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Acellular Organisms

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Viruses and Other Acellular Microorganisms Kristina C. Erasmo, M.D.
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Viruses and Other Acellular


Kristina C. Erasmo, M.D.

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Very small, acellular infectious agentsVirions – complete virus particles10 to 300 nm in diameterCan infect virtually all organisms • Humans, animals,

plants, fungi, protozoa, algae, and bacterial cells

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Specific Properties of Viruses

Possess either DNA or RNA.Unable to replicate (multiply) on their own. Do not divide by binary fission, mitosis, or meiosis. Lack genes and enzymes necessary for energy production.Depend on ribosomes, enzymes, and metabolites of the host cell for protein and nucleic acid production.

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Viral Structure

DNA or RNAProtein coat (capsid) composed of many small protein units (capsomeres)

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Classification of Viruses

Type of genetic material (DNA or RNA)Shape of capsid (icosahedral, helical)Shape of capsomeresSize of capsidPresence or absence of an envelopeType of host it infectsType of disease it producesTarget cellImmunologic or antigenic properties

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DNA vs. RNA Viruses

Herpes virusHepadnavirus AdenovirusPapovavirusParvovirusPoxvirus

ArenavirusCalicivirusCoronavirusFilovirus OrthomyxovirusParamyxovirusPicornavirusReovirus RetrovirusRhabdovirusTogavirus

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Icosahedral vs. Helical Viruses

Polyhedral capsids (20 sides or facets)DNA or RNA viruses

Mostly spherical in shapeRhabdovirus – bullet-shapedRNA viruses only

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Naked vs. Enveloped Viruses

Outer envelope composed of lipids and polysaccharidesAcquired as viruses escape from the nucleus or cytoplasm of the host cell by budding (derived from the host cell’s nuclear or cell membrane)

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Viruses that infect bacteria

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2 categories:• Virulent bacteriophages

• Lytic cycle – ends with lysis of bacterial cell• Temperate bacteriophages

• Do not immediately initiate lytic cycle• DNA remains integrated in bacterial cell

chromosome after many generations

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Multiplication of Bacteriophages (Lytic Cycle)


1. Attachment (adsorption)

Phage attaches to protein or polysaccharide molecule (receptor) on surface of bacterial cell

2. Penetration Phage injects DNA into bacterial cell; capsid remains on the outer surface of the cell

3. Biosynthesis Phage genes are expressed → production of phage DNA and proteins

4. Assembly Phage parts (DNA, protein, etc.) are assembled to create complete phages

5. Release Complete phages escape from bacterial cell by lysis of the bacterial cell

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Multiplication of Animal Viruses


1. Attachment (adsorption)

Virus attached to a protein/polysaccharide molecule (receptor) on the surface of host cell

2. Penetration Entire virus enters host cell (sometimes, it is phagocytized by cell)

3. Uncoating Viral nucleic acid escapes from capsid

4. Biosynthesis Viral genes are expressed → production of phage DNA and proteins

5. Assembly Viral parts are assembled to create complete virions

6. Release Complete virions escape from host cell by lysis or budding

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Inclusion Bodies

Remnants or collections of virusesUsed as diagnostic toll to identify certain viral diseasesCytoplasmic inclusions• e.g. Guarnieri bodies (smallpox),Negri bodies

(rabies)Intranuclear inclusions• Herpes, poliomyelities

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Host Cell Outcome

ResolutionDeathTransformation: oncogenesLatent infectionsChronic infection

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Latent Viral Infections

Virus remains in host in a “sleeping state”(surviving but not producing clinically overt infection)Viral is “re-activated’ by various factors (e.g. fever, stress, excessive sunlight)e.g. cold sores (herpes virus), shingles (herpes zoster)

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Herpes Zoster

Causes shinglesInitial: chickenpox (varicella zoster) Virus becomes latent in the nerve cell bodiesSymptoms: flu-like symptoms followed by rash, pain, itching (dermatomal distribution)

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Oncogenic Viruses


Epstein-Barr virus Nasopharyngeal cancerBurkitt’s lymphomaB-cell lymphoma

Human herpesvirus 8 Kaposi sarcoma

Hepatitis B and C viruses Hepatocellular carcinoma

Human papilloma virus (HPV)

Cervical cancer

HTLV-1 Adult T-cell leukemia

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Viruses and Viral Diseases


Poxvirus Variola Smallpox

Herpesvirus Herpes simplex IHerpes simplex IIHerpes zosterVaricella

Cold soresGenital herpesShinglesChickenpox

Myxovirus Myxovirus parotidisParamyxovirusRhabdovirusOrthomyxoviruses types A and B

MumpsMeasles (rubeola)RabiesInfluenza

Retrovirus HTLV virusHIV


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Short, naked fragments of single-stranded RNANo protein coatInterfere with the metabolism of plant cells

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Small infectious proteinsCause fatal neurologic diseases in humans and animals

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Scrapie Sheep, goats Infected animals scrape themselves against object to relieve itching (pruritus)

Bovine spongiform encephalopathy (“mad cow disease”)

Cattle May be due to ingestion of cattle feed that contained group-up parts of prion-infected sheep

Kuru Humans Caused by eating human brains infected with prions

Creutzfeldt-Jakob disease

Humans May be transmitted to human beings who eat the brain or spinal cord of infected carcassesLoss of coordination, dementia (impaired memory, judgment, and intellect)

Fatal familial insomnia

Humans Autosomal dominentDifficulty of sleeping followed by insomnia and dementia