Which of the Following Represents a Virus Family Name?

Learning Objectives

• Describe the general characteristics of viruses as pathogens
• Describe viral genomes
• Depict the general characteristics of viral life cycles
• Differentiate among bacteriophages, plant viruses, and animal viruses
• Depict the characteristics used to identify viruses as obligate intracellular parasites

Clinical Focus: Joaquim, Part ane

Joaquim, a 45-year-old announcer, has merely returned to the U.S. from travels in Russia, China, and Africa. He is non feeling well, so he goes to his general practitioner complaining of weakness in his arms and legs, fever, headache, noticeable agitation, and small-scale discomfort. He thinks it may be related to a dog bite he suffered while interviewing a Chinese farmer. He is experiencing some prickling and itching sensations at the site of the bite wound, simply he tells the medico that the dog seemed healthy and that he had not been concerned until now. The doctor ordered a civilisation and sensitivity test to rule out bacterial infection of the wound, and the results came back negative for any possible pathogenic leaner.

• Based on this information, what additional tests should be performed on the patient?
• What type of treatment should the doctor recommend?

We'll render to Joaquim's instance later on on this page.

Despite their small size, which prevented them from being seen with low-cal microscopes, the discovery of a filterable component smaller than a bacterium that causes tobacco mosaic disease (TMD) dates back to 1892.^[1] At that fourth dimension, Dmitri Ivanovski, a Russian botanist, discovered the source of TMD by using a porcelain filtering device first invented by Charles Chamberland and Louis Pasteur in Paris in 1884. Porcelain Chamberland filters have a pore size of 0.one µm, which is pocket-size enough to remove all bacteria ≥0.2 µm from whatever liquids passed through the device. An excerpt obtained from TMD-infected tobacco plants was made to decide the cause of the disease. Initially, the source of the disease was thought to be bacterial. Information technology was surprising to anybody when Ivanovski, using a Chamberland filter, found that the crusade of TMD was not removed afterward passing the extract through the porcelain filter. So if a bacterium was not the cause of TMD, what could be causing the disease? Ivanovski concluded the cause of TMD must be an extremely pocket-sized bacterium or bacterial spore. Other scientists, including Martinus Beijerinck, continued investigating the crusade of TMD. It was Beijerinck, in 1899, who eventually ended the causative agent was not a bacterium simply, instead, possibly a chemical, similar a biological poison nosotros would draw today as a toxin. As a consequence, the word virus, Latin for poisonous substance, was used to depict the crusade of TMD a few years subsequently Ivanovski's initial discovery. Fifty-fifty though he was not able to come across the virus that caused TMD, and did not realize the cause was not a bacterium, Ivanovski is credited as the original discoverer of viruses and a founder of the field of virology.

Today, nosotros can run into viruses using electron microscopes (Figure 1) and we know much more about them. Viruses are distinct biological entities; however, their evolutionary origin is all the same a matter of speculation. In terms of taxonomy, they are not included in the tree of life because they are acellular (non consisting of cells). In gild to survive and reproduce, viruses must infect a cellular host, making them obligate intracellular parasites. The genome of a virus enters a host cell and directs the production of the viral components, proteins and nucleic acids, needed to course new virus particles called virions. New virions are made in the host cell by associates of viral components. The new virions send the viral genome to some other host jail cell to carry out some other circular of infection. Table 1 summarizes the backdrop of viruses.

Table one. Characteristics of Viruses
Infectious, acellular pathogens
Obligate intracellular parasites with host and cell-type specificity
DNA or RNA genome (never both)
Genome is surrounded past a protein capsid and, in some cases, a phospholipid membrane studded with viral glycoproteins
Lack genes for many products needed for successful reproduction, requiring exploitation of host-prison cell genomes to reproduce

Figure 1. (a) Tobacco mosaic virus (TMV) viewed with transmission electron microscope. (b) Plants infected with tobacco mosaic disease (TMD), caused by TMV. (credit a: modification of work past USDA Agronomical Research Service—scale-bar data from Matt Russell; credit b: modification of work past USDA Wood Service, Department of Plant Pathology Annal North Carolina State University)

Recall nigh It

• Why was the first virus investigated mistaken for a toxin?

