Viruses and Other Acellular

Microorganisms

Kristina C. Erasmo, M.D.

Microorganisms

Cellular

ProkaryotesArchaeaBacteria

Cyanobacteria

EukaryotesAlgae

ProtozoaFungi

Acellular

VirusesPrions

Viroids

VIRUSES

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

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.

Viral Structure

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

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

DNA vs. RNA Viruses

Herpes virusHepadnavirus AdenovirusPapovavirusParvovirusPoxvirus

ArenavirusCalicivirusCoronavirusFilovirus OrthomyxovirusParamyxovirusPicornavirusReovirus RetrovirusRhabdovirusTogavirus

Icosahedral vs. Helical Viruses

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

Mostly spherical in shapeRhabdovirus – bullet-shapedRNA viruses only

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)

Bacteriophages

Viruses that infect bacteria

Bacteriophages

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

Multiplication of Bacteriophages (Lytic Cycle)

STEPS WHAT HAPPENS?

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

Multiplication of Animal Viruses

STEPS WHAT HAPPENS?

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

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

Host Cell Outcome

ResolutionDeathTransformation: oncogenesLatent infectionsChronic infection

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)

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)

Oncogenic Viruses

VIRUS CANCER

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

Viruses and Viral Diseases

VIRUS TYPE VIRUS DISEASE

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

LeukemiaAIDS

VIROIDS

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

PRIONS

Small infectious proteinsCause fatal neurologic diseases in humans and animals

PRIONS

DISEASE AFFECTED CHARACTERISTICS

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