Viruses and PrionsViruses and Prions

Objectives:Objectives:

1. Discuss the key differences between prokaryotic and eukaryotic cellular components

2. Describe the basic structure of bacteria.

3. Discuss the taxonomy of bacteria.

4. Explain how do organisms get their names.

VIRUSESVIRUSES* One of the smallest forms of * One of the smallest forms of

microorganisms and infects other forms microorganisms and infects other forms of live:of live:

Animal --- Plants --- BacteriaAnimal --- Plants --- Bacteria

* They can cause oral and orofacial * They can cause oral and orofacial disease - - produces oral signs of disease - - produces oral signs of

systemic infection and transmitted to systemic infection and transmitted to patients and dental staffpatients and dental staff

Main Features:Main Features:* Small size (10-100nm).* Small size (10-100nm).* Genome: either DNA or RNA but * Genome: either DNA or RNA but never both- ss (single strand) or ds never both- ss (single strand) or ds (double strand), linear or circular.(double strand), linear or circular.* Metabolic activity: Obligate * Metabolic activity: Obligate intracellular parasites.intracellular parasites.

StructureStructure* Viruses consist of a nucleic acid core containing

the viral genome, surrounded by a protein shell called a capsid.

* The capsid consist of repeating units of one or more protein molecules called capsomeres.

* Nucleic acid + Capsid + Capsomere = Nucleocapsid

* Nucleocapsid may be naked or enveloped within a lipoprotein sheath. * In enveloped viruses the protein units called

capsomeres.

Viral Nucleic acidViral Nucleic acid• DNA or RNA never both.• The RNA, in turn, may be ss or ds and the

genome may consist of one or several molecules of nucleic acid.

• If the genomes composed of a single molecules , this may linear or have a circular configuration.

Viral ProteinViral Protein• Made up of 2 or 3 different polypeptide

chains (some only 1 polypeptide). • Virion surface proteins may have a special

affinity for receptors on the surface of host cells.

• Viral lipid and Carbohydrate: The lipids and carbohydrates of viruses are found in the envelopes and mostly derived from the host cells.

• Virus symmetry: arranging of nucleocapsids:

1- Icosahedral symmetry: ex. Herpesviruses.

2- Helical symmerty: Most mammalian RNA viruses.

3- Complex symmetry: ex. Poxviruses.

TaxonomyTaxonomyThe attributes used in classification:

Symmetry

Presence or absence of an envelope

Nucleic acid composition (DNA or RNA).

The number of nucleic acid strands

polarityde

DNA virusesDNA virusesPapovaviruses:

Small, icosahedral DNA viruses with capacity to produce tumours in vivo and to transform cultured cell lines.

Papilomavirus:

This genus contain human serotypes which cause benign skin tumours or warts and both oral and skin papilomas (e.g. hand and plantar warts)- it is now known that the papilomaviruses may be involved in genital and oral cancers.

DNA virusesDNA virusesAdenoviruses:Icosahedral DNA viruses, commonly associated

with respiratory and eye infections in humans.Syndromes associated with adenoviruses include: * acute febrile pharyngitis (primarily in infants and children). * acute adult respiratory disease, ranging from phryngitis to pneumonia. * ocular infections.

DNA virusesDNA virusesHerpesviruses:Predominant viral cause of oral infections in

humans, often the infections are recurrent, and latent.

Structure of HerpesvirusesStructure of Herpesviruses• Enveloped, linear, double-stranded DNA molecule.• Has over 100 members • Spread among vertebrates and invertebrates• Unstable at room temperature • Rapidly inactivated by lipid solvents ex: alcohol.

During Reproduction:

Typical and highly intracellular inclusions are found in cells that have undergo active virus replication.

|* Polykaryotes or giant cells readily appear in tissue lesions.

Different herpesviruses cause a variety of infections diseases, some localized and some generalized, often with vesicular rash.

Herpesviruses establish latent infection, which can be readily reactivated by immunosuppression.

The herpesviruses that commonly infect humans can be distinguished by their antigenic and genomic profiles.

They cannot be differentiated by electron microscopy owing to identical capsid morphology.

Type of herpesviruses that can cause infection in Type of herpesviruses that can cause infection in oral and perioral tissues:oral and perioral tissues:

Herpes simplex virus

Herpes zoster virus

Epstein-Bar virus

Human cytomegalovirus

Herpesviruses 6 and 8

DNA VirusesDNA Viruses

Poxviruses:• Largest viruses to infect humans or

animals.• Cause: Molluscum contagiosum in human• Human occasionally acquire infection by

animal poxviruses, e.g. cowpox.

DNA VirusesDNA VirusesParvoviruses: • Icosahedral viruses• ss DNA• The B19 virus is responsible for a febrile illness.

particularly in children, manifesting as a maculopapular rash.

DNA VirusesDNA VirusesHepdnaviruses:• small, spherical DNA viruses causing

hepatitis.• Chronic liver infections and possibly liver

cancer.• They are of particular interest in dentistry

because of their mode of transmission via blood and saliva.

RNA VirusesRNA VirusesPicornaviruses:• Smallest family of RNA viruses• Include the genus: Enteroviruses • Respiratory infections and pharyngitis

caused by human enteroviruses (coxsackie A).

• Coxsackie B cause more serious disease: carditis, which may be lethal in the newborn.

RNA VirusesRNA Viruses Orthomyxoviruses:• Have tubular nucleocapsid and

lipoprotein envelope.• Influenza A viruses of birds, mammals

and human are in this category.

RNA VirusesRNA Viruses Paramyxoviruses:• Large, pleomorphic envelopd RNA

viruses.• This family contains four common and

important pathogens:• Measles, mumps, parainfluenza and

respiratory syncytial viruses.

RNA VirusesRNA Viruses Retroviruses:• Large, spherical enveloped • RNA tumour viruses characterized by• Unique genome, a unique enzyme and a

unique mode of replication.

Retroviruses:Retroviruses:Viral genome:

The viral genome RNA is first transcribed into DNA by a virus-specific enzyme, reverse transcriptase.

This DNA can serve as a templet for mRNA synthesis.

Medically important viruses in this familyMedically important viruses in this family

HIV TYPES 1 and 2.HTLV-I (Human T-cell leukaemia virus)

cause tumours.ATLL (adult T-cell leukaemia-lymphomaHTLV-II associated with hairy cell leukaemia.

Viral ReplicationViral Replication

1- Adsorption or attachment of the virus particle to the specific receptors of the host cell plasma membrane.

2- Penetration or uptake: the process by which the virus or its genome enters the host cell cytoplasm by three mechanisms:

Endocytosis, fusion and translocation.

3- Uncoating and eclipse: The eclipse phase= begins with uncoating of the lipid membrane and protein capsid surrounding the nucleic acid viral core.

4- Transcription: synthesis of viral mRNA then transcription and synthesis of viral proteins.

5- Synthesis of viral components.

6- Assembly

7- Release

Prion and prion-induced diseasePrion and prion-induced diseaseProteinaceus infectious particlesMajor features:

1- They are neither viruses nor viroids.

2- do not have either DNA or RNA

3- have ability to self replicate

4- very long incubation period

5- highly resistant to heat, chemical agents and irradiation

6- Iatrogenic transmission of prion disease by neurological instruments has been reported.

pathogenesispathogenesisPrions appear to replicate incessantly,

first in lymphoid tissue, then brain cells

The vacuoles in brain cells give rise to the sponge-like appearance of the brain.