic acid molecules with capsid proteins and the formation of nucleocapsids (for example, polio viruses). In complex viruses, nucleocapsids are first formed, with which supercapsid shell proteins interact (for example, influenza viruses);
3. The formation of viruses does not occur in the intracellular fluid, but on the nuclear or cytoplasmic membranes of the cell;
4. Complexly organized viruses during the process of formation include components of the host cell (lipids, carbohydrates).
Exit of viruses from the cell. There are two main types of release of viral progeny from the cell. The first type - explosive - is characterized by the simultaneous release of a large number of viruses. In this case, the cell quickly dies. This exit method is typical for viruses that do not have a supercapsid shell. The second type is budding. It is characteristic of viruses that have a supercapsid shell. At the final stage of assembly, the nucleocapsids of complex viruses are fixed on the cell plasma membrane, modified by viral proteins, and gradually protrude it. As a result of protrusion, a “bud” containing a nucleocapsid is formed. The “bud” is then separated from the cell. Thus, the outer shell of these viruses is formed as they exit the cell. With this mechanism, a cell can produce a virus for a long time, maintaining to one degree or another its basic functions.
The time required to complete the full cycle of virus reproduction varies from 5-6 hours (influenza viruses, smallpox, etc.) to several days (measles viruses, adenoviruses, etc.). The resulting viruses are able to infect new cells and undergo the above-mentioned reproduction cycle in them.

The process of viral reproduction can be divided into 2 phases . The first phase includes 3 stages: 1) adsorption of the virus on sensitive cells; 2) penetration of the virus into the cell; 3) deproteinization of the virus . The second phase includes the stages of implementation of the viral genome: 1) transcription, 2) translation, 3) replication, 4) assembly, maturation of viral particles and 5) virus exit from the cell.

The interaction of a virus with a cell begins with the adsorption process, i.e., with the attachment of the virus to the cell surface.

Adsorption is a specific binding of the virion protein (antireceptor) to the complementary structure of the cell surface - the cell receptor. According to their chemical nature, the receptors on which viruses are fixed belong to two groups: mucoprotein and lipoprotein. Influenza viruses, parainfluenza, and adenoviruses are fixed on mucoprotein receptors. Enteroviruses, herpes viruses, arboviruses are adsorbed on lipoprotein receptors of the cell. Adsorption occurs only in the presence of certain electrolytes, in particular Ca2+ ions, which neutralize excess anionic charges of the virus and cell surface and reduce electrostatic repulsion. Adsorption of viruses depends little on temperature. The initial processes of adsorption are nonspecific in nature and are the result of electrostatic interaction of positively and negatively charged structures on the surface virus and cell, and then a specific interaction occurs between the virion attachment protein and specific groups on the plasma membrane of the cell. Simple human and animal viruses contain attachment proteins as part of the capsid. In complex viruses, attachment proteins are part of the supercapsid. They can have the form of filaments (fibers in adenoviruses), or spikes, mushroom-like structures in myxo-, retro-, rhabdo- and other viruses. Initially, a single connection of the virion with the receptor occurs - such attachment is fragile - adsorption is reversible. For irreversible adsorption to occur, multiple connections must appear between the viral receptor and the cell receptor, i.e., stable multivalent attachment. The number of specific receptors on the surface of one cell is 10 4 -10 5. Receptors for some viruses, for example, arboviruses. are contained on the cells of both vertebrates and invertebrates; for other viruses only on the cells of one or more species.

Penetration of human and animal viruses into cells occurs in two ways: 1) viropexis (pinocytosis); 2) fusion of the viral supercapsid shell with the cell membrane. Bacteriophages have their own penetration mechanism, the so-called syringe, when, as a result of contraction of the protein appendage of the phage, the nucleic acid is injected into the cell.

Deproteinization of the virus, the release of the viral hemome from the viral protective shells occurs either with the help of viral enzymes or with the help of cellular enzymes. The end products of deproteinization are nucleic acids or nucleic acids associated with the internal viral protein. Then the second phase of viral reproduction takes place, leading to the synthesis of viral components.

