An Introduction to the Viruses Microbiology: A Systems Approach An Introduction to the Viruses Chapter 6 Part 3 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display. Chapter 6, pages 139 to 167

Damage to the Host Cell and Persistent Infections Cytopathic effects- virus-induced damage to the cell that alters its microscopic appearance Inclusion bodies- compacted masses of viruses or damaged cell organelles Synctia- fusion of cells Latent infections Tranformation – oncogenic, cancer causing (oncoviruses)

Cytopathic Effects

Results of Viral Infections- Examples Death of host cell = Most viruses Proliferation of cells = Papillomavirus Fusion of host cells = Respiratory syncytial virus Transformation of cells = Adenovirus No obvious changes = Rubella

Viruses that Infect Bacteria Bacteriophage Most contain dsDNA Often make the bacteria they infect more pathogenic for humans Some are named “T-even phages”, such as T2 and T4 Attach, but do not enter host bacterial cell

T-even Bacteriophages Icosahedral capsid head containing DNA Central tube surrounded by a sheath Collar Base plate Tail pins and fibers Inject nucleic acid into bacterial cell

Bacteriophages Entry of the nucleic acid causes the cessation of host cell DNA replication and protein synthesis The host cell machinery is then used for viral replication and synthesis of viral proteins As the host cell produces new parts, they spontaneously assemble Eventually host cell lyses

Lysogeny: The Silent Virus Infection Temperate phages- special DNA phages that undergo adsorption and penetration but are not replicated or released immediately Instead the viral DNA enters an inactive prophage stage Lysogeny: the cell’s progeny will also have the temperate phage DNA Lysogenic conversion: when a bacterium acquires a new trait from its temperate phage

Lytic and Lysogenic Cycles

Bacteriophage vs. Animal Virus Multiplication

An Introduction to the Viruses Concept Check Microbiology: A Systems Approach Corynebacterium diphtheriae becomes more pathogenic when a bacteriophage called beta is incorporated into its genome. What is this process called? A. Transposition B. Transformation C. Lysogenic conversion D. Lytic cycle Correct answer is C; the bacterium has been converted to a more toxic form by the incorporated viral genes UNDERSTANDING (Bloom’s taxonomy) – must use the provided vocabulary terms in a new and specific setting Chapter 6, pages 139 to 167

An Introduction to the Viruses Concept Check Microbiology: A Systems Approach How are the infectious cycles of viruses different in bacterial and eukaryotic host cells? The virus genome does not enter a eukaryote. The virus genome does not enter a bacterium. The capsid does not enter a eukaryote. The capsid does not enter a bacterium. Correct answer is D; The genome is injected into the bacterial cell by the capsid; it remains outside KNOWLEDGE (Bloom’s taxonomy): Simple recall of material from the previous slides. Chapter 6, pages 139 to 167

6.6 Techniques in Cultivating and Identifying Animal Viruses Primary purposes of viral cultivation To isolate and identify viruses in clinical specimens To prepare viruses for vaccines To do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells Using Live Animal Inoculation Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits Occasionally invertebrates or nonhuman primates are used Animal is exposed to the virus by injection

Viral Culture Using Bird Embryos Enclosed in an egg- nearly perfect conditions for viral propagation Chicken, duck, and turkey are most common Egg is injected through the shell using sterile techniques

Using Cell (Tissue) Culture Techniques Most viruses are propagated in some sort of cell culture The cultures must be developed and maintained Animal cell cultures are grown in sterile chambers with special media Cultured cells grow in the form of a monolayer Primary or continuous

Detecting Viruses in the Lab Plaque assays (used to visualize the presence of virus) Bacteria cultures used for bacteriophage Tissue culture used for eukaryotes

6.7 Medical Importance of Viruses Infectious diseases Chronic affliction association Type I diabetes, multiple sclerosis Cancer

6.8 Noncellular Infectious Agents Viroids Plant pathogens Naked RNA, Single-stranded, Circles, Very small (100 to 300 nucleotides) Prions Animal pathogens, “Slow viruses” Spongiform encephalopathy Mad Cow Disease, Creutzfeld-Jakob Disease Defectively folded host protein