Micro Test Two: Chapter 10 (Viruses)

Influenza A

Influenza A antigenic variation due to 1. Antigenic drift - mutations in genes for (H) and (N) - Surface antigens change 2. Antigenic shift - gene reassortment of two different viruses infecting the same cell. Bird and human virus mix in a pig cell. New virus has mixed genome - different surface antigens. Antigenic shift before a pandemic. Rare. --causes epidemics and pandemics at irregular intervals because it periodically undergoes antigenic variation -various species of birds and mammals are infected with strains of influenza A and may serve as reservoirs of infection

Prions

Proteinaceous infectious particles (Fig. 10.25, 10.26) Cause slow progressive neurological degeneration (Fig. 24.14) Discovered by Prusiner 1982 Creutzfeldt-Jacob disease (CJD) (genetic) Kuru in New Guinea (Fig. 24.15) -transmitted through small breaks in the skin -muscle weakness, incoordination leading to inability to walk -reach a point where they must be fed prechewed food New-variant CJD Animals: scrapie in sheep (Fig. 24.16). Bovine spongiform encephalopathy (BSE) (Fig. 24.17). Chronic wasting disease in elk , deer (Fig. 24.18) Prions are misfolded proteins. (Fig. 10.25, T. 10.6) Resistant to heating up to 90 C Resistant to radiation (UV) Resistant to enzyme digestion (nucleases) Sensitive to protein denaturing agents (phenol, urea) Have direct pairing of AA Transmission through eating infected meat Normal prion protein found in membrane of neurons Abnormal prion protein attaches to normal protein and converts it to abnormal

Viruses and Cancer

(Fig.10.28) Cancer - uncontrolled division of cells, do not respond to normal growth controls. Mutations or viruses inserted in genome can disrupt genes and cause uncontrolled growth Epstein-Barr virus (EBV) - Burkitts lymphoma -this is a herpes virus, tumor in the jaw that causes swelling and eventual destruction of the jaw Human papilloma virus (HPV) - cervical cancer Hepatitis B (HBV) liver cancer Human herpes virus 8 - Kaposis's sarcoma HTLV-1 (retrovirus) - adult T cell leukemia -Cancer: uncontrolled, invasive growth of abnormal cells, cancer cells divide repeatedly -in many cases they cannot stop dividing, this is called a neoplasm, or localized accumulation of cells known as tumor -a neoplasm can be benign -but if the cells invade and interfere with the functioning of surrounding normal tissue, the tumor is malignant How cancer viruses cause cancer -DNA tumor viruses contain viral genes whose protein products disrupt the activities of normal host cell proteins that control cell division -RNA tumor viruses contain viral genes used for neoplastic transformation and viral replication

Sexually Transmitted Diseases

1. Herpesvirus infections - enveloped, DNA virus (T. 10.3, Fig. 20.20, 20.21, 20.22, 20.23) Herpes simplex type 1 (HPV-1) - fever blisters Herpes simplex type 2 (HPV-2) - genital herpes Latency - recurrence (eruption) of infection. Stay in ganglia (Fig. 20.20) -Agents: Herpes Simplex Viruses -Characteristics: fever blisters usually caused by HSV-1; genital herpes usually caused by HSV-2 (both are latent viruses); recurrent, painful vesicular lesions;neonatal herpes, and a variety of other manifestations 2. Genital warts Human papilloma virus (HPV) naked, circular DNA virus (fig. 20.24, 20.25) Agents: Human Papillomavirus Characteristics: warts on genitals, vagina, and cervix; irritation and sometimes intense itching;causes 99% of all cervical carcinoma 3. Acquired Immune Deficiency Syndrome (AIDS): Retroviruses enveloped, (+) sense RNA 2 copies Human immunodeficiency virus HIV-1, HIV-2 Destroys immune system, can't fight opportunistic infections (T. 18.7)(Fig. 18.25) Infects T helper cell, macrophages, dendritic cells gp120 binds to CD4 receptors and coreceptor CXCR4 on T helper cells, and to CD4 receptors and coreceptor CCR5 on macrophages Cells without these receptors do not get infected Infection, disease progression (Fig. 18.25) Epidemiology (p. 569-570) --caused by the human immunodeficiency virus (HIV) -HIV destroys T helper cells and eventually impairs all immune functions -HIV-infected individuals progress through a series of stages that lead to AIDS -Group 4 individuals suffer from opportunistic infections and malignancies such as Kapok's sarcoma -the AIDs pandemic will have over 35.3 million people infected with HIC by the end of 2012 -HIV infection can be acquired by: sexual contact, needle sharing, blood transfusions, or products, and transfer across the placenta from mother to fetus -Health care personnel must practice Universal precautions at ALL times -vaccine, development remains unpromising -social, economic, legal, and ethical problems are becoming increasingly significant

Classification of Viruses

1. Host infected: bacteriophages, plant viruses, animal viruses • Bacteriophages: infect bacteria • Plant infect plant • Animal infect animal 2. Type of host structure infected: Dermotropic - skin Neurotropic - nervous tissue Viscerotropic - digestive system Pneumortopic - respiratory system 3. Genome structure: RNA or DNA, single stranded or double stranded • Virus particle structure (capsid shape) • Presence or absence of viral envelope

Purine and Pyrimidine Analogs:

A. Purine and pyrimidine analogs: ( toxic) incorporate into nucleic acid during replication and stop replication (this is why we don't like giving these drugs), these drugs inhibit replications of the virus and can therefore inhibit a lot of the replications of the things that take place in the body. 1. Idoxuridine - thymine analog 2. Vidarabine - adenine analog 3. Ribavarine - guanine analog. Low toxicity 4. Acyclovir - guanine analog (less toxic than other analogs) 5. Ganciclovir - guanine analog 6. Zidovudine (AZT) - thymine analog. Inhibits reverse transcription in HIV -several purine and pyrimidine analogs are effective antiviral agents -all cause the virus to incorporate erroneous information (the analog) into a nucleic acid and thereby interfere with the replication of viruses -the drugs include idoxuridine, virarabine, acyclovir, ganiclovir, and azidothymidine -idoxuridine and trifluridine, both analogs of thymine, are administered in eye drops to treat inflammation of the cornea caused by a herpes virus -they should not be used internally because they suppress bone marrow -Vidarabine (ARA-A), an analog of adenine, has been used effectively to treat viral encephalitis, an inflammation of the brain caused by herpesvirus and by cytomegalovirus infections acquired before birth -vidarabine is less toxic than either idoxuridine or cytarabine, but it sometimes causes GI disturbances -Ribavirin (Virazole), a synthetic nucleotide analog of guanine, blocks replication of certain viruses -in an aerosol spray, it can combat influenza viruses; in an ointment, it can help to heal herpes lesions -although it has low toxicity, it can induce birth defects and should not be given to pregnant women -it has been found to be effective against hantaviruses, such as those that caused the deadly outbreak of respiratory disease on the Navajo reservation in the four corners region of the American Southwest in 1993 -Ribavarin has show activity against a wide variety of unrelated viruses, raising hopes of finding a broad spectrum antiviral drug -Axyclovir (Zovirax), an analog of guanine, is much more rapidly incorporated into virus-infected cells than into normal cells -thus, it is less toxic than other analogous -it can be applied topically or given orally or intravenously -it is especially effective in reducing pain and promoting healing of primary lesions in a new case of genital herpes it is given prophylactically to reduce the frequency and severity of recurrent lesions, which appear periodically after a first attack -it does not, however, prevent the establishment of latent viruses in nerve cells -acyclovir is more effective than vidarabine against herpes encephalitis and neonatal herpes, an infection acquired at birth, but is not effective against other herpesviruses -Ganciclovir is an analog of guanine similar to acyclovir -the drug is active against several kinds of herpesvirus infections, particularly cytomegalovirus eye infections in patients with AIDS -Zidovudine (AZT) interferes with reverse transcriptase making DNA from RNA -it is used in treating AIDS

