chapter 13-viruses of bacteria. general characteristics of all viruses contain a single type of...
TRANSCRIPT
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Chapter 13-Viruses of Bacteria
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General Characteristics of all viruses
• Contain a single type of nucleic acid
• Contain a protein coat
• Obligate intracellular parasites
• Are viruses the only obligate intracellular parasites?
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History began with the Tobacco Mosaic Virus (TMV)
• 1886 Aldolf Mayer showed that a virus was transmissable between plants
• 1892 Iwanowski tried to isolate it by filtering with porcelain filter
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Sizes of viruses
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Polyhedral virus
• Capsid coat made of capsomeres
• Nucleic acid inside
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Parts of a virus
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Relationship of virus with host cell
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Shapes of a virus
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Bacterial viruses
• Known as bacteriophages or phages
• Two different life cycles1. Lytic cycle (virulent)-results in lysis of the
cell
2. Lysogenic cycle (temperate)-may result in lysis of the cell or the virus becomes a permanent part of the chromosome by integrating
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T4 phage replication
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Lambda Phage
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Lambda integrates into the chromosome
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Properties conferred by prophage
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Some phage are filamentous
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Replication of filamentous phage
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M13 is ssDNA…how does it replicate the ssDNA?
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Chapter 14: Animal Viruses
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How do animal viruses differ from bacterial viruses?
• Attachment
• Replication of nucleic acid
• Uncoating
• Exit by budding
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Entry by membrane fusion
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Entry by endocytosis
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Replication of nucleic acid
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Release by budding
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Persistant infections
• Virus is continually present in the body, released by budding
• Three categories– Latent infections– Chronic infections– Slow infections
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Latent Infections
• Persistant infection with symptomless period followed by reactivation of virus and symptoms
• Herpesviridae – Herpes simplex virus -1– Herpes simplex virus -2
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Latent Viral infections
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DNA Animal Viruses
• Herpesviridae (dsDNA, enveloped virus)
-simplex 1(cold sores)
-simplex 2 (genital herpes)
-chicken pox, shingles
-epstein barr
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Herpes Simplex virus-1
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HSV-1 reactivation
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Herpes simplex-1
• HHV-1 causes fever blisters, HHV-2 genital herpes
• Symptoms: fluid filled skin lesions
• Treatment: Acyclovir
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Varicella (chickenpox) and Herpes Zoster (Shingles)
• HHV-3 causes chicken pox and latent activation known as shingles
• Acquired by respiratory route, 2 weeks later see vesicles on skin
• Vaccine established in 1995 for chickenpox
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Epstein Barr
• Causes infectious mononucleosis
• Acquire by saliva, incubation period is 4-7 weeks
• Identify by
-lobed lymphocytes
-heterophile antibodies
-fluorescent antibody tests
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Chronic infections
• Infectious virus present at all times
• Disease may be present or absent
• Best example is Hepatitis Type B virus– Carriers produce virus detected in blood, saliva,
and semen– Unique replication of dsDNA
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Chronic Viral infections
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Hepadnaviridae
• dsDNA, enveloped• Hepatitis B
-passes through intermediate stage (RNA)-three particles in blood Danefilamentoussphericle-exposure through blood/body fluids
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Hepatitis B
• Incubation period is ~12 weeks
• 10% of cases become chronic, mortality rate is less than 1%
• About 40% of the chronic cases die of liver cirrhosis
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Slow Infections
• Infectious agent increases in amount over a long time during which there are no symptoms
• Examples are HIV found in the Retroviridae family
• Retroviruses use reverse transcriptase to replicate ssRNA
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Retrovirus
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Viruses and tumors
• dsDNA viruses are most common to cause viral-induced tumors
• Cancer is result of integration of viral genes into the host chromosome
• Transforming genes are called oncogenes
• Examples: papillomavirus, herpesvirus
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Viruses associated with cancers
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Viruses can alter their genome by reassortment
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Orthomyxoviridae-multiple strands of (-)RNA
• Influenza virus– Consists of 8 segments of RNA– Envelope has H spikes (hemagglutinin) and N
spikes (neuraminidase)– Incubation is 1-3 days– Symptoms include: chills, fever, headache,
muscle aches, may lead to cold-like symptoms
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Influenza virus
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Ways to study viruses
• Since viruses grow in living cells….need a live cell to culture them– Cell culture/tissue culture– Embryonated chicken eggs
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Cell Culture
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Quantitation of viruses
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Proteinaceous infectious particles: PRIONS
• 1982 Stanley Prusiner proposed that there were infectious proteins
• Caused the disease “scrapie” in sheep
• Caused the “mad-cow”disease in 1987
• Human forms suggest a genetic component
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Prions
• Contain no nucleic acid
• Abnormal protein promotes conformational change to normal protein
• Results in damage to neurons…transmissable spongiform encephalopahthies
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Brain with spongiform encephalopathy
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Infections caused by prions
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Mechanism of prion replication
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Picornaviridae (+) ssRNA
• Hepatitis A
-obtain through fecal-oral route, enters GI tract and multiplies
-incubation period is ~4 weeks
-symptoms include: anorexia, malaise, nausea, diarrhea, abdominal discomfort, fever, and chills lasting 2-21 days
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Rhabdoviridae (-)ssRNA, enveloped
• Rabies virus
-enters the skin and multiplies in skeletal muscle and connective tissue
-virus travels along nerves to the CNS causing encephalitis
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Flaviviridae (+) ssRNA, enveloped
• Hepatitis C virus– Obtain from blood/body fluids– Incubation period averages 6 weeks– Hard to screen blood for the virus– 85% of all cases become chronic
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Pathology of rabies
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Retroviridae-multiple strands of (-)RNA
• HIV
-infects Helper T cells
-requires the enzyme reverse transcriptase
-integrates as a provirus
-is released by budding, or lyses the cell