microbiology unit 4: viruses
TRANSCRIPT
Test your memory
A. True or False
1. Cocci are rod-shaped bacteria
2. A cell uses pili and flagella to move
3. Spores are formed to survive extreme conditions
4. During binary fission, the chromosome is duplicated
5. Bacterial growth refers to an increase in size
B. Multiple choice
1. Acid-fast staining is used to stain which bacteria
a) Gram positive; b) Gram negative; c) Gram variable
2. Bacteria which can only grow in the absence of oxygen
a) Obligate aerobes; b) Obligate anaerobes; c) Aerotolerant anaerobes
3. Which statement about Rickettsias is true
a) Rickettsias are no real bacteria; b) Rickettsias can only live within a host cell; c) Rickettsias produce no proteins
Variety of Microbes
• Categories
– Acellar: Viroids, Virus, Prion
– Prokaryotes (cellular without nucleus): Bacteria, Archaea, Cyanobacteria
– Eukaryotes (cellular with nucleus): Algae, Fungi, Protozoa
Microbes
Acellular
Viroids
Prions
Virusus
Cellular
Prokaryotes
Archaea
Bacteria
Cyanobacteria
Eukaryotes
Algae
Fungi
Protozoa
Today
• Structure
• Classification
– Nucleic acids
– Shape
– Size
– Disease
• Bacteria viruses
– Replication
• Animal viruses
– Replication
• Latent virus infections
• Different types of viruses
Acellular microbes
• No-one is safe from viral infections
– Humans, animals, plants, fungi, algae, bacteria
• Different from cellular microbes (bacteria, fungi)
– No cells: no living organisms
• Correctly referred to as acellular microbes or infectious particles
– Possess either DNA or RNA
– Dependent on host cell (invade and take over)
• Unable to replicate on their own (no binary fission)
• Lack genes/enzymes required for energy production
• Lack ribosomes/enzymes/metabolites for protein and nucleic acid production
Burton’s Microbiology: Chapter 3,4
Structure
• Typical virion (virus particle)
– Genome
• DNA or RNA (nucleid acid)
– Surrounded by capsid (protein coat)
• Many small protein units: capsomers
– Optional
• Outer envelope
• Bacteria viruses: tail, tail fibers, sheath
– Absent
• Ribosomes or sites for energy production: invade and take over functioning cell
Burton’s Microbiology: Chapter 3,4
Nucleocapsid
Classification of viruses
• Viruses can be classified by following characteristics
– Genetic material (DNA or RNA)
– Shape of capsid
– Size of capsid or number of capsomers
– Presence/Absence envelope
– Type of host (or cell) that it infects
– Type of disease it produces
Burton’s Microbiology: Chapter 4
Classification by shape
Burton’s Microbiology: Chapter 4
• Viral shape and symmetry
– Polyhedral
• 20 sides: icosahedrons
– Helical
– Bullet shaped
– Spherical
– Complex
Classification by size
• Typical size
– Much smaller than bacteria
– Most virus particles (virions): 10-300 nm (1 billion nanometers = 1 meter)
– Exception: Ebola virus is 1 μm (1 million micrometers = 1 meter)
• Size of virus related to number of capsomers
• Small viruses only visible with electron microscope (since 1940)
Burton’s Microbiology: Chapter 4
Classification by disease
• Viral diseases
– No organism is free of viral infections
– Classify by type of disease it causes
Burton’s Microbiology: Chapter 4
Bacteria viruses
• Viruses that infect bacteria
– Bacteriophages (or phages)
• Icosahedron (nearly spherical)
• Filamentous (long tubes)
• Complex (see figure)
– Obligate intracellular pathogens (need host cell to replicate)
• All viruses
• Rickettsias & Chlamydias
Burton’s Microbiology: Chapter 4
Attachment and penetration
• Attachment
– Attach to cell with specific protein receptor (surface of cell recognized by surface of phage)
– Therefore most phages are species specific
• Penetration
– Core penetrates bacterial cell wall
– DNA is injected from head to cell
– Phage DNA taking over host cell
• “DNA will give orders to bacteria”
Burton’s Microbiology: Chapter 4
Replication of phages (2)
Burton’s Microbiology: Chapter 4 Burton’s Microbiology: Chapter 4
• Via lytic cycle (within hour)
Replication of animal viruses (1)
• Attachment
– Viruses can only attach/invade cells that bear receptor (protein) they recognize
• Virus is species specific (even cell specific)
• Penetration (entire virus)
Burton’s Microbiology: Chapter 4
Penetration
Burton’s Microbiology: Chapter 4
Penetration by nonenveloped virus
Penetration by enveloped virus
Herpes virus
Replication of animal viruses (2)
• Uncoating, biosynthesis and assembly
• Release (lyis or budding)
Burton’s Microbiology: Chapter 4
Release via budding
Burton’s Microbiology: Chapter 4
Enveloped virus: escape via “budding”
Herpes virus
Inclusion bodies
• Aggregates or collections of viruses (in infected cells): inclusion bodies
– Used as diagnostic tool to identify viral diseases
• Rabies: cytoplasmic inclusion bodies in nerve cells: Negri bodies
Burton’s Microbiology: Chapter 4
Latent virus infections
• Usually, virus is limited by defense system
• Herpes virus infection (cold sores/fever blisters)
1. Infected person always harbors the virus in cells
2. Fever, stress, sunlight can trigger viral genes to take over cells
3. Cells are destroyed, cold sore develops
Burton’s Microbiology: Chapter 4
Treat viral infections
• Antibiotics
– Inhibit activities of cellular pathogens (bacteria)
– Virus are no cells
– Why treating viral infections with antibiotics?
• Prevent secondary bacterial infections that follow virus infection
• Antiviral agents (recently)
– Chemicals that interfere with:
• Virus-specific enzymes
• Virus production
Burton’s Microbiology: Chapter 4
Oncogenic viruses
• Oncogenic- or oncoviruses
– Viruses that cause cancer
– In Rodents, frogs, cats, chickens and humans
• e.g. HTLV-1, retrovirus closely related to HIV adult T-cell leukemia (ATL)
Burton’s Microbiology: Chapter 4
Human Immunodeficiency virus
• HIV
– Cause of AIDS
– Enveloped
– Single-stranded RNA
– Reverse transcriptase
• Enzyme: DNA from RNA
– Family: retroviruses
• Attack
– Attach and invade cells bearing CD4 receptors
– CD4+ cells
• Helper T cells
• Macrophages
– Destroys important cells for immune system!
Burton’s Microbiology: Chapter 4
Immune system
Mimivirus
• Extremely large virus: Mimivirus
– Diameter of 750 nm
• Mimics bacteria (in size): mimivirus
• Observed with standard light microscope
– Double-stranded DNA
• Genome 10x larger than other large viruses (and larger than smallest bacteria)
• Genes code for a lot of functions!
Burton’s Microbiology: Chapter 4
Plant viruses
• Viruses that cause diseases of:
– Trees: citrus, cocoa
– Vegetables: cauliflower, potatoes
– Fruits: tomatoes
– Grains: rice
• Transmitted via:
– Insects, mites, worms
– Infected seeds
– Contaminated tools
• Economic losses
– >$70 billion per year worldwide
Burton’s Microbiology: Chapter 4
Viroids & Prions
• Viroids
– Infectious RNA molecules
• Plant diseases (interfere with metabolism)
– Transmitted like viruses
Burton’s Microbiology: Chapter 4
• Prions
– Infectious protein molecules
– Animal/human diseases
• Insomnia, mad cow disease
– Very resistant to disinfectants
– Exact mechanism unknown:
• Change shape of proteins? Become non-functional?