Selected Viral Diseases:

A. Upper respiratory system: 1. Common cold: Rhinoviruses, Coronaviruses (naked, (+) sense RNA) 2. Parainfluenza: Parainfluenza viruses (enveloped, (-) sense RNA)

Components of Viruses

These components are a nucleic acid core and a surrounding protein coat called a capsid. In addition, some viruses have a surrounding lipid bilayer membrane called an envelope. A complete virus particle, including its envelope, if it has one, is called a virion

The Treatment of AIDS

Treatment of HIV (AIDS) (Fig. 18.27) AZT and other reverse transcriptase inhibitors Protease inhibitors (can't make capsid proteins) HAART (highly active retroviral therapy) - combination of protease inhibitors and 2 nucleotide analogs. Integrase inhibitors (prevent integration of DNA) Entry inhibitors (can't enter cell) -several agents are being tested for the treatment of AIDS -new information about AIDS, its complications, and its treatment is becoming available with great rapidity

Culturing Animal Viruses

Whole animals Fertilized (embryonated) chicken eggs (Fig. 10.21) Cell cultures (tissue culture or cell culture monolayers) Three types of cultures used in clinical and research virology: Primary cell cultures Diploid fibroblast strains Continues cell lines Cytopathic effects (CPE) - visible effect (change) of virus on cells used for preliminary virus identification (Fig. 10.22)

Latent Viral infections

all herpesviruses have the ability to become latent, remaining dormant -activation usually involves changes in cell-mediated immunity

General Characteristics of Replication

in general, viruses go through the following five steps in their replication cycles to produce more virions 1.) Adsorption: the attachment of viruses to host cells 2.) Penetration: the entry of visions (or their genome) into host cells 3.) Synthesis: the synthesis of new nucleic acid molecules, capsid proteins, and other viral components within host cells while using the metabolic machinery of those cells 4.) Maturation: the assembly of newly synthesized viral components into complete virions 5.) release: the departure of new virions from host cells. Release generally, but not always, kills (lyses) host cells

Latent Infection

After a lytic cycle (acute phase) virus goes into latent phase, as a provirus It can reemerge - activated and replicate - lytic phase All herpes viruses (in nerve cells) -many individuals experience the recurrence of skin eruptions commonly called cold sores or fever blisters -these are caused the the herpes simplex virus, a member of the herpesviruses -as we saw earlier, these are dsDNA viruses that can exhibit a lytic cycle -they can also remain latent within the cells of the host organism throughout the individuals life--not in the skin cells we associate them with, but in the nerve cells -when activated, whether by a cold or fever or by stress or immunosupression, they once again replicate resulting in cell lysis -the ability to become latent is held by all of the herpesviruses -another herpesvirus, the one thats responsible for chicken pox, can also remain dormant within the central nervous system -when it becomes activated, usually due to changes in cell-mediated immunity, the virus causes a rash to form along the nerve where it lay latent -this reactivation is known as shingle -many individuals carry these viruses throughout their lives, never exhibiting any symptoms

Parainfluenza

Agents: Parainfluenza viruses Characteristics: Nasal inflammation, pharyngitis, bronchitis, croup, sometimes pneumonia

Common Cold

Agents: Rhinoviruses, coronaviruses Characteristics: sore throat, malaise, headache and cough Naked, positive sense RNA

Read the history

Emerging Viruses: many are due to demographic changes, travel, commerce, ecological changes, viral mutations, environmental changes

The Functions of Bacteriophage Structural Components

Genome: Carries genetic information necessary for replication of new phage particles Tail Sheath: Retracts so that the genome can move from the head into the host cell's cytoplasm Plate and Tail fibers: Attach phage to specific receptor sites on the cell wall of a susceptible host bacterium

T4

obligate parasite of E.Coli, dsDNA -capsid: Head: contains DNA -----------collar, tail sheet: allows genome to enter cell -----------plate, tail fibers: attach to receptors to enter cell

Viruses

-submicroscopic obligate intracellular parasites--they replicate only inside a living host cell -infectious agents that are too small to be seen with a light microscope and that are not cells -they have no nucleus, organelles or cytoplasm -when they invade susceptible host cells, viruses display some properties of living organisms and so appear to be on the borderline between living and nonliving -viruses can replicate, or multiply, only inside a living host cell --as such they are called obligate intracellular parasites, a distinction they share with chlamydia and rickettsias - we may have to reconsider the traditional definition of viruses following the announcement in December 1991 by E. Wimmer, A. Molla, and A. Paul that they successfully grew entire polioviruses in test tubes containing ground up human cells, but no live cells. -RNA from polioviruses was added to the cell free extract, and about 5 hours later complete new virus particles began to appear -this work has been replicated by many other groups, but has not yet been duplicated with other viruses -in november 2003, Dr. Craig Venter (already famous for his role in completing the human genome project) led a team at the institute for biological energy alternatives in creating a brand new, man made virus -they didn't even make the parts themselves -they ordered them from various commercial companies -then they put the over 5,000 DNA building blocks plus proteins together, creating a bacteriophage -they hope to eventually design genetically modified organisms that can eat carbon dioxide and clean the environment -some environmentalists are very upset, fearing that such organisms could run amok -viruses differ from cells in important ways -whereas prokaryotic and eukaryotic cells contain both DNA and RNA, individual virus particles contain only one kind of nucleic acid--either DNA or RNA but never both -but now researchers, working at boiling springs lake in lassen volcanic park in northern california, have discovered a new type of virus--a DNA-RNA hybrid virus -sequencing the entire genome of this virus clearly showed that there was a lateral transfer of genes from RNA-only virus to a DNA-only virus -such a recombination may be an important step in understanding the transition from the ancient RNA world, which came first, to the current DNA world -Cells can grow and divide, but viruses do neither -viral replications requires that a virus particle infect a cell and program the host cell's machinery to synthesize the components required for the assembly of new virus particles -the infected cell may produce hundred to thousands of new viruses and then usually dies -tissue damage as a result of cell death accounts for the destructive effects seen in many viral diseases.

