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