Microbiology:
Week Two

Chapter 4: Microbial Diversity

Microbes can be divided into those that are truly cellular (bacteria, archaea, algae, protozoa, and fungi) and those that are acellular (viruses, viroids, and prions).
Cellular microbes (microorganisms) can be divided into those that are procaryotic (bacteria and archaea) and those that are eucaryotic (algae, protozoa, and fungi).
Viruses, viroids and prions are often referred to as acellular microbes or infectious particles.

Viruses

Complete virus particles are called virions.
Most viruses are from 10 to 300 nm in diameter.
Viruses infect humans, animals, plants, fungi, protozoa, algae and bacterial cells.
Some viruses, called oncogenic viruses or oncoviruses, cause specific types of cancer.
A typical virion consists of a genome of either DNA or RNA, surrounded by a capsid (protein coat) which is composed of protein units called capsomeres.
Some viruses (enveloped viruses) have an outer envelope composed of lipids and polysaccharides.

Viruses have 5 properties that distinguish them from living cells:

They possess either DNA or RNA – living cells possess both.
They are unable to replicate on their own.
Unlike cells, they do not divide by binary fission, mitosis, or meiosis.
They lack the genes and enzymes necessary for energy production.
They depend on the ribosomes, enzymes, and metabolites of the host cell for protein and nucleic acid production.

Viruses are classified by:

Viruses that infect bacteria are known as bacteriophages or simply phages.
There are two categories of bacteriophages: virulent bacteriophages and temperate bacteriophages.
Virulent bacteriophages always cause what is known as the lytic cycle, which ends with the destruction of the bacterial cell.

Viral infections in which the virus is able to hide from a host’s immune system by entering cells and remaining dormant.
Herpes viral infections are examples.
Once acquired, herpes virus infections (e.g., those that cause cold sores, genital herpes, and chickenpox/shingles) never completely go away; for example, chickenpox may be followed, years later, by shingles - both the result of the same virus.

Antibiotics are not effective against viral infections.
Antiviral agents are drugs that are used to treat viral infections.
These agents interfere with virus-specific enzymes and virus production by disrupting critical phases in viral multiplication or inhibiting synthesis of viral DNA, RNA, or proteins.

Viroids

Viroids are short, naked fragments of single-stranded RNA, which can interfere with the metabolism of plant cells.
Viroids are transmitted between plants in the same manner as viruses.
Examples of plant diseases caused by viroids: potato spindle tuber and citrus exocortis.

Prions

Prions are small infectious proteins that cause fatal neurologic diseases in animals; examples: Scrapie, Bovine Spongiform Encephalopathy (“Mad Cow Disease”) and Creutzfeldt-Jacob disease.
Of all pathogens, prions are the most resistant to disinfectants.
The mechanism by which prions cause disease remains a mystery.