Size and Shapes

--viruses have a range of sizes -the largest are ones between 1200 nm and 1500 nm such as Megavirus chillness, a giant virus found off the coast of chile -the complex bacteriophages are about 65 nm by 200nm -among the smallest viruses known are the enteroviruses, which are less than 30 nm in diameter -most viruses are quite small when compared with bacteria or eukaryotic cells -although some viruses are variable in shape, most viruses have a specific shape that is determined by the capsomeres of the envelope -a helical capsid consists of a ribonlike protein that forms a spiral around the nucleic acid -the tobacco mosaic virus is a helical virus -polyhedral viruses are many sided -the picornavirus and the human adenoviruses are polyhedral viruses -one of the most common polyhedral capsid shapes is the icosahedron; icosahedral viruses have 20 triangular faces -a complex capsid is a combination of helical and icosahedral shapes, and some viruses have a bullet shaped capsid -most viruses with envelopes have a somewhat spherical shape -for example, the herpes viruses has a polyhedral capsid and an envelope -the film viruses are threadlike in shape -the poxviruses and many bacterial viruses are called complex viruses because they have a more elaborate coat or capsid -many bacteriophages, or viruses that infect bacteria, have a complex shape that incorporates specialized structures such as heads, tails, and tail fibers -like spikes, the tail fibers are used by visions to attach to host bacteria -other specialized bacteriophage structures are used to infect bacterial cells

Envelopes

-Enveloped viruses have a typical bilayer membrane outside their capsids -such viruses acquire their envelope after they are assembled in a host cell as they bud, or move through, one or several membranes -a virions nucleocapsid comprises the viral genome together with the capsid -viruses with only a nucleocapsid and no envelope are known as naked, or non enveloped, viruses -the composition of an envelope generally is determined by the viral nucleic acid and by the substances derived from host membranes -combinations of lipids, proteins, and carbs make up most envelopes -depending on the virus, projections referred to as spikes may or may not extend from the viral envelope -these surface projections are glycoproteins that serve to attach visions to specific receptor sites on susceptible host cell surfaces -in certain viruses the possession of spikes causes various types of red blood cells to clump, or hemagglutinate--a property that is useful in viral identification -what advantages might envelopes have for viruses? -because envelopes are acquired from and are therefore similar to host cell membranes, viruses may be "hidden" from attack by the host's immune system -also, envelopes help viruses infect new cells by fusion of the envelope with the host's cell or plasma membrane. -conversely, enveloped viruses are damaged easily -environmental conditions that destroy membranes--increased temperature, freezing and thawing, pH below 6 or above 8, lipid solvents, and some chemical disinfectants such as chlorine, hydrogen peroxide, and phenol will also destroy the envelope -naked viruses generally are more resistant to such environmental conditions envelope: lipid bilayer membrane derived from host cell membrane spikes: glycoproteins attach virus to cell receptors

RNA viruses

-RNA animal viruses classified according to their nucleic acid content, capsid shape and presence or absence of envelope, (+) or (-) sense RNA

Capsids

-The nucleic acid of an individual vision is in most cases enclosed within a capsid that protects it and determines the shape of the virus -capsids also play a key role in the attachment of some viruses to host cells -each capsid is composed of protein subunits called capsomeres -in some viruses, the proteins found in the capsomere are of a single type -in other viruses several different proteins may be present -the number of proteins and the arrangement of viral capsomeres are characteristic of specific viruses and thus can be useful in virus identification and classification capsid: a protein coat made of subunits (capsomeres)

Influenza B

-also undergo antigenic changes, but less extensively and at a slower rate than do influenza A viruses -epidemics caused by influenza B viruses are limited geographically and tend to center around schools and other instittutions -these viruses are found only in humans

Replication of Bacteriophages

-bacteriophages, or simply phages, are viruses that infect bacterial cells -phages were first observed in 1915 by Frederic Twort in england and in 1917 by Felix d'Herelle in France -d'Herelle and eliava founded an institute in soviet georgia for the study of phages and phage therapy of bacterial diseases -this institute became the largest in the world devoted to development and production of phage therapy products

Synthesis in DNA Animal Viruses

-generally DNA animal viruses replicate their DNA in the host cell nucleus with the aid of viral enzymes and synthesize their capsid and other proteins in the cytoplasm by using host cell enzymes -the new viral proteins move to the nucleus, where they combine with the new viral DNA to form virions -this pattern is typical of adenoviruses, hepadnaviruses, herpesviruses, and papoviruses -poviruses are the only exception, their parts are synthesize in the host cells cytoplasm -in dsDNA viruses, replication proceeds in a complex series of steps designated as early and late transcription and translation -the early events take place before the synthesis of viral DNA and result in th production of the enzymes and other proteins necessary for viral DNA replication -the late events occur after the synthesis of viral DNA and result in the production of structural proteins needed for building new capsids -copmared with bacteriophage replication, synthesis in animal virus replication can take much longer -the capsids of the herpesvirus, for example, contain so many proteins that their synthesis requires 8 to 16 hours -some viruses, such as the adenovirus, contain only ssDNA -before viral replication can be initiated, the viral DNA must be copied, forming a dsDNA viral genome

