Module 6: Microbial Diversity - Viruses

Virus

extremely small, aceulluar infectious agents that require a living host cell for reproduction, as well as rely on host for energy, synthesis, metabolic intermediates, and protein synthesis.

Virion

extracellular form of a virus, that is essentially inert

Structure of the Virion

• nucleic acid genome

• capsid that surrounds the nucleic acid

Capsid

the protein coat that surrounds the nucleic acid of a virus

Capsomeres

subunits of a capsid that are arranged in precise and highly repetitive pattern

Nucleocapsid

the capsid + nucleic acid of a virus

Naked Viruses

viruses consisting of only the nucleocapsid

Envelope

found on some viruses; an additional later composed of a phospholipid bilayer

Glycoproteins

on the envelope virus surface that are important for host cell attachment

Enveloped Viruses

viruses that have envelope surrounding the nucleocapsid

Icosahedral symmetry

spherical viruses

Role of Lysozyme (bacteriophages)

enables nucleic acid entry into bacterial cellls

Role of Neuraminidase

facilitate the release of visions from host cells

Role of Reverse Transcriptase (retroviruses)

are required for replication of viral genome

Viral genomes are typically _____ than cellular genomes

smaller

Characteristics of Virus Classification

1. type of genome

2. host or cell type infected

3. size and shape of virion

4. presence or absence of envelope

Positive (+) Strand Virus

contains a ss genome taht had identical sequence as the mRNA

Negative (-) Strand Virus

contains a ss genome that has the complementary sequence to the mRNA

Plaques

zones of cell lysis; formed when lytic phages infect a lawn of bacterial cells on a solid plate.

You can estimate the number of infectious viral particles by counting the number of plaques formed because…

each plaque results from an original infection of 1 cell by 1 virus

Titer

the number of infectious visions per volume of liquid (Plaque forming units / ml)

Steps in the replication cycle of a virus

1. attachement of the virion

2. Penetration of viral nucleic acid
   a. Prokaryotic → only nucleic acid enters cell
   b. Eukaryotic → entire virion is taken up by cell

3. Synthesis of viral nucleic acid and proteins by host cell

4. Assembly of capsids and packaging of viral genomes into new visions

5. release of new visions from host cell

Attachment

Maturation Phase of Lytic Replicatoin

assembly of new viral particles from replicated nucleic acid and synthesized capsids.

Later Period

The eclipse and early maturation of lytic replication of virus. The number of visions within the cell increases, but none are outside the cell (appears as a flat line on the growth curve)

Burst Size

the number of visions released per cell

Mature Virions are released at the end of the ________ phase

maturation

Virulent (lytic) phages

always lyse and kill host cell after infection

Temperate Phages

can choose between lytic and lysogenic pathways

Lysogenic Pathway

phage genome becomes part of host genome (prophage). This is site-specific (always in the same place)

Lysogen

a cell containing prophage, and is immune to further infection by the same virus

Induction

In response to cell stress, the prophage is induced to enter the lytic cycle

Generalized Transduction

transfer of bacterial genes from one host to another

Lysogenic Conversion

prophages can confer new or altered phenotypes to host cells → provides immunity to infection by related phages

Phage Therapy

using lytic phages to kill specific bacterial species or strains without affecting other bacteria or eukaryotic cells

Challenges in Developing Phage Theraputics

• need to find the right phage (time sink)

• temperate phages don’t kill immediately and can harbour toxins

• many bacteria have defence systems to prevent phage infection

Bacterial Defence to Prevent Attachment of Phages

• loss, modification of phage receptors

• host is inaccessible to infection

Bacterial Defence to Restrict Modification (R-M) Systems

• restriction endonuclease enzyme recognize foreign DNA and cleave the DNA at specific sequences

• The host DNA is protected by chemical modification (methylation) at recognition sites

Phage Exlusion

Bacterial Defence against bacteriophages;

• cellular enzymes recognize and modify foreign DNA. Modifications prevents the replication of viral DNA

Abortive Infection

Bacterial Defence against Bacteriophages;

• infection induced cell death or dormancy in host ell

• prevents phage from replicating and spreading to other cells

Production of Chemical compounds with antiphage activity

a bacterial defence mechanisms against bacteriophages that involves DNA intercalating agents produced by Streptomyces can inhibit phage replicatoin

CRISPR-Cas System

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