Viruses, Viroids, and Prions Unit 03 Microbiology

History of Viruses  By the 1800s, many bacteria, fungi, and protozoa were identified as infectious agents.  Most of these organisms could be easily seen with a light microscope in a lab.  In the 1890s, D.M. Iwanowsky and Martinus Beijerinck found that mosaic disease of the tobacco plant was caused by an agent too small to be seen with a light microscope.  They could be grown only in media that contained living cells.

Tobacco Mosaic Virus

History of Viruses  Beijerinck called the agent a filterable virus.  About 10 years later, F.W. Twort and F. d’Herelle discovered a filterable virus that destroyed bacteria.  With time, filterable was dropped and virus remained.  Virus means poison.

Viruses  Viruses can be viewed as genetic information- either DNA or RNA- conained within a protective coat.  They are inert particles, incapable of metabolism, replication, or motility.  When a viral genome finds its way into a host cell, however, it can hijack that cell’s replication machinery causing it to produce more virus.

Viruses Have two main groups- those that infect prokaryotic cells and those that infect eukaryotic cells Bacteriophages: A virus that infects bacteria: often shorted to phage. Phage means to eat.

Viruses  The fact that viruses are obligate intercellular parasites makes them very difficult to study. They can be seen only with an electron microscope.

Listeria monocytogenes  In 2006, the FDA approved a preparation of bacteriophages that infect the food-borne pathogen L. monocytogenes for use as an antimicrobial agent on ready to eat meat and poultry products.  L. monocytogenes multiplies at refrigeration temperatures and causes a potentially lethal disease.

Characteristics of Viruses  They are noted for their small size.  They are app. 100 to 1000 times smaller than the cells they infect.  Smallest 10 nm in diameter, and contain very little nucleic acid.  The largest is 800 nm. One was so large it was identified as a bacterium.

Viral Architecture  At a minimum, a viral particle consists of nucleic acid surrounded by a protein coat.  The coat is a capsid.  The capsid together with the nucleic acid it encloses is a nucleocapsid.

Viral Architecture  Some viruses have an envelope, a lipid bilayer outside of the capsid.  Those that have it are enveloped viruses.  Those that don ’ t are naked viruses. Nearly all phages are naked.  Enveloped viruses are more susceptible to disinfectants.

Viral Architecture  Viruses contain only a single type of nucleic acid- either RNA or DNA- but never both.  They are classified as either RNA or DNA viruses.  The type of genome has important implications with the virus replication.

Naked and Enveloped Viruses  The image on the left is a naked virus West Nile  The image on the right is of influenza an enveloped virus

Shapes of Viruses  Viruses generally have three shapes  Icosahedral  Helical  Complex

Shapes of Viruses

Viral Taxonomy  Although viruses are not living organisms they are classified to provide easy identification and study.  The International Committee on Viral Taxonomy keeps an online database.  They describe over 6,000 viruses belonging to 2,288 species, 348 genera, 87 families, and 6 orders. Key characteristics used are the genome structure and the hosts they infect. Shape and symptoms are also considered.

Virus Transmission  Viruses are often grouped according to how they are transmitted.  Zoonotic viruses cause zoonoses- transmission from an animal to a human  Arboviruses- transmitted by arthropods- mosquitoes and ticks- West Nile, Yellow Fever, Dengue Fever  Enteric viruses- fecal-oral route  Respiratory viruses- respiratory or salivary route

Infections  Lytic phages exit the host at the end of the infection cycle by lysing the cell.  These viral infections result in the formation of new virus particles and are called productive infections.

Infection Cycle- Viral Replication  Has five stages  Attachment  Genome Entry- uncoating releases the nucleic acid from the protein coat  Sythesis of proteins and genome- genes are transcribed and translated  Assembly (maturation)  Release-  enveloped virions by budding  Naked virions are released when the host cell dies

Categories of Animal Virus Infections  Acute- sudden onset of symptoms of relatively short duration  Persistent- can continue for the life of the host, with or without symptoms  Chronic infections  Latent infections- remains silent, but can reactivate

Viroids and Prions  Viroids are plant pathogens that consist of small circular, single stranded RNA molecules  Prions are composed solely of protein, and a cause a number of transmissible spongiform encephalopathies.