Hosts and Viral Manual

Viruses can infect every type of host cell, including those of plants, animals, fungi, protists, bacteria, and archaea. Most viruses will simply exist able to infect the cells of one or a few species of organism. This is called the host range. Notwithstanding, having a wide host range is not mutual and viruses will typically just infect specific hosts and only specific cell types within those hosts. The viruses that infect leaner are chosen bacteriophages, or just phages. The word phage comes from the Greek give-and-take for devour. Other viruses are just identified by their host group, such as animal or found viruses. Once a cell is infected, the effects of the virus tin can vary depending on the type of virus. Viruses may cause aberrant growth of the cell or cell death, alter the cell's genome, or cause piffling noticeable effect in the prison cell.

Viruses can exist transmitted through directly contact, indirect contact with fomites, or through a vector: an creature that transmits a pathogen from 1 host to another. Arthropods such as mosquitoes, ticks, and flies, are typical vectors for viral diseases, and they may deed as mechanical vectors or biological vectors. Mechanical manual occurs when the arthropod carries a viral pathogen on the outside of its body and transmits it to a new host by physical contact. Biological manual occurs when the arthropod carries the viral pathogen inside its body and transmits it to the new host through biting.

In humans, a wide variety of viruses are capable of causing various infections and diseases. Some of the deadliest emerging pathogens in humans are viruses, nonetheless nosotros accept few treatments or drugs to deal with viral infections, making them difficult to eradicate.

Viruses that can be transmitted from an animate being host to a human host tin cause zoonoses. For example, the avian influenza virus originates in birds, but can crusade disease in humans. Reverse zoonoses are caused by infection of an animate being by a virus that originated in a human being.

Fighting Bacteria with Viruses

The emergence of superbugs, or multidrug resistant bacteria, has get a major claiming for pharmaceutical companies and a serious health-intendance trouble. According to a 2013 report by the US Centers for Disease Command and Prevention (CDC), more than than 2 million people are infected with drug-resistant bacteria in the The states annually, resulting in at least 23,000 deaths.^[2] The continued use and overuse of antibiotics will likely atomic number 82 to the evolution of even more drug-resistant strains.

1 potential solution is the use of phage therapy, a procedure that uses leaner-killing viruses (bacteriophages) to treat bacterial infections. Phage therapy is non a new idea. The discovery of bacteriophages dates back to the early 20th century, and phage therapy was kickoff used in Europe in 1915 by the English bacteriologist Frederick Twort.^[3] However, the subsequent discovery of penicillin and other antibiotics led to the virtually abandonment of this grade of therapy, except in the old Soviet Spousal relationship and a few countries in Eastern Europe. Involvement in phage therapy outside of the countries of the former Soviet Union is but recently re-emerging because of the rise in antibiotic-resistant leaner.^[4]

Phage therapy has some advantages over antibiotics in that phages kill but ane specific bacterium, whereas antibiotics kill not only the pathogen simply also benign bacteria of the normal microbiota. Development of new antibiotics is besides expensive for drug companies and for patients, especially for those who live in countries with high poverty rates.

Phages have also been used to forbid nutrient spoilage. In 2006, the The states Food and Drug Assistants canonical the use of a solution containing six bacteriophages that tin can be sprayed on lunch meats such every bit bologna, ham, and turkey to impale Listeria monocytogenes, a bacterium responsible for listeriosis, a form of food poisoning. Some consumers take concerns near the use of phages on foods, however, peculiarly given the rising popularity of organic products. Foods that have been treated with phages must declare "bacteriophage grooming" in the list of ingredients or include a label declaring that the meat has been "treated with antimicrobial solution to reduce microorganisms."^[5]

Think about It

• Why do humans not take to exist concerned almost the presence of bacteriophages in their food?
• What are three ways that viruses can be transmitted betwixt hosts?

Viral Structures

In general, virions (viral particles) are modest and cannot be observed using a regular light microscope. They are much smaller than prokaryotic and eukaryotic cells; this is an accommodation allowing viruses to infect these larger cells (see Figure 2). The size of a virion can range from twenty nm for small-scale viruses upwards to 900 nm for typical, large viruses (see Effigy 3). Recent discoveries, notwithstanding, accept identified new giant viral species, such equally Pandoravirus salinus and Pithovirus sibericum, with sizes budgeted that of a bacterial prison cell.^[6]

Figure ii. (a) In this manual electron micrograph, a bacteriophage (a virus that infects leaner) is dwarfed by the bacterial cell it infects. (b) An illustration of the bacteriophage in the micrograph. (credit a: modification of piece of work past U.Due south. Section of Energy, Office of Science, LBL, PBD)

Figure iii. The size of a virus is minor relative to the size of most bacterial and eukaryotic cells and their organelles.