Transcription is the rewriting of information from DNA or RNA of a virus into mRNA according to the laws of the genetic code.

Translation is the process of translating genetic information contained in mRNA into a specific sequence of amino acids.

Replication is the process of synthesis of nucleic acid molecules homologous to the viral genome.

The implementation of genetic information in DNA-containing viruses is the same as in cells:

DNA transcription i-RNA translation protein

RNA transcription i-RNA translation protein

Viruses with a positive RNA genome (togaviruses, picornaviruses) lack transcription:

RNA translation protein

Retroviruses have a unique way of transmitting genetic information:

RNA reverse transcription DNA transcription mRNA translation protein

The DNA integrates with the genome of the host cell (provirus).

After the cell has accumulated viral components, the last stage of viral reproduction begins: the assembly of viral particles and the release of virions from the cell. Virions exit the cell in two ways: 1) by “exploding” the cell, as a result of which the cell is destroyed. This path is inherent in simple viruses (picorna-, reo-, papova- and adenoviruses), 2) exit from cells by budding. Inherent in viruses containing a supercapsid. With this method, the cell does not die immediately and can produce multiple viral offspring until its resources are depleted.

Virus cultivation methods

To cultivate viruses in laboratory conditions, the following living objects are used: 1) cell cultures (tissues, organs); 2) chicken embryos; 3) laboratory animals.

Cell culture

The most common are single-layer cell cultures, which can be divided into 1) primary (primarily trypsinized), 2) semi-continuous (diploid) and 3) continuous.

By origin they are classified into embryonic, tumor and from adult organisms; by morphogenesis- fibroblastic, epithelial, etc.

Primary Cell cultures are cells of any human or animal tissue that have the ability to grow in the form of a monolayer on a plastic or glass surface coated with a special nutrient medium. The lifespan of such crops is limited. In each specific case, they are obtained from the tissue after mechanical grinding, treatment with proteolytic enzymes and standardization of the number of cells. Primary cultures obtained from monkey kidneys, human embryonic kidneys, human amnion, and chicken embryos are widely used for the isolation and accumulation of viruses, as well as for the production of viral vaccines.

Semi-leathered (or diploid ) cell cultures - cells of the same type, capable of withstanding up to 50-100 passages in vitro, while maintaining their original diploid set of chromosomes. Diploid strains of human embryonic fibroblasts are used both for the diagnosis of viral infections and in the production of viral vaccines.

Continuous cell lines are characterized by potential immortality and a heteroploid karyotype.

The source of transplantable lines can be primary cell cultures (for example, SOC, PES, BHK-21 - from the kidneys of one-day-old Syrian hamsters; PMS - from the kidney of a guinea pig, etc.) individual cells of which show a tendency to endless reproduction in vitro. The set of changes leading to the appearance of such features from cells is called transformation, and the cells of continuous tissue cultures are called transformed.

Another source of transplantable cell lines is malignant neoplasms. In this case, cell transformation occurs in vivo. The following lines of transplanted cells are most often used in virological practice: HeLa - obtained from cervical carcinoma; Ner-2 - from laryngeal carcinoma; Detroit-6 - from lung cancer metastasis to the bone marrow; RH - from human kidney.

For cell cultivation, nutrient media are required, which, according to their purpose, are divided into growth and supporting media. Growth media must contain more nutrients to ensure active cell proliferation to form a monolayer. Supporting media should only ensure that cells survive in an already formed monolayer during the multiplication of viruses in the cell.

Standard synthetic media, such as synthetic media 199 and Eagle's media, are widely used. Regardless of the purpose, all cell culture media are formulated using a balanced salt solution. Most often it is Hanks solution. An integral component of most growth media is animal blood serum (veal, bovine, horse), without 5-10% of which cell reproduction and monolayer formation do not occur. Serum is not included in the maintenance media.

Isolation of viruses in cell cultures and methods for their indication.