Teratogenesis

-induction of defects during embryonic development -Teratogen - drug or other agent causing defects. -Some viruses act as teratogens, cross placenta, infect fetus. -Damage to cells, tissues, organs more extensive if infection occurs early in pregnancy. -Cytomegalovirus (CMV), Herpes simplex 1 and 2, Rubella -the induction of defects during embryonic development -a teratogen: a drug or other agent that induces such defects -certain viruses are known to act as teratogens and can be transmitted across the placenta and infect the fetus -the earlier in the pregnancy the embryo is infected, the more extensive the damage is likely to be -during the early stages of embryoloc development, when an organ or body system may be represented by only a few cells, viral damage to those cells can interfere with the development of that organ or body system -viral infections occurring later in the development may damage fewer cells and thus have a proportionately smaller effect -that is because, by then, the total cell population in the fetus has greatly increased, and each organ or body system consists of thousands of cells -three human viruses-cytomegalovirus (CMV), herpes simplex virus (HSV) types 1 and 2, and rubella--account for a large number of teratogenic effects -CMV infections are found in about 1% of live births; of those, about 1 in 10 eventually die from the CMV infection -most of the defects are neurological, and the children have varying degrees of mental retardation -some also have enlarged spleens, liver damage, and jaundice -HSV infections usually are acquired at or shortly after birth -infections acquired before birth are rare -in cases of disseminated infections (those that spread through the body), some infants die and survivors have permanent damage to the eyes and central nervous system -rubella virus infections in the mother during the first 4 months of pregnancy are most likely the result in fetal defects referred to as the "rubella syndrome" -these defects include deafness, damage to other sense organs, heart and other circulatory defects, mental retardation -the degree of impairment is highly variable -some children adapt to their disabilities and live productive lives -in other cases the fetus is so impaired that death and natural abortion occur -a series of blood tests often referred to as the TORCH series is sometimes used to identify possibly teratogenic diseases in pregnant women and newborn infants -these tests detect antibodies made against toxoplasma, other disease causing viruses (usually including hepatitis B virus and the varicella, or chickenpox virus), rubella virus, CMV, HSV -all these diseases can be transmitted to the fetus via the placenta -intrauterine diseases other than those tested for in the TORCH series may also exist in a newborn -therefore, passing the TORCH tests does not guarentee a health baby

Viral Specificity

-infect specific tissues or species (cell receptors, enzyme s for replication) -another important property of viruses, refers to the specific kinds of cells a virus can infect -viral specificity is determined mainly by whether a virus can attach to a cell -attachment depends on the presence of specific receptor sites on the surfaces of the host cells and on specific attachment structures on viral capsids or envelopes -specificity is also affected by whether appropriate host enzymes and other proteins the virus needs in order to replicated are available inside the cell -finally, specificity is affected by whether replicated viruses can be released from the cell to spread the infection to other cells

Antiviral agents

-inhibit viral replication but not kill the viruses -until recent years no chemotherapeutic agents effective against viruses were available -one reason for the difficulty in finding such agents is that the agent must act on viruses within cells without severely affecting the host cells -currently available antiviral agents inhibit some phase of viral replication, but they do not kill the viruses -latent, non-replicating viruses such as the Herpes cold sore virus are impossible to destroy -they must be actively replicating for drugs to interfere with their replication -identifying ways of activating them are being researched, so that some day your cold sores may be gone forever

Development of Culturing Methods

-initially, if a virologist wanted to study viruses, the viruses had to be grown in whole animals -this made it difficult to observe specific effects of the viruses at the cellular level -in the 1930s virologists discovered that embryonate (intact, fertilized) chickenegges could be used to grow herpesvirus, poxviruses, and influenza viruses -although the chick embryo is simpler in organization than a whole mouse or rabbit, it is still a complex organism -the use of embryos did not completely solve the problem of studying cellular effects caused by viruses -another problem was that bacteria also grow well in embryos, and the effect of viruses often could not be determined accurately in bacterially contaminated embryos -virology progressed slowly during these years until techniques for growing viruses in cultures improved -two discoveries greatly enhanced the usefulness of cell cultures for virologists and other scientists -first, the discovery and use of antibiotics made it possible to prevent bacterial contamination -second, biologists found that proteolytic enzymes, particularly trypsin, can free animal cells from the surrounding tissues without injuring the freed cells -after the cells are washed, they are counted and then dispensed into plastic flasks, tubes, petri dishes, or roller bottles -cells in such suspensions will attach to the plastic surface, multiply, and spread to form sheets one cell thick, called monolayers -these monolayers can be subcultured -subculturing is the process by which cells from an existing culture are transferred to new containers with fresh nutrient media -a large number of separate subcultures can be made from a single tissue sample, thereby assuring a reasonably homogenous set of cultures with which to study viral effects -the term tissue culture remains in widespread usage to describe the preceding technique, although the term cell culture is perhaps more accurate -today, the majority of cultured cells are in the form of monolayers grown from enzymatically dispersed cells -with a wide variety of cell cultures available and with antibiotics to control contamination, virology entered its "Golden Age" -in the 1950s and 1960s more than 400 viruses were isolated and characterized -although new viruses are still being discovered, emphasis is now on characterizing the viruses in more detail and on determining the precise steps in viral infection and viral replication -plaque assays similar to those used to study phages can be used for animal viruses -for example, cultures of susceptible human cells are grown in cell monolayers and then inoculated with viruses -if the viruses lyse cells, several rounds of infection will produce plaques

Properties of bacteriophages

-like other viruses, bacteriophages can have their genetic information in the form of either double stranded or singled stranded RNA or DNA -they can be relatively simple or complex in structure -to understand phage replication, we will examine the T-even phages -these phages, designated T2, T4 and T6 (T stands for"type") are complex but well studied naked phages that have dsDNA as their genetic material -the most widely studied is the T4 phage, an obligate parasite of the common enteric bacterium E.coli -T4 has a distinctly shaped capsid made of a head, collar, and tail -the DNA is packaged in the polyhedral head, which is attached to a helical tail

General properties of DNA viruses

-like the RNA viruses, the animal DNA viruses are grouped into families according to their DNA organization -the dsDNA viruses are further separated into families on the basis of the shape of their DNA (linear or circular), their capsid shape, and the presence or absence of an envelope -only one family of viruses has ssDNA

Host Range

-refers to the spectrum of hosts that a virus can infect -different viruses can infect bacteria, fungi, algae, protozoa, plants, vertebrates, or even invertebrates -however most viruses are limited to only one host and to only specific cells and/or tissues of that host -the host range of the rabies virus is much more extensive than that of polioviruses

Influenza C

-structurally different from A and B viruses -infections with C viruses are rarely recognized -typically limited to children in a single family or single classroom -the low infectivity of C viruses, which lack neuraminidase suggests that this enzyme may enhance infectivity in viruses that have it

