Overview of Microbiology B

History of Viruses

What is poliomyelitis?



Poliomyelitis is a disease caused by infection with the poliovirus. The virus spreads by direct person-to-person contact, by contact with infected mucus or phlegm from the nose or mouth, or by contact with infected faeces.

The virus enters through the mouth and nose, multiplies in the throat and intestinal tract, and then is absorbed and spread through the blood and lymph system. The time from being infected with the virus to developing symptoms of disease (incubation) ranges from 5 - 35 days (average 7 - 14 days).

Risks include:

• Lack of immunization against polio and then exposure to polio
• Travel to an area that has experienced a polio outbreak
  

In areas where there is an outbreak, those most likely to get the disease include children, pregnant women, and the elderly. The disease is more common in the summer and fall.

Between 1840 and the 1950s, polio was a worldwide epidemic. Since the development of polio vaccines, the incidence of the disease has been greatly reduced. Polio has been wiped out in a number of countries. There have been very few cases of polio in the Western hemisphere since the late 1970s. Children in the United States are now routinely vaccinated against the disease.

Outbreaks still occur in the developed world, usually in groups of people who have not been vaccinated. Polio often occurs after someone travels to a region where the disease is common. Thanks to a massive global campaign over the past 20 years, polio exists only in a few countries in Africa and Asia.

What is smallpox?

Smallpox is a contagious, disfiguring and often deadly disease caused by the variola virus. Few other illnesses have had such a profound effect on human health and history. In the 20th century alone, an estimated 300 million people died of smallpox.

The initial signs and symptoms of smallpox, which appear about two weeks after infection, resemble those of the flu — fever, fatigue and headache. Later, severe pus-filled blisters appear on the skin that eventually leave deep, pitted scars. Once symptoms develop, there's no effective treatment for smallpox and no known cure.

Naturally occurring smallpox was finally eradicated worldwide by 1980 — the result of an unprecedented immunization campaign. But the virus didn't disappear entirely. Stocks of smallpox virus, set aside for research purposes, are officially stored in two high-security labs — one in the United States and one in Siberia. This has lead to concerns that smallpox someday may be used as a biological warfare agent.

What is rabies?

Rabies infections in people are rare in the United States. However, worldwide about 50,000 people die from rabies each year, mostly in developing countries where programs for vaccinating dogs against rabies don't exist. But the good news is that problems can be prevented if the exposed person receives treatment before symptoms of the infection develop.

Rabies is a virus that in the U.S. is usually transmitted by a bite from a wild infected animal, such as a bat, raccoon, skunk, or fox. If a bite from a rabid animal goes untreated and an infection develops, it is almost always fatal.

If you suspect that your child has been bitten by a rabid animal, go to the emergency department immediately. Any animal bites — even those that don't involve rabies — can lead to infections and other medical problems. As a precaution, call your doctor any time your child has been bitten.

Who was Edward Jenner?

Edward Jenner was an English country doctor who pioneered vaccination. Jenner's discovery in 1796 that inoculation with cowpox gave immunity to smallpox, was an immense medical breakthrough and has saved countless lives.

Edward Jenner was born on May 17 1749 in the small village of Berkeley in Gloucestershire. From an early age Jenner was a keen observer of nature and after nine years as a surgeon's apprentice he went to St George's Hospital, London to study anatomy and surgery under the prominent surgeon John Hunter. After completing his studies, he returned to Berkeley to set up a medical practice where he stayed until his death in 1823.

What he did?
In 1796 he carried out his now famous experiment on eight-year-old James Phipps. Jenner inserted pus taken from a cowpox pustule and inserted it into an incision on the boy's arm. He was testing his theory, drawn from the folklore of the countryside, that milkmaids who suffered the mild disease of cowpox never contracted smallpox, one of the greatest killers of the period, particularly amongst children. Jenner subsequently proved that having been inoculated with cowpox Phipps was immune to smallpox. He submitted a paper to the Royal Society in 1797 describing his experiment but was told that his ideas were too revolutionary and that he needed more proof. Undaunted, Jenner experimented on several other children, including his own 11-month-old son. In 1798 the results were finