MICROBIOLOGY Study Guide - 2012

CHAPTER 6. VIRUSES

Viruses are a group of extremely small microorganisms invisible under a Light Microscope, with sizes measured in nanometers. Viruses are Obligate Intracellular Parasites that cause diseases in humans, animals, plants, and microorganisms.

Even before the microbial world was discovered, the term virus (from Latin for poison) was used to describe any pathogenic agent.

In 1892, the Russian botanist D. I. Ivanovsky discovered the CAUSATIVE AGENT OF tobacco mosaic disease. It turned out to be a certain pathogen present in the leaves of infected plants that could pass through bacterial filters and infect healthy plants.

Soon, other viruses causing diseases in animals and humans were also discovered. In 1898, German scientists F. Löffler and P. Frosch discovered the FOOT-and-Mouth disease virus; in 1900, the yellow fever virus was discovered; and in 1911, the Rous Sarcoma virus.

Further research established the existence of many other viruses. Viruses are the causative agents of Influenza, measles, rabies, poliomyelitis, encephalitis, smallpox, chickenpox, trachoma, herpes, foot-and-mouth disease in cattle and goats, ornithosis in birds, and various plant diseases such as mosaic, streak, and leaf curl. Viruses also infect microorganisms: Bacteria (Bacteriophages), actinomycetes (actinophages), cyanobacteria (cyanophages), and Fungi (mycophages).

In 1971, phytopathogenic Viroids were discovered. These are short RNA molecules without a protein coat, having a molecular mass of 100–130 kDa. Viroids infect potatoes, cucumbers, tomatoes, citrus fruits, and other plants.

In recent years, pathogens causing new diseases in humans, animals, and plants have been identified. The most widely known are the AIDS viruses—HIV-1 and HIV-2. The AIDS virus was isolated in 1983 by the French scientist L. Montagnier and in 1984 by the American scientist R. Gallo.

New infectious agents include Prions, the causative agents of spongiform encephalopathy, or "mad cow disease." An infectious protein with a molecular mass of approximately 30 kDa and dimensions smaller than a virus was isolated from the affected Brain tissue of an animal. The isolated protein did not elicit an Immune Response and was not inactivated by agents that destroy Nucleic Acids. Attempts to infect healthy animals using the protein isolated from the brains of sick animals were unsuccessful. S. Prusiner hypothesized that depending on the environment, the protein undergoes a genetic mutation, resulting in A change in its stereostructure and the acquisition of infectious properties. Prions cause the destruction of Neurons, leaving behind cavities and resulting in a spongy appearance, hence the name of the disease—spongiform encephalopathy.

6.1. BASIC PROPERTIES AND CHARACTERISTICS OF VIRUSES

Viruses are characterized by extremely small dimensions. Unlike bacteria, they cannot be seen under a light microscope and become visible only with an Electron microscope; they pass through bacterial filters and cannot be sedimented in a standard centrifuge (sedimentation occurs only in an ultracentrifuge).

Viruses lack cellular Structure and are devoid of any cellular components. A viral particle consists of two primary components: nucleic acid and protein, which allows viruses to be classified as non-Cellular forms of life.

Viruses contain only a single type of nucleic acid—either DNA or RNA. Furthermore, viruses feature unique forms of nucleic acids, such as single-stranded DNA (in small and filamentous bacteriophages) and double-stranded DNA (in certain plant tumor viruses).

Viruses do not grow on artificial nutrient media and are capable of replicating only inside the Cells of a susceptible host or in tissue culture. Thus, they are obligate intracellular parasites operating at the genetic level.

Some small viruses are capable of crystallization.

Viruses possess heredity and Variability, which enable them to persist in the Earth's biosphere.

Based on the type of nucleic acid as well as biological and physicochemical properties, viruses are generally divided into two broad groups: RNA-containing and DNA-containing. Currently, viruses are grouped into 19 families, 12 of which contain RNA and the other 7 contain DNA. Among these, 6 families have genomes containing double-stranded DNA, and 1 family contains single-stranded DNA.

Viruses cannot be considered fully living organisms because they lack an independent METABOLISM—the fundamental property of living matter. Deprived of a protein-synthesizing apparatus and the Enzymes responsible for The Biosynthesis of polymers, viruses outside a living host Cell exist as inert particles and can remain in this state for a prolonged period. At the same time, viruses possess certain properties that place them alongside living organisms. Viruses consist of ordered PROTEIN AND NUCLEIC acid molecules, reproduce inside a host cell, and are capable of variability and transmitting hereditary traits to their offspring.