Synthesis in RNA animal viruses

-synthesis in RNA animal viruses takes place in a greater variety of ways than is found in DNA animal viruses -in RNA viruses such as the picornaviruses, the positive sense RNA acts as mRNA, and viral proteins are made immediantly after penetration and uncaring -the nucleus of the host cell is not involved -viral proteins also play key roles in the synthesis of these viruses -one protein inhibits synthetic activites of the host cell -for synthesis, an enzyme uses the positive sense RNA as a template to make a negative sense RNA -this negative sense RNA in turn acts as a template RNA to replicate many positive sense RNA molecules for virion formation -in the retroviruses, such as HIC, the two copies of positive sense RNA do not act as mRNA -rather, they are transcribed into sand with the help of reverse transcriptase -the ssDNA then is replicated through complementary base pairing to make dsDNA molecules -once in the cell nucleus, this molecule inserts itself as a provirus into a host cell chromosome -the provirus can remain there for an indefinite period of time -when infected cells divide, the provirus is replicated along with the rest of the host chromosomes -thus, the viral genetic information is passed to progeny host cells -unlike prophages, however, the provirus cannot be excised -if an event occurs that activates the provirus, its genes are expressed; that is, the genes are used to make viral mRNA, which directs synthesis of viral proteins -full length positive sense RNA molecules also are transcribed from the prophage -two copies of the positive sense RNA are packaged into each vision -in negative sense RNA animal viruses, such as the viruses causing measles and influenza A, a packaged transcriptase uses the negative sense RNA to make positive sense RNA molecules (mRNA) -prior to assembly, new negative sense RNA is made from positive sense RNA templates -the process is essentially the same regardless of whether the viral RNA is in one segment (measles) or in many segments (influenza A) -in the reoviruses, the dsRNA codes for several viral proteins -each strand of the dsRNA acts as a template for its partner -like DNA replication, RNA replication is semiconservative, so the molecules produced have one strand of old RNA and one strand of new RNA -These viruses have a double walled capsid that is never completely removed, and replication takes place within the capsid

General Properties of RNA Viruses

-the different families of RNA viruses are distinguished from one another by their nucleic acid content, their capsid shape, and the presence or absence of an envelope -most families of RNA viruses contain either one positive sense RNA or one negative sense RNA molecule -however, some RNA viruses are placed in separate families if the RNA exists as two complete copies of positive sense RNA or contains small segments of negative sense RNA -finally, one family has segmented dsRNA

The Cytopathic Effect

-the visible effect viruses have on cells is called the cytopathic effect (CPE) -cells in culture show several common effects, including changes in cell shape and detachment from adjacent cells or the culture container -however, CPE can be so distinctive that an experienced virologist often can use it to make a preliminary identification of the infecting virus -for example, human adenoviruses and herpesviruses cause infected cells to swell because of fluid accumulation whereas picornaviruses arrest cell functions when they enter and lyse cells when they leave -the paramyxoviruses cause adjacent cells in culture to fuse, forming giant, multinucleate cells called syncytia -synctia can contain 4 to 100 nuclei in a common cytoplasm -another type of CPE produced by some viruses is transformation: the conversion of normal cells into malignant ones

types of cell cultures

-three basic types of cell cultures are widely used in clinical research virology: 1) primary cell cultures 2) diploid fibroblast strains 3) continuous cell lines Primary cell cultures: come directly from the animal and are not subcultured -the younger the source animal, the longer the cell types will survive in culture -they typically consist of a mixture of cell types, such as muscle and epithelial cells -although such cells usually do not divide more than a few times, they support growth of a wide variety of viruses -if primary cell cultures are repeatedly subcultured, one cell type will become dominant, and the culture is called a cell strain -in cell strains all the cells are genetically identical to one another -they can be subcultured for several generations with only a very small likelihood that changes in the cells themselves will interfere with the determination of viral effects -among the most widely used strains are diploid fibroblast strains -fibroblasts are immature cells that produce collagen and other fibers as well as the substances of connective tissues, such as the dermis of the skin -derived from fetal tissues, these strains retain the fetal capacity for rapid, repeated cell division. -such strains support growth of a wide range of viruses and are usually free of contaminating viruses often found in cell strains from mature animals -for this reason they are used in making viral vaccines -the third type of cell culture in extensive use is the continuous cell line -a continuous cell line consists of cells that will reproduce for an extended number of generations -the most famous of such cultures is the HeLa cell line, which has been maintained and grown in culture since 1951 and has been used by many researchers worldwide -the original cells of the HeLa cell line came from a woman with cervial cancer and are named from the first letters of her name -in fact, many of the early continuous cell lines used malignant cells because of their capacity for rapid growth -such immortal cell lines grow in the lab without aging, divide rapidly and repeatedly, and have simpler nutritional needs than normal cells -the HeLa cell line, for example, contains two viral genes necessary for its own immortality -immortal cell lines are heteroploid (have different numbers of chromosomes) and are therefore genetically diverse -cell cultures have largely replaced animals and embryonate eggs for studies in animal virology -yet, the embryonated chicken egg remains one of the best host systems for influenza A viruses -in addition, young albino Swiss mice are still used to culture arboviruses (arthropod-borne viruses), and other mammalian cell lines--as well as mosquito cell lines---have been used for some time

Origin of Viruses

-viruses have arisen, and probably continue to arise, by multiple origins -viruses act as agents of evolution by their participation in lateral gene transfer -free viruses are incapable of reproduction: they must infect host cells, uncoat their genetic material, and then use the host's machinery to copy or transcribe the viral genetic material -one hypothesis proposes that viruses and cellular organisms evolved together, with both viruses and cells originating from self replicating molecules present in the pre cellular world -another idea, sometimes referred to as reverse evolution, is that viruses were once cells that lost all cell functions, retaining only that information to replicate themselves by using another cells metabolic machinery -a third hypothesis proposes that viruses evolved within the cells they infect, possibly from plasmids, the independently replicating DNA molecules found in many bacterial cells or from retrotransposons

Nucleic Acids

-viruses use their genome, their genetic information, to replicate themselves in host cells -the result is often a disruption of host cellular activities or death of the host -viral genomes consist of either DNA or RNA -Viral replication depends on the expression of the viral genome for the formation of viral proteins and the replication of new viral genomes within the infected host cell -viral nucleic acid can be single-stranded or double stranded, and linear, circular, or segmented (existing as several fragments) -all genetic information in RNA viruses is carried by RNA -RNA genomes occur only in viruses and a virus like agent called the viroid core: nucleic acid DNA or RNA, single or double stranded