When isolating viruses from various infectious materials from a patient (blood, urine, feces, mucous discharge, organ washings), cell cultures that are most sensitive to the suspected virus are used. For infection, cultures in test tubes with a well-developed monolayer of cells are used. Before infecting the cells, the nutrient medium is removed and 0.1-0.2 ml of a suspension of the test material, pre-treated with antibiotics to destroy bacteria and fungi, is added to each test tube. After 30-60 min. After contact of the virus with cells, excess material is removed, a supporting medium is added to the test tube and left in a thermostat until signs of virus replication are detected.

An indicator of the presence of a virus in infected cell cultures can be:

1) the development of specific cell degeneration - the cytopathic effect of the virus (CPE), which has three main types: round or small cell degeneration; formation of multinucleated giant cells - symplasts; development of foci of cell proliferation, consisting of several layers of cells;

2) detection of intracellular inclusions located in the cytoplasm and nuclei of affected cells;

3) positive hamagglutination reaction (RHA);

4) positive hemadsorption reaction (RHAds);

5) plaque formation phenomenon: a monolayer of virus-infected cells is covered with a thin layer of agar with the addition of a neutral red indicator (background - pink). In the presence of a virus, colorless zones (“plaques”) form on the pink agar background in the cells.

6) in the absence of CPD or GA, an interference reaction can be performed: the culture under study is re-infected with the virus that causes CPD. In a positive case, there will be no CPP (the interference reaction is positive). If there was no virus in the test material, CPE is observed.

Isolation of viruses in chicken embryos.

For virological studies, chicken embryos 7-12 days old are used.

Before infection, the viability of the embryo is determined. During ovoscoping, living embryos are mobile and the vascular pattern is clearly visible. The boundaries of the air sac are marked with a simple pencil. Chicken embryos are infected under aseptic conditions, using sterile instruments, after pre-treating the shell above the air space with iodine and alcohol.

Methods for infecting chicken embryos can be different: applying the virus to the chorion-allantoic membrane, into the amniotic and allantoic cavities, into the yolk sac. The choice of infection method depends on the biological properties of the virus being studied.

Indication of the virus in a chicken embryo is made by the death of the embryo, a positive hemagglutination reaction on glass with allantoic or amniotic fluid, and by focal lesions (“plaques”) on the chorion-allantoic membrane.

III. Isolation of viruses in laboratory animals.

Laboratory animals can be used to isolate viruses from infectious material when more convenient systems (cell cultures or chicken embryos) cannot be used. They take mainly newborn white mice, hamsters, guinea pigs, and rat pups. Animals are infected according to the principle of virus cytotropism: pneumotropic viruses are injected intranasally, neurotropic viruses - intracerebrally, dermatotropic viruses - onto the skin.

Indication of the virus is based on the appearance of signs of disease in animals, their death, pathomorphological and pathohistological changes in tissues and organs, as well as a positive hemagglotination reaction with extracts from organs.

Abstract Viruses

Viruses play a big role in human life. They are the causative agents of a number of dangerous diseases - smallpox, hepatitis, encephalitis, rubella, rabies, influenza, etc.

In 1892, Russian scientist D.I. Ivanovsky described the unusual properties of the causative agent of tobacco disease - tobacco mosaic. This pathogen passed through bacterial filters and, moreover, did not grow on artificial nutrient media. Thus, healthy tobacco plants could be infected with cell-free filtrate from the juice of a diseased plant.

How many years later was the causative agent of foot-and-mouth disease discovered, which also passed through bacterial filters.

In 1898, Beijerinck coined the new word "virus" (from the Latin poison) to indicate the infectious nature of certain filtered plant liquids.

In 1917, F. d'Herrel discovered a bacteriophage - a virus that infects bacteria. However, for a long time the structure of the virus remained a mystery to scientists. That is why viruses were among the first objects examined under the electron microscope immediately after its discovery in the 30s.

Differences between viruses and other organisms:

1. Viruses are the smallest organisms (on average they are 50 times smaller than bacteria), they cannot be seen with a light microscope. 30-300 nm.

2. Viruses do not have a cellular structure. If we consider cellular structure as a mandatory sign of life, then viruses are not alive. However, they possess genetic material and are capable of self-reproduction. There is an assumption that viruses are genetic material that once escaped from a cell and retained the ability to reproduce when returning to the cellular environment.

