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Chapter 13-Viruses of Bacteria 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? 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 Sizes of viruses Polyhedral virus • Capsid coat made of capsomeres • Nucleic acid inside Parts of a virus Relationship of virus with host cell Shapes of a virus Bacterial viruses • • Known as bacteriophages or phages Two different life cycles 1. 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 T4 phage replication Lambda Phage Lambda integrates into the chromosome Properties conferred by prophage Some phage are filamentous Replication of filamentous phage M13 is ssDNA…how does it replicate the ssDNA? Chapter 14: Animal Viruses How do animal viruses differ from bacterial viruses? • • • • Attachment Replication of nucleic acid Uncoating Exit by budding Entry by membrane fusion Entry by endocytosis Replication of nucleic acid Release by budding Persistant infections • Virus is continually present in the body, released by budding • Three categories – Latent infections – Chronic infections – Slow infections Latent Infections • Persistant infection with symptomless period followed by reactivation of virus and symptoms • Herpesviridae – Herpes simplex virus -1 – Herpes simplex virus -2 Latent Viral infections DNA Animal Viruses • Herpesviridae (dsDNA, enveloped virus) -simplex 1(cold sores) -simplex 2 (genital herpes) -chicken pox, shingles -epstein barr Herpes Simplex virus-1 HSV-1 reactivation Herpes simplex-1 • HHV-1 causes fever blisters, HHV-2 genital herpes • Symptoms: fluid filled skin lesions • Treatment: Acyclovir 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 Epstein Barr • Causes infectious mononucleosis • Acquire by saliva, incubation period is 4-7 weeks • Identify by -lobed lymphocytes -heterophile antibodies -fluorescent antibody tests 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 Chronic Viral infections Hepadnaviridae • dsDNA, enveloped • Hepatitis B -passes through intermediate stage (RNA) -three particles in blood Dane filamentous sphericle -exposure through blood/body fluids 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 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 Retrovirus 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 Viruses associated with cancers Viruses can alter their genome by reassortment 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 Influenza virus Ways to study viruses • Since viruses grow in living cells….need a live cell to culture them – Cell culture/tissue culture – Embryonated chicken eggs Cell Culture Quantitation of viruses 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 Prions • Contain no nucleic acid • Abnormal protein promotes conformational change to normal protein • Results in damage to neurons…transmissable spongiform encephalopahthies Brain with spongiform encephalopathy Infections caused by prions Mechanism of prion replication 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 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 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 Pathology of rabies 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