Microbiology - The Viruses | Biology

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Study GuideMicrobiology–The Viruses1.Viral Cultivation and PhysiologyViruses arenot living cells, so they cannot grow on their own. To multiply, they must infect alivinghost cell. Because of this, special methods are used to grow and study viruses in the laboratory.1.1Cultivation of VirusesCultivating BacteriophagesBacteriophages are viruses that infectbacteria. To study them:1.Bacteria are grown in a suitable nutrient medium.2.Bacteriophages are added to the culture.3.The viruses infect the bacteria, multiply inside them, and increase rapidly in number.1.2Cultivating Animal and Plant VirusesAnimal and plant viruses are usually grown incell cultures.Acell cultureis prepared by growing living cells outside their original plant or animal source. Thesecells are kept alive in a nutrient-rich solution inside aPetri dish. When the cells spread out and forma thin layer, called amonolayer, viruses are added. The viruses then infect the cells and replicate.Viruses can also be grown using:•Fertilized eggs•Living animalsThese methods are especially useful for studying viruses that do not grow well in cell cultures.1.3Large-Scale Virus ProductionFor research purposes, viruses can be grown inlarge tissue culture systems. After viral replication:

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Study Guide•The host cells break down•Viruses are collected (harvested)•Viral particles are concentrated usingprecipitation•Further purification is done usingrepeated centrifugationIn some cases, viruses can behighly purifiedby crystallization and concentration under controlledconditions.Viral MeasurementsViruses aretoo small to be seen with a light microscope, so anelectron microscopeis usuallyrequired.Instead of counting viruses directly, scientists measure them usingvirus infectious units. A virusinfectious unit is thesmallest amount of virus that produces a detectable effectin a susceptiblehost. These units are expressedper volume of fluid.Plaque AssayOne common way to measure virus numbers is theplaque assay.•Viruses are spread over alawn of host cells•As viruses infect and destroy cells,clear areascalledplaquesappear•Each plaque represents viral replication and cell destruction•By counting plaques, scientists can estimate the number of infectious viral unitsAnimal-Based MeasurementAnother method involves infectingliving animalswith different virus dilutions. Scientists determine:•Which dilution causes death•Theend-point dilution, which indicates viral concentration

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Study GuideAntiviral AgentsAntibiotics cannot treat viral infectionsbecause they target bacterial structures and processes thatviruses do not have. For example, penicillin disrupts bacterial cell wall synthesis, butviruses have nocell wall.However, some drugs can blockviral replication:•Acyclovirinterferes with DNA synthesis in herpes viruses•Azidothymidine (AZT)blocks DNA synthesis in HIV•Gancicloviris used against cytomegaloviruses•Amantadineis effective against influenza virusesInterferonsInterferonsare natural antiviral proteins produced by host cells after viral infection.•They donot protect the infected cell•Theyprotect nearby cellsby preventing viral replication•Interferons can be produced usinggenetic engineering techniquesViral VaccinesVaccines provide protection by preparing the immune system to recognize viruses.Inactivated (Killed) VaccinesThese vaccines contain viruses thatcannot replicatebecause they have been chemically orphysically treated.•Example:Salk polio vaccine•Example:Yellow fever vaccineAttenuated (Live) VaccinesAttenuated viruses areweakened formsof the virus.

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Study Guide•They replicate very slowly•They usually donot cause disease•Examples include:oSabin polio vaccineoVaccines formeaslesandrubellaRecombinant VaccinesModern vaccines often containviral proteins produced by genetic engineeringrather than wholeviruses.•Example:Hepatitis B vaccineViral InactivationViruses are made ofnucleic acid,protein, and sometimes alipid envelope. Because of this, theycan be inactivated by substances that damage these components.Viruses can be destroyed by:•Chemical agentssuch as chlorine, iodine, phenol, detergents, and heavy metals•Heat, similar to methods used for other microbes•Ultraviolet (UV) light•Filtration, if the filter pores are small enough to trap virus particlesKey Points to Remember•Viruses must grow insideliving cells•Cell cultures, eggs, and animals are used for cultivation•Viral numbers are measured usinginfectious units•Antibiotics do not work against viruses•Vaccines and antiviral drugs are key tools in viral control

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