In 1935, after the development of the electron microscope, Wendell Stanley was the first scientist to crystallize the structure of the tobacco mosaic virus and discovered that information technology is composed of RNA and protein. In 1943, he isolated Influenza B virus , which contributed to the development of an influenza (flu) vaccine. Stanley's discoveries unlocked the mystery of the nature of viruses that had been puzzling scientists for over twoscore years and his contributions to the field of virology led to him existence awarded the Nobel Prize in 1946.

As a outcome of continuing enquiry into the nature of viruses, we now know they consist of a nucleic acrid (either RNA or DNA, merely never both) surrounded past a poly peptide coat called a capsid (run across Effigy four). The interior of the capsid is not filled with cytosol, as in a cell, but instead it contains the bare necessities in terms of genome and enzymes needed to direct the synthesis of new virions. Each capsid is composed of protein subunits called capsomeres made of 1 or more unlike types of capsomere proteins that interlock to form the closely packed capsid.

There are ii categories of viruses based on full general composition. Viruses formed from simply a nucleic acid and capsid are called naked viruses or nonenveloped viruses. Viruses formed with a nucleic-acid packed capsid surrounded past a lipid layer are chosen enveloped viruses (run into Figure 4). The viral envelope is a pocket-size portion of phospholipid membrane obtained as the virion buds from a host cell. The viral envelope may either be intracellular or cytoplasmic in origin.

Extending outward and abroad from the capsid on some naked viruses and enveloped viruses are protein structures called spikes. At the tips of these spikes are structures that permit the virus to attach and enter a cell, like the influenza virus hemagglutinin spikes (H) or enzymes like the neuraminidase (N) flu virus spikes that permit the virus to detach from the cell surface during release of new virions. Influenza viruses are ofttimes identified by their H and N spikes. For example, H1N1 influenza viruses were responsible for the pandemics in 1918 and 2009,^[7] H2N2 for the pandemic in 1957, and H3N2 for the pandemic in 1968.

Effigy 4. Click for a larger image. (a) The naked atadenovirus uses spikes made of glycoproteins from its capsid to bind to host cells. (b) The enveloped man immunodeficiency virus uses spikes made of glycoproteins embedded in its envelope to bind to host cells (credit a "micrograph": modification of work by NIAID; credit b "micrograph": modification of work by Centers for Disease Control and Prevention)

Viruses vary in the shape of their capsids, which can be either helical, polyhedral, or complex. A helical capsid forms the shape of tobacco mosaic virus (TMV), a naked helical virus, and Ebola virus, an enveloped helical virus. The capsid is cylindrical or rod shaped, with the genome fitting but inside the length of the capsid. Polyhedral capsids form the shapes of poliovirus and rhinovirus, and consist of a nucleic acid surrounded past a polyhedral (many-sided) capsid in the form of an icosahedron. An icosahedral capsid is a three-dimensional, 20-sided structure with 12 vertices. These capsids somewhat resemble a soccer ball. Both helical and polyhedral viruses can have envelopes. Viral shapes seen in certain types of bacteriophages, such as T4 phage, and poxviruses, similar vaccinia virus, may have features of both polyhedral and helical viruses and so they are described as a complex viral shape (see Effigy 5). In the bacteriophage complex form, the genome is located inside the polyhedral head and the sheath connects the caput to the tail fibers and tail pins that help the virus attach to receptors on the host cell'due south surface. Poxviruses that have complex shapes are ofttimes brick shaped, with intricate surface characteristics not seen in the other categories of capsid.

Figure five. Viral capsids can be (a) helical, (b) polyhedral, or (c) have a circuitous shape. (credit a "micrograph": modification of piece of work by USDA ARS; credit b "micrograph": modification of work by U.S. Department of Energy)

Think about It

• Which types of viruses have spikes?

Nomenclature and Taxonomy of Viruses

Although viruses are not classified in the three domains of life, their numbers are great enough to require classification. Since 1971, the International Union of Microbiological Societies Virology Division has given the task of developing, refining, and maintaining a universal virus taxonomy to the International Committee on Taxonomy of Viruses (ICTV). Since viruses can mutate then chop-chop, it can be hard to classify them into a genus and a species epithet using the binomial nomenclature system. Thus, the ICTV's viral nomenclature system classifies viruses into families and genera based on viral genetics, chemistry, morphology, and machinery of multiplication. To appointment, the ICTV has classified known viruses in seven orders, 96 families, and 350 genera. Viral family names end in –viridae (e.g, Parvoviridae) and genus names end in −virus (e.g., Parvovirus). The names of viral orders, families, and genera are all italicized. When referring to a viral species, we oft use a genus and species epithet such every bit Pandoravirus dulcis or Pandoravirus salinus.

