General Microbiology - Schlegel, H. 1987

Viruses: Distribution and Structure

The term "virus" (from Latin virus meaning poison) was originally used to describe various poorly understood disease-causing agents. Later, it became exclusively associated with a group of pathogens discovered in 1892 by Ivanovsky, which were found to be capable of passing through bacterial filters. Consequently, they came to be known as "filterable Viruses" or simply "viruses." Viruses differ from microorganisms in several key ways: 1) they contain only one type of nucleic acid—either DNA or RNA; 2) only nucleic acid is required for their reproduction; and 3) they are incapable of Replication outside a living Cell. Thus, viruses are not independent organisms; instead, they hijack living Cells to reproduce, with replication taking place within the host cell. Cellular mechanisms are required both for the replication of viral nucleic acid and for the Synthesis of the viral protein coat. Viral development ultimately results in the death of the host cell. Outside of a cell, a virus exists as a viral particle, or virion, which consists of nucleic acid enclosed in a protein coat known as a capsid. For this reason, a viral particle is also referred to as a nucleocapsid.

Viruses are identified by the consequences of their development within host cells. They destroy entire cell complexes and cause tissue damage that leads to The formation of necrotic spots or zones of lysis (Fig. 4.1). The typical hosts for viruses are plants, animals, and microorganisms.

Plant viruses (phytopathogenic viruses). These viruses enter plant cells through existing wounds rather than by active penetration. The Quantitative determination of phytopathogenic viruses is based on counting the necrotic lesions that appear at sites where primary damage was artificially inflicted. Under natural conditions, plant viruses are spread through direct contact or via vectors. Viruses are frequently introduced into leaves through abrasions caused by leaves rubbing against one another. Parasitic plants can also facilitate viral transmission. For example, dodder (Cuscuta), by penetrating host plants with its haustoria, establishes a direct connection between plants via its Vascular System, allowing viruses to spread. Insects serve as vectors for many viruses. In some cases, viruses replicate within the insect's digestive tract (persistent viruses); under such conditions, infection of a new plant is only possible after a certain incubation period within the insect. Non-persistent viruses are transmitted directly during mechanical plant tissue Damage caused by insect mouthparts. Viruses cause a wide range of plant diseases. Viruses that infect potatoes are of major economic importance. The best-studied plant virus is the tobacco mosaic virus. The genetic material of phytopathogenic viruses is most commonly RNA.

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Fig. 4.1. Detection of viruses via local necrosis, zones of lysis, or clearings. A. Local necrotic lesions caused by tobacco mosaic virus on a tobacco leaf. B. Zones of lysis formed in a tissue culture under the action of poliovirus. C. Clearings, or "plaques," on a bacterial lawn resulting from bacteriophage infection.

Viruses pathogenic to animals and humans. In humans and animals, viruses cause diseases such as smallpox, chickenpox, measles, rabies, poliomyelitis (infantile paralysis), Influenza infections, the common cold, FOOT-and-Mouth disease, and others. Much like plant viruses, they are transmitted either through contact or via vectors such as insects, and they apparently enter cells via phagocytosis or pinocytosis. In laboratory research, the propagation of viruses requires The Use of experimental animals or chicken embryos. Certain animal viruses can also be cultured and quantified using tissue cultures. The genetic material of these viruses may be either DNA or RNA. While viral DNA is almost invariably double-stranded, viral RNA typically consists of a single polynucleotide chain.

Bacterial viruses. Viruses that infect Bacteria are known as Bacteriophages. It is unlikely that there are any bacteria for which a corresponding phage cannot be found with sufficiently diligent searching. Bacteriophages are detected by the formation of "clearings" or "plaques" on a confluent bacterial lawn. In a bacterial suspension, they multiply so rapidly that they are capable of lysing all cells within a short period of time. The nucleic acid of a phage is either double- or single-stranded DNA, or single-stranded RNA. Bacteriophages infecting Escherichia coli serve as standard model organisms. The Study of phages and their various developmental cycles has significantly contributed to elucidating the Mechanisms of Genetic material transfer from Cell to Cell.