Replication of a virulent bacteriophage

1. Adsorption:phage is adsorbed onto bacterial cell wall 2. Penetration: phage penetrates bacterial cell wall and cell membrane. Phage DNA is injected -bacterial DNA is disrupted 3. Biosynthesis: the phage DNA directs the cell's metabolism to produce viral components--proteins and copies of phage DNA -empty phage heads and pieces of phage DNA are synthesized 4.Maturation -collars, sheats, and base plates have been attached to heads. Tail fibers are added last 5. Release: bacterial cell lyses, releasing mature phages

replication of a temperate bacteriophage

1&2: Adsoprtion and Penetration: phage is adsorbed to receptor sites on bacterial cell wall, penetrates it, and inserts its DNA -phage DNA inserts itself as a prophage into a bacterial chromosome -phage is replicated along with bacterial DNA prior to binary fission -binary fission is completed; each cell has the phage DNA incorporated 3. Biosynthesis: the phage DNA directs the cell's metabolism to produce viral components--proteins and copies of phage DNA -empty phage heads and pieces of phage DNA 4. Maturation: heads are packed with DNA -Collars, sheaths, and base plates have been attached to heads. tail fibers are added last 5. release: bacterial cell lyses, releasing completed infective phages --following adsorption and penetration, the virus undergoes prophage formation -in the lysogenic cycle, temperate phages can exist harmlessly as a prophage within the host cell for long periods of time -each time the bacterial chromosome is replicated, the prophage also is replicated; all daughter bacterial cells are "infected" with the prophage -induction innvolves either a spontaneous or an environmentally induced exision of the prophage from the bacterial chromosome -a typical lytic cycle, involving biosynthesis and maturation, occurs, and new temperate phages are released

Amantadine

B. Amantadine - tricyclic amine. Prevents penetration of virus - Influenza A Rimantadine - less toxic Tamiflu - for flue -the tricyclic amine amantadine prevents influenza A viruses from penetrating cells -given orally, it is readily absorbed and can be used from a few days before to a week after exposure to influenza A viruses to reduce the incidence and severity of symptoms -unfortunately, it causes insomnia and ataxia (inability to coordinate voluntary movements, especially in elderly patients, who also are often severly affected by influenza infections -rimantadine, a drug similar to amantadine, may be effective against a wider variety of viruses and may be less toxic as well

Comparison of Bacteriophage and Animal Virus Replication

Bacteriophage: Attachment Sites: attachment of tail fibers to cell wall proteins Penetration: injection of viral nucleic acid through bacterial cell wall Uncoating: none needed Synthesis: in cytoplasm, bacterial synthesis ceased, viral DNA or RNA replicated, formation of viral mRNA, viral components synthesized Maturation: addition of collar, sheet, base plate, and tail fibers to nucleic acid--containing head Release: host cell lysis Chronic Infection: lysogeny Animal virus: Attachment sites: attachment of spikes, capsid or envelope to plasma membrane proteins Penetration: endocytosis or fusion Uncoating: enzymatic digestion of viral proteins Synthesis: in cytoplasm (RNA viruses) or nucleus (DNA viruses), host cell synthesized, viral DNA or RNA replicated, formation of viral RNA, viral components synthesized Maturation: insertion of viral nucleic acid into capsid Release: budding (enveloped viruses), cell rupture (nonenveloped viruses) Chronic infection: latency, chronic infection, cancer

Nervous System

D. Nervous system: 1. Viral meningitis - self limiting, non fatal. Enteroviruses 2. Rabies - Rhabdovirus enveloped, (-) sense RNA Preference for nervous tissue (Fig. 24.3) World wide distribution (Fig. 24.4). Transmitted by bite of animal -agents: rabies virus -characteristics: invades nerves and brain; headache, fever, nauseous, partial paralysis, coma, and death, ensue unless patient has immunity 3. Encephalitis - inflammation of the brain Togaviruses - enveloped (+) sense RNA Eastern equine encephalitis (EEE) Western equine encephalitis (WEE) Venezuelan equine encephalitis (VEE) Flaviviruses - enveloped, (+) sense RNA St. Louis encephalitis (SLE) Transmitted by bite of mosquito -agents: seceral encephalitis viruses -characteristics: shrinkage, and lysis of neurons of the CNS; headache, fever, and sometimes brain necrosis and convulsions 4. West Nile Fever (Fig. 24.5) Neuroinvasive, Encephalitis, Transmitted by mosquito agents 5. Poliomyelitis - Picornaviruses - naked, (+) sense RNA 3 strains of Enterovirus Affinity for motor neurons - spinal cord, brain. Damage (lysis) of motor neurons Paralysis (Fig. 24.12, 24.13) Transmission - fecal-oral route agents: several types of polioviruses characteristics: fever, back pain, muscle spasms, partial or complete flaccid paralysis from destruction of motor neurons

DNA viruses

DNA viruses can be dsDNA or ssDNA DNA animal viruses are grouped according to DNA organization dsDNA viruses classified according to DNA shape, capsid shape, presence or absence of envelope

DNA viruses

DNA viruses: (Fig. 10.16) most replicate in nucleus of host cell. Transcription in cytoplasm Maturation in nucleus. Release may or may not kill host cell. Envelope derived from host membranes during budding. Shedding of new virions. Lysis of cell produces symptoms of infection. Some viruses insert their DNA into host genome and can not be excised.

Systemic Diseases

Denque fever (Fig. 23.17) Agents: denque fever virus characteristics: severe bone and joint pain, high fever, headache, loss of appetite, weakness, sometimes a rash Yellow fever agents: yellow fever virus characteristics: fever, anorexia, nauseau, vomiting, liver damage, jaundice Infectious mononucleosis - Epstein-Barr virus (EBV) (Fig. 23.18) agents: epstein-barr virus characteristics: headache, fatigue, malaise, usually sore throat, secondary streptococcal infections common Chikungunya Ebola

T4 replication (virulent)

Lytic Cycle: figure 10.11 adsorption: tail fibers attach to specific receptors on host cell penetration: genome enters host cell synthesis: host cell us used to make viral DNA and proteins maturation: assembly of viral particle release: lysozyme break cell wall, phages released -phages such as T4 are called virulent (lytic) phages because they lyse and destroy the bacteria they infect -such infections by virulent phages represent a lytic cycle of infection -burst time: the time from adsorption to release, varies from 20 to 40 minutes for different phages -the number of new visions released from each bacterial host represents the viral yield or burst size -in phages such as T4, anywhere from 50 to 200 new phages may be released from one infected bacterium

