Microbiology Virology

Sanitary Engineering

Microbiology

Virology

• Virology is the scientific study of viruses and viral diseases, including their distribution, biochemistry, physiology, molecular biology, ecology, evolution, and clinical aspects.

Virus Definition and History

• The term "virus" comes from the Latin word meaning "slimy liquid" or "poison."

• A virus is a small infectious agent that replicates only inside the living cells of an organism.

• Viruses can infect all types of life forms, including animals, plants, microorganisms, bacteria, and archaea.

• A virus is an acellular organism whose genome consists of nucleic acid, either RNA or DNA.

• Viruses cannot be grown on sterile media and require specific host cells to replicate.

Early Virology Experiments

• Experiments in virology began with Edward Jenner in 1798.

• Jenner inoculated a 13-year-old boy with vaccinia virus (cowpox) and demonstrated immunity to smallpox.

• Jenner observed that individuals exposed to cowpox did not suffer from smallpox, though he did not know the cause.

Smallpox Eradication

• On May 8, 1980, the World Health Organization (WHO) announced the world was free of smallpox.

• The last known case of smallpox was reported in the summer of 1978, when medical photographer Janet Parker died after contracting the disease.

Louis Pasteur's Contributions

• In 1881-1885, Louis Pasteur used animals as models for growing and studying viruses.

• He found that the rabies virus could be cultured in rabbit brains and developed the rabies vaccine.

• On July 6, 1885, Pasteur administered the rabies vaccine to Joseph Meister, a 9-year-old boy attacked by a rabid dog. The boy survived and avoided contracting rabies.

• Rabies is derived from the Latin word "rabere," meaning "rave" or "mad."

Discovery of Tobacco Mosaic Virus (TMV)

• In 1879, Adolf Mayer observed dark and light spots on infected tobacco plant leaves and named it tobacco mosaic disease.

• Mayer demonstrated the infectious nature of the disease by inoculating juice extract from diseased plants into healthy ones.

• In 1890, Dimitri Ivanovsky showed that the sap of leaves infected with tobacco mosaic disease retained its infectious property even after filtration through a Chamberland filter.

• In 1898, Martinus Beijerinck replicated Ivanovsky's experiments and concluded that the filtered solution contained a new form of infectious agent, which he named virus, or contagium vivum fluidum.

Discovery of Animal Viruses

• Friedrich Loeffler and Paul Frosch discovered an animal virus that causes foot-and-mouth disease in cattle in 1898.

• Foot-and-mouth disease (FMD) is a highly contagious viral disease of animals, caused by a picornavirus.

• Walter Reed and his team discovered the yellow fever virus in Cuba in 1901.

• In 1900, yellow fever became the first human disease shown to be caused by a filterable agent by Walter Reed. He found the yellow fever virus present in the blood of patients during the fever phase and that the virus spread via mosquitoes.

• Yellow fever virus is an arbovirus.

Discovery of Poliovirus

• Poliovirus was discovered by Karl Landsteiner and Erwin Popper in 1909.

• Poliovirus was not visible to researchers until the 1950s, when the electron microscope became available.

Virus Characteristics

• Viruses are classified based on their viral genome.

• Viroids are plant pathogens.

• A virion is one complete virus unit, consisting of an outer shell (capsid) made up of subunits called capsomeres, which dictate the shape of the virus.

Examples of Viruses

• HPV (human papillomavirus) can cause cancer.

• Retroviruses can convert their genome from RNA to DNA (or vice versa).

Viral Multiplication

Adsorption

• Adsorption is the recognition process between a virus and the host cell, resulting in the virus's attachment to the external surface of the host cell.

Penetration

• Penetration is the entrance of the virion (either a whole virus or just its nucleic acid) into the host cell.

Replication

• Replication involves the copying and expression of the viral genome at the expense of the host's synthetic equipment, resulting in the production of various components.

Assembly and Maturation

• Assembly and maturation involve the organization of individual viral parts into whole, intact virions.

Release

• Release is the escape from the host cell of active, infectious viral particles.

• Eclipse is the early stage of viral replication, during which no mature virions can be detected within the host cell. It corresponds with the period of penetration, replication, and early assembly.

Two Stages of Viral Multiplication

Lytic Infection

• Lytic infection is an active, infectious stage. An example is the influenza virus.

• A virus cannot reproduce by itself; it must invade a host cell and take over the cell's activities, eventually causing destruction of the cell and killing it.

• During the lytic cycle, the virus enters a cell, makes copies of itself, and causes the cell to burst, releasing more viruses.

• Some viruses, like HIV, do not immediately enter the lytic cycle.

Lysogenic Infection

• Lysogenic infection is a dormant (sleeping) stage, representing the period between an infection and the onset of disease. An example is the Human Immunodeficiency Virus.

• Viral genetic material (DNA) incorporates into a host's DNA and is called a prophage.