The Baltimore classification organization is an culling to ICTV nomenclature. The Baltimore arrangement classifies viruses according to their genomes (Dna or RNA, unmarried versus double stranded, and way of replication). This arrangement thus creates vii groups of viruses that have mutual genetics and biology.

Explore the latest virus taxonomy at the ICTV website.

Aside from formal systems of nomenclature, viruses are often informally grouped into categories based on chemistry, morphology, or other characteristics they share in common. Categories may include naked or enveloped construction, single-stranded (ss) or double-stranded (ds) Dna or ss or ds RNA genomes, segmented or nonsegmented genomes, and positive-strand (+) or negative-strand (−) RNA. For example, canker viruses can be classified equally a dsDNA enveloped virus; human immunodeficiency virus (HIV) is a +ssRNA enveloped virus, and tobacco mosaic virus is a +ssRNA virus. Other characteristics such as host specificity, tissue specificity, capsid shape, and special genes or enzymes may also exist used to depict groups of similar viruses. Tabular array 2 lists some of the well-nigh common viruses that are human pathogens past genome type.

Table 2. Mutual Pathogenic Viruses
Genome	Family unit	Instance Virus	Clinical Features
dsDNA, enveloped	Poxviridae	Orthopoxvirus	Skin papules, pustules, lesions
	Poxviridae	Parapoxvirus	Skin lesions
	Herpesviridae	Simplexvirus	Common cold sores, genital herpes, sexually transmitted disease
dsDNA, naked	Adenoviridae	Atadenovirus	Respiratory infection (common cold)
	Papillomaviridae	Papillomavirus	Genital warts, cervical, vulvar, or vaginal cancer
	Reoviridae	Reovirus	Gastroenteritis astringent diarrhea (breadbasket flu)
ssDNA, naked	Parvoviridae	Adeno-associated dependoparvovirus A	Respiratory tract infection
	Parvoviridae	Adeno-associated dependoparvovirus B	Respiratory tract infection
dsRNA, naked	Reoviridae	Rotavirus	Gastroenteritis
+ssRNA, naked	Picornaviridae	Enterovirus C	Poliomyelitis
	Picornaviridae	Rhinovirus	Upper respiratory tract infection (common cold)
	Picornaviridae	Hepatovirus	Hepatitis
+ssRNA, enveloped	Togaviridae	Alphavirus	Encephalitis, hemorrhagic fever
	Togaviridae	Rubivirus	Rubella
	Retroviridae	Lentivirus	Acquired allowed deficiency syndrome (AIDS)
−ssRNA, enveloped	Filoviridae	Zaire Ebolavirus	Hemorrhagic fever
	Orthomyxoviridae	Influenzavirus A, B, C	Influenza
	Rhabdoviridae	Lyssavirus	Rabies

Think virtually It

• What are the types of virus genomes?

Classification of Viral Diseases

While the ICTV has been tasked with the biological classification of viruses, information technology has also played an important office in the classification of diseases caused by viruses. To facilitate the tracking of virus-related homo diseases, the ICTV has created classifications that link to the International Classification of Diseases (ICD), the standard taxonomy of disease that is maintained and updated past the Earth Health Organisation (WHO). The ICD assigns an alphanumeric code of up to vi characters to every type of viral infection, also as all other types of diseases, medical conditions, and causes of decease. This ICD code is used in conjunction with ii other coding systems (the Current Procedural Terminology, and the Healthcare Common Process Coding Organization) to categorize patient atmospheric condition for treatment and insurance reimbursement.

For case, when a patient seeks treatment for a viral infection, ICD codes are routinely used by clinicians to club laboratory tests and prescribe treatments specific to the virus suspected of causing the illness. This ICD code is then used by medical laboratories to identify tests that must exist performed to confirm the diagnosis. The ICD code is used by the health-care management system to verify that all treatments and laboratory work performed are advisable for the given virus. Medical coders apply ICD codes to assign the proper code for procedures performed, and medical billers, in plough, use this data to process claims for reimbursement by insurance companies. Vital-records keepers use ICD codes to record cause of death on death certificates, and epidemiologists used ICD codes to calculate morbidity and mortality statistics.

Think almost Information technology

• Identify ii locations where you would likely detect an ICD code.

Clinical Focus: Joaquim, Function 2

This example continues Joaquim's story that started earlier on this page.