3. Viruses can reproduce themselves only inside a living cell and are not independent organisms.

4. Viruses consist of one small nucleic acid molecule (DNA or RNA) and are surrounded by a protein shell

5. Unlike cellular organisms, viruses cannot synthesize proteins on their own. The virus introduces only its nucleic acid into the cell, which turns off the host DNA and gives the cell the command to synthesize the proteins it needs (for the assembly and release of new copies of the virus).

Structure of viruses

A viral particle, also called virion, consists of a nucleic acid (DNA or RNA) surrounded by a protein shell. This shell is called capsid. The capsid consists of subunits - capsomeres. Capsid with nucleic acid – nucleocapsid- may be naked or have an additional shell ( influenza and herpes viruses).

The simplest viruses, such as tobacco mosaic virus, have only a protein capsid. The structure of the wart virus and adenoviruses are similar..

Viral particles can be rod-shaped or filamentous, or have a polyhedron shape.

Virus-cell interaction

1. Recognition by the virus of its cell. As a rule, virus entry is preceded by its binding to a special receptor protein on the cell surface.

2. Adsorption – attachment of the virion to the cell surface. Binding is carried out using special proteins on the surface of the viral particle, which recognize the corresponding receptor on the cell surface. Like a key to a lock.

3. Penetration through the membrane. The section of the membrane to which the virus has attached is immersed in the cytoplasm and turns into a vacuole, which can then merge with the nucleus.

4. “Undressing” - release from the capsid. Occurs either on the cell surface or as a result of destruction of the capsid by cell enzymes in the cytoplasm.

5. Copying (reduplication) of viral nucleic acid.

6. Synthesis of viral proteins.

7. Assembly of virions in the nucleus or cytoplasm.

8. Exit of virions from the cell. For some viruses, this occurs by “explosion,” as a result of which the integrity of the cell is disrupted and it dies. Other viruses are released in a manner reminiscent of budding. In this case, the host cells can maintain their viability for a long time. Virions are released from the cell at different rates. In some types of infection, virions can remain inside the cell for quite a long time without destroying it.

Bacteria viruses

Viruses that attack bacteria , are called bacteriophages or simply phages.

Structure of bacteriophages mainly studied using T-phage as an example Eccherichia coli(coliphage). Coliphage consists of a polyhedral head and tail. The head consists of capsomeres and contains DNA inside. The tail has a complex structure and consists of a hollow rod, a contractile sheath surrounding it, and a basal plate with spines and filaments (necessary for adsorption on the host cell).

Penetration of a bacteriophage into a cell

The thick cell walls of bacteria do not allow the virus to enter the cytoplasm, as when infecting animal cells. Therefore, the bacteriophage inserts a hollow rod into the cell and pushes out the nucleic acid located in the head through it.

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 - 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 - capsomeres. There are two ways of packing 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 genome complex 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 of the virus with 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 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 interaction between viruses and host cells

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, the delta (D) hepatitis virus can replicate only in the presence of the hepatitis B virus, its Hbs - antigen, adeno-associated virus - in the presence of an 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 that can integrate into the genome of the host cell are retroviruses, which have a special mechanism for this. The peculiarity of their reproduction is the synthesis of provirus 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 the 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 cultures), capable of repeated dispersion and transplantation, i.e., multiple passages, are 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, Igla, 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 proliferation 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. Plaque method (plaque formation). As a result of the cytolytic effect of many viruses on cell cultures, zones of mass cell death are formed. Plaques are identified - viral “cell-negative” colonies.

Nomenclature of viruses.

The name of the family of viruses ends in “viridae”, the 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 a head - an icosahedron, a process (tail) with spiral symmetry (often has a hollow rod and a contractile sheath), spines and processes (filaments), i.e., they are somewhat reminiscent of a spermatozoon in appearance.

The interaction of phages with a cell (bacterium) is strictly specific, i.e. bacteriophages are able to infect only certain species and phagotypes bacteria.

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. Exit 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 host cell chromosome (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 go into 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. The temperate phage tox causes lysogenic conversion of nontoxigenic diphtheria bacillus into toxigenic one.

According to their spectrum of action on 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.