Natural Defense by Immune System Interferons and Immunoenhancers

Natural defense by immune system Interferon - proteins produced by infected cells, stimulate other cells to produce proteins and prevent infection. Levamisole, inosiplex - drugs stimulate immune system and enhance natural defenses against viruses -cells infected with viruses produce one or more proteins collectively referred to as interferons -when released, these proteins induce neighboring cells to produce antiviral proteins, which prevent these cells from becoming infected -thus, interferons represent a natural defense against viral infection -some interferons are currently being genetically engineered and tested as antiviral agents -some positive results have been obtained in controlling chronic viral hepatitis and warts and arresting virus-related cancers, such as Kapok's sarcoma -because cells produce interferons naturally, a possible way to combat viruses is to induce cells to produce interferons -synthetic double stranded RNA has been shown to increase the quantity of interferon in the blood -experiments with one such substance in virus-infected monkeys have shown sufficient increase in interferon to prevent viral replication -two other agents, levamisole and inosiplex, appear to stimulate the immune system to resist viral and other infection -both seem to stimulate activity of leukocytes called T lymphocytes rather than to stimulate interferon release -levamisole appears to be effective prophylactically in reducing the incidence and severity of chronic upper respiratory infections, which are probably viral in nature -it also reduces symptoms of autoimmune disorders such as rheumatoid arthritis, in which the body reacts against its own tissues -inosiplex has a more specific action; it stimulates the immune system to resist infection with certain viruses that cause colds and influenza -although efforts to improve antiviral therapies by enhancing natural defenses have been somewhat successful, none is yet in widespread use -more research is needed to identify or synthesize effective agents, to determine how they act, and to discover how they can be most effectively used

Oncogenes

Oncogenes: are mutated proto-oncogenes (growth regulating genes) Inserted in genome by virus, and disrupt genes. Originated from proto-oncogenes taken by a retrovirus exiting a cell. Mutated in virus and became oncogenes -oncogenes are viral genes that cause host cells to divide uncontrollably -proto-oncogenes are normal genes that, when under the control of a virus, act as oncogenes, causing uncontrolled cell division -oncogenes in RNA tumor viruses produce proteins in excessive amounts or produce proteins at the wrong times -in either case, infected host cells start uncontrolled cell division

Nucleic Acid classification

RNA: ssRNA or dsRNA (single stranded or double stranded) two types of ssRNA: • Positive (+) sense RNA: RNA acts like mRNA, is transcribed by host's ribosomes • Negative (-) sense RNA: RNA acts as a template during transcription to make complementary (+) sense mRNA which is translated by host's ribosomes (-) sense RNA viruses carry RNA polymerase for transcription. --major groups of viruses are distinguished first by their nucleic acid content as either RNA or DNA viruses -subsequent divisions are based largely on other properties of nucleic acids -the RNA viruses can be single stranded (ssRNA) or double stranded (dsRNA), although most are single stranded -because most eukaryotic cells do not have the enzymes to copy viral RNA molecules, the RNA viruses must either carry the enzymes or have the genes for those enzymes as part of their genome -two types of single stranded RNA viruses- positive sense and negative sense -many ssRNA viruses contain positive sense RNA: meaning that during an infection the RNA acts like mRNA and can be translated by the host's ribosomes -other ssRNA viruses have negative sense RNA -in such viruses the RNA acts as a template during the transcription to make a complementary positive sense mRNA after a host cell has been entered -this strand is translated by host ribosomes -in order to perform the transcription step, negative sense RNA viruses must carry an RNA polymerase within the virion -like RNA viruses, DNA viruses can also occur in single stranded or double stranded form -for example, the human adenovirus, responsible for some common colds, and the herpesvirus are double stranded (dsDNA) viruses -only one single stranded DNA (ssDNA) virus is currently known to produce human disease

Animal Viruses

Replication follows same steps but process is different (T. 10.5) 1. Adsorption: Virus attaches to specific receptors on cell surface. Glycoprotein spikes (Fig. 10.18) 2. Penetration: nucleic acid and capsid enter by endocytosis.(Fig. 10.19) Uncoating 3. Synthesis: Different in DNA and RNA viruses Adsorption: animal cells lack cell walls but have different ways of attaching to host cells -naked viruses have attachment sites (proteins) on the surfaces of their capsids that bind to corresponding sites on appropriate host cells -enveloped viruses, such as HIV, have spikes that recognize, in part, a membrane protein receptor on the surface of certain immune defense cells Penetration: -animal cells do not have a mechanism for injecting nucleic acid into host cells -thus, both the nucleic acid and the capsid usually penetrate animal host cells -most naked viruses enter the cell by endocytosis, in which virions are captured by pitlike regions on the surface of the cell and enter the cytoplasm within a membranous vesicle -enveloped viruses may fuse their envelope with the hosts plasma membrane or enter by endocytosis -in the latter case, the envelope fuses with the vesicle membrane -once the animal virus enters the host cell's cytoplasm, the viral genome must be separated from its protein coat (released) through a process called uncaring -naked viruses are uncoated by proteolytic enzymes from host cells or from the viruses themselves -the uncaring of viruses such as the poxvirus is completed by a specific enzyme that is encoded by viral DNA and formed soon after infection -polioviruses begin uncaring even before penetration is complete Synthesis: -the synthesis of new genetic material and proteins depends on the nature of the infecting virus (look at next slides for synthesis Maturation: -once an abundance of viral nucleic acid, enzymes, and other proteins have been synthesized, assembly of components into complete virions starts -this step constitutes maturation or assembly of progeny viruses -the cellular site of maturation varies depending on the virus type -for example, Human adenovirus nucleocapsids are assembled in the cell nucleus, whereas viruses such as HIV are assembled at the inner surface of the host cells plasma membrane -the poxviruses, polioviruses, and picornaviruses are assembled in the cytoplasm Release -the budding of new virions through a membrane may or may not kill the host cell -human adenoviruses, for example, bud from the host cell in a controlled manner --this shedding of new virions does not lyse the host cell -when an infected animal cell is filled with progeny virions, the plasma membrane lyses and the progeny are released -lysis of cells often produces the clinical symptoms of the infection or disease -the herpesvirus that cause cold sores and poxviruses destroy skin cells as a result of virion release -and the poliovirus destroy nerve cells during the release process

Replication of RNA viruses

Replication of RNA viruses: (Fig. 10.17) (+) sense RNA viruses: (+) RNA acts as mRNA to form viral proteins (+) RNA to (-)RNA template to (+) viral RNA Maturation (assembly) and release (-) sense RNA viruses: Enzyme transcriptase carried by the virus (-)RNA to (+)RNA (mRNA) to proteins then (+)RNA to (-)RNA Assembly and release dsRNA viruses: codes for proteins, each strand replicates, assembly and release Retroviruses HIV (Fig. 10.17) • (+) sense RNA, 2 strands Carries reverse transcriptase, integrase, protease enzymes. gp 120 glycoproteins (spikes) attach to CD4 receptors on host cell. Also coreceptors CXCR4 on T cells and CCR5 on macrophages are required. • (+)RNA with reverse transcriptase forms ssDNA then synthesize complementary strand dsDNA. Enters nucleus, inserts in cell chromosome by integrase - Provirus. Replicates with host genome. Can not be excised. Activated - genes expressed. DNA to (-)RNA transcribed to polyprotein which is cleaved by protease to capsid proteins, envelope proteins (spikes), reverse transcriptase Assembly and release by budding.