• Lysogeny is the potential ability to produce phage.

• The virus replicates its genetic material without destroying the host.

• Every time the host divides, it copies the viral DNA and passes the copies.

• Various factors, such as UV light, can trigger a virus to enter the lytic cycle, causing the cell to burst.

Lytic vs. Lysogenic Cycles

Lytic

• Causes disease right away.

• Example: Influenza.

• Does NOT involve integration of viral genetic material into the host genetic material.

• Results in host destruction: Host cell lyses (bursts).

Lysogenic

• Long period between infection and onset of disease (dormant period).

• Example: HIV.

• Involves integration of viral genetic material into the host's genetic material forming a prophage.

• Eventually enters the Lytic Cycle and host cell lyses.

Bacteriophage Life Cycle

1. Attachment: Phage attaches to host cell.

2. Penetration: Phage penetrates host cell and injects its DNA.

3. Biosynthesis: Phage DNA directs synthesis of viral components by the host cell.

4. Maturation: Viral components are assembled into virions.

5. Release: Host cell lyses and new virions are released.

Lysogeny: The Silent Virus Infection

• Phage DNA integrates within the bacterial chromosome by recombination, becoming a prophage.

• Temperate phages are special DNA phages that undergo adsorption and penetration in the bacterial host but are not replicated or released.

• Instead, the viral DNA enters an inactive prophage state, in which it is inserted into the bacterial chromosome.

• The viral DNA will be retained by the bacterial cell and copied during its normal cell division, so that the cell's progeny will also have the temperate phage DNA.

• Lysogeny is the condition in which the host chromosome carries bacteriophage DNA.

• Because of the intimate association between the genetic material of the virus and host, phages occasionally serve as transporters of bacterial genes from one bacterium to another and consequently can play a profound role in bacterial genetics. This phenomenon is called transduction.

• Transduction is the process by which foreign DNA is introduced into a cell by a virus or viral vector. An example is the viral transfer of DNA from one bacterium to another.

Multiplication Cycles in Animal Viruses

1. Adsorption (Attachment)

• Invasion begins when the virus encounters a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane.

• The virus attaches to a protein or polysaccharide molecule (receptor) on the surface of a host cell.

2. Penetration of Animal Viruses

• Unlike bacteriophages, animal viruses have no mechanism to inject their nucleic acids. Instead, the flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid.

• The entire virus enters the host cell, in some cases because it was phagocytized by the cell.

3. Uncoating

• When enzymes in the vacuole dissolve the envelope and capsid, the virus is said to be uncoated.

• The viral nucleic acid escapes from the capsid.

4. Biosynthesis

• Viral genes are expressed, resulting in the production of pieces or parts of viruses (i.e., viral DNA and viral proteins).

5. Assembly

• The viral pieces or parts are assembled to create complete virions.

6. Release

• The complete virions escape from the host cell by lysis or budding (or exocytosis).

Comparison of Bacteriophage and Animal Virus Multiplication

• Note: For enveloped viruses, budding and exocytosis are interchangeable. They mean the release of a virus from an animal cell by enclosing it in a portion of membrane derived from the cell.

Miscellaneous Viral Information

• Cytomegalovirus causes distinct diseases.

• Chickenpox (child) can convert to shingles (adult, elderly).

Prions

Transmissible mink encephalopathy
Ungulate spongiform encephalopathy

• Prions are infectious proteins.

• The normal form of the protein is called $PrP^{c}$, while the infectious form is called $PrP^{sc}$. The c refers to cellular PrP, while the sc refers to scrapie, the prototypic prion disease, occurring in sheep.

• Scrapie is a neurodegenerative disease of sheep and goats and is also caused by prions. Experimental scrapie has been extensively studied in hamsters and mice.

• The scrapie prion protein ($PrP^{sc}$) is the only component of the infectious scrapie prion identified.

Families of DNA Viruses

Families of RNA Viruses

Classification of Virus Related Diseases

• Measles, Rubella, Chicken Pox, Dengue, Enteroviruses

• Respiratory Tract

• Mucous Membranes

• Ivory Glands

• Intestinal Track

• Transmitted

• Poliomyelitis, Rabies, Herpes Simplex, Meningoencephalitis of Mumps, Measles, Slow Viral Infections

• Influenza, RSV, Adenovirus, Coronavirus, Rhinovirus

• HSV Types 1 and 2, Molluscum Contagiosum, Enteroviruses (coxsackie)

• Adenovirus conjunctivitis, herpes keratoconjunctivitis, Epidemic hemorrhagic conjunctivitis (enterovirus)

• Hepatitis, Enteroviruses, Herpesviruses (Epstein-Barr, CMV)

• Mumps, CMV

• Rotavirus, Norwalk, Enteric Adenovirus

• HSV, Hep B, papillomavirus, molluscum contagiosum, HIV