Joaquim's doctor was concerned that his symptoms included prickling and itching at the site of the dog bite; these sensations could be early symptoms of rabies. Several tests are available to diagnose rabies in live patients, just no single antemortem test is acceptable. The doctor decided to take samples of Joaquim'south blood, saliva, and peel for testing. The skin sample was taken from the nape of the neck (posterior side of the neck near the hairline). It was about half-dozen-mm long and contained at least 10 hair follicles, including the superficial cutaneous nervus. An immunofluorescent staining technique was used on the skin biopsy specimen to detect rabies antibodies in the cutaneous nerves at the base of the hair follicles. A test was too performed on a serum sample from Joaquim's blood to determine whether any antibodies for the rabies virus had been produced.

Meanwhile, the saliva sample was used for reverse transcriptase-polymerase chain reaction (RT-PCR) assay, a examination that tin observe the presence of viral nucleic acid (RNA). The blood tests came back positive for the presence of rabies virus antigen, prompting Joaquim's doctor to prescribe prophylactic treatment. Joaquim is given a series of intramuscular injections of human rabies immunoglobulin along with a serial of rabies vaccines.

• Why does the immunofluorescent technique expect for rabies antibodies rather than the rabies virus itself?
• If Joaquim has contracted rabies, what is his prognosis?

We'll return to Joaquim's example in later pages.

Key Concepts and Summary

• Viruses are more often than not ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have been found.
• Virions are acellular and consist of a nucleic acid, Deoxyribonucleic acid or RNA, but not both, surrounded by a poly peptide capsid. There may also be a phospholipid membrane surrounding the capsid.
• Viruses are obligate intracellular parasites.
• Viruses are known to infect various types of cells found in plants, animals, fungi, protists, bacteria, and archaea. Viruses typically have limited host ranges and infect specific cell types.
• Viruses may have helical, polyhedral, or complex shapes.
• Classification of viruses is based on morphology, type of nucleic acrid, host range, cell specificity, and enzymes carried inside the virion.
• Like other diseases, viral diseases are classified using ICD codes.

Multiple Option

The component(s) of a virus that is/are extended from the envelope for zipper is/are the:

1. capsomeres
2. spikes
3. nucleic acid
4. viral whiskers

Show Reply

Answer b. The components of a virus that are extended from the envelope for zipper are the spikes.

Which of the following does a virus lack? Select all that apply.

1. ribosomes
2. metabolic processes
3. nucleic acid
4. glycoprotein

Bear witness Answer

Answer b. A virus lacks ribosomes and metabolic processes.

The envelope of a virus is derived from the host'southward ________.

1. nucleic acids
2. membrane structures
3. cytoplasm
4. genome

Show Respond

Answer b. The envelope of a virus is derived from the host'south membrane structures.

In naming viruses, the family proper name ends with ________ and genus proper noun ends with _________.

1. −virus; −viridae
2. −viridae; −virus
3. −virion; virus
4. −virus; virion

Show Answer

Answer b. In naming viruses, the family name ends with −viridae and genus name ends with −virus.

What is another name for a nonenveloped virus?

1. enveloped virus
2. provirus
3. naked virus
4. latent virus

Testify Reply

Respond c. "Naked virus" is another name for a nonenveloped virus.

Truthful/False

True or False: Scientists have identified viruses that are able to infect fungal cells.

Fill in the Bare

A virus that infects a bacterium is called a/an __________.

Show Reply

A virus that infects a bacterium is called a bacteriophage.

A/an __________ virus possesses characteristics of both a polyhedral and helical virus.

Testify Answer

A complex virus possesses characteristics of both a polyhedral and helical virus.

A virus containing only nucleic acid and a capsid is called a/an __________ virus or __________ virus.

Evidence Respond

A virus containing only nucleic acid and a capsid is called a naked virus or nonenveloped virus.

The __________ on the bacteriophage allow for binding to the bacterial cell.

Prove Answer

The tail fibers on the bacteriophage permit for bounden to the bacterial prison cell.

Call back nearly It

1. Talk over the geometric differences among helical, polyhedral, and complex viruses.
2. What was the meaning of the word "virus" in the 1880s and why was it used to draw the cause of tobacco mosaic disease?
3. In terms of evolution, which do you call up arises beginning? The virus or the host? Explain your respond.
4. Do you think it is possible to create a virus in the lab? Imagine that you are a mad scientist. Draw how you would get about creating a new virus?
5. Name each labeled part of the illustrated bacteriophage.

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Source: https://courses.lumenlearning.com/microbiology/chapter/viruses/

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