Lower Respiratory System

SARS (severe acute respiratory syndrome): -caused by coronavirus -symptoms: high fever, dry cough, shortness of breath, difficulty breathing, animist often X-rays indicating pneumonia -spread by close contact with an infected person, usually exhaled or coughed aerosol droplets Hantavirus Pulmonary Syndrome: -caused by hantavirus -enveloped, negative sense RNA -carried by rodents that shed the virus in urine, feces, and saliva -virus from dried excreta becomes airborne and is inhaled Influenza: -caused my orthomyxoviruses which are enveloped negative sense RNA -epidemics and pandemics ---H1N1 ---etc ---avian flu Surface antigens (spikes): (Fig. 21.20) Hemagglutinin (H) - for infectivity, attaches to receptors on cells Neuraminidase (N) enzyme helps penetration On basis of nucleoprotein antigens - 3 major serotypes A, B, C Antigenic variation - mutations affect viral antigens Influenza A (Fig. 21.18) - epidemics and pandemics - periodic antigenic variation, strains Influenza B - some antigenic variation, not extensive Influenza C - structurally different from A and B. No neuraminidase

Lysogeny: General properties of lysogeny

Temperate phage. Phage nucleic acid incorporated in host's DNA Lambda phage of E. coli. (Fig. 10.14, 10.15) Phage DNA inserts into bacterial chromosome - Prophage Lysogenic conversion - alter cell's characteristics Toxic effects : production of toxins due to a phage - Clostridium botulinun Lysogenic cycle - phage replicates with host DNA. Prophage can excise and enter the lytic cycle -the bacteriophages we have been discussing, virulent phages, destroy their host cells -Temperare phages: do not always undergo a lytic cycle -the majority of the time they will exhibit lysogeny, a stable long term relationship between the phage and its host in which the phage nucleic acid becomes incorporated into the host nucleic acid -such participating bacteria are called lysogenic cells -one of the most widely studied lysogenic phages is the lambda phage of E.Coli -lambda phages attach to bacterial cells and insert their linear DNA into the bacterial cytoplasm -however, once in the cytoplasm, the phage DNA circularizes and then integrates into the circular bacterial chromosome at a specific location -this viral DNA within the bacterial chromosome is called a prophage -the combination of a bacterium an a temperate phage is called a lysogen -insertion of a lambda phage into a bacterium alters the genetic characteristics of the bacterium -two genes present in the prophage produce proteins that repress virus replication -the prophage also contains another gene that provides"immunity" to infection by another phage of the same type -this process called lysogenic conversion, prevents the absorption or biosynthesis of phages of the type whose DNA is already carried by the lysogen -the gene responsible for such immunity does not protect the lysogen against infection by a different type of temperate phage or by a virulent phage -lysogenic conversion can be of medical significance because the toxic effects of some bacterial infections are caused by the prophages they contain -without the prophages, bacteria do not cause disease -clostridium botulinum contain prophages that have a gene that codes for the production of a toxin ---the conversion from non toxin production to toxin production is largely responsible for the tissue damage that occurs in diphtheria and botulism -once established as a prophage, the virus can remain dormant for a long time -each time a bacterium divides, the prophage is copied and is part of the bacterial chromosome in the progeny bacteria -thus, this period of bacterial growth with a prophage represents a lysogenic cycle -however, either spontaneously or in response to some outside stimulation, the prophage can become active and initiate a typical lytic cycle -this process, called induction, may be due to a lack of nutrients for bacterial growth or the presence of chemicals toxic to the lysogen -the provirus seems to sense that "living" conditions are deteriorating and that it is time to find a new home -through induction, the provirus removes itself from the bacterial chromosome -the phage DNA then codes for viral proteins to assemble new temperate phages in a manner similar to that used by lytic phages -as a result, new temperate phages mature and are released through cell lysis

Phage growth and the estimation of phage numbers

Viral growth is biosynthesis and maturation Replication curve: (Fig. 10.12) Eclipse period - penetration to biosynthesis Latent period - penetration to point of phage release Plaque assay: estimate number of virus in a sample. (Fig. 10.13) -like bacterial growth, viral growth (biosynthesis and maturation) can be described by a replication curve, which generally is based on observations of phage-infected bacteria in lab cultures -the replication curve of a phage includes an eclipse period, which spans from penetration through biosynthesis -during the eclipse period, mature virions cannot be detected in host cells -the latent period stands from penetration up to the point of phage release --the latent period is longer than--and includes--the eclipse period -the number of viruses per infected host cell rises after the eclipse period and eventually levels off -viriologists and microbiologists use a different approach to estimate phage number -the viral assay method used is called a plaque assay ---to perform aplaque assay, virologists start with a suspension of phages. serial dilutions, like those described for bacteria, are prepared..a sample of each dilution is inoculated onto a plate containing a susceptible bacterial lawn: a layer of bacteria ---ideally, virologists want a dilution that will permit only one phage to infect one bacterial cell ---as a result of infection, new phages are produced from each infected bacterial cell, lysing the cell ---these phages then infect surrounding susceptible cells and lyse them ---after incubation and several rounds of lysis, the bacterial lawn shows clear areas called plaques ---plaques represent areas where viruses have lysed host cells --in other parts of the bacterial lawn, uninfected bacteria multiply rapidly and produce turbid growth layer -each plaque should represent the progeny from one infectious phage -therefore, by counting the number of plaques and multiplying that number by the dilution factor, virologists can estimate the number of phages in a milliliter of suspension -sometimes however, two phages are deposited so close together that they produce a single plaque -and not all phages are infective -thus, counting the number of plaques on a plate will approximate, but may not exactly equal, the number of infectious phages in the suspension -therefore, such counts usually are reported as plaque-forming units rather than as the number of phages

Group disease causing viruses according to route of transmission

• Enteric viruses - ingested (contamination) fecal-oral route o Get infected through eating and ingesting, usually get GI problems o Can stay in the digestive system or leave o They can leave GI system and become systemic infection ♣ Polio: get through contaminated water • Respiratory viruses - inhaled, replicate in respiratory tract o Some can move farther away and become systemic o Measles, mumps • Viral zoonoses - zoonotic viruses, transmitted from animal to human o Rabies • Arboviruses - transmitted by bite of infected arthropod (arthropod borne) ♣ Arthropod transmitted • Flies, mosquitoes • Malaria • Yellow fever • West nile • Sexually transmitted viruses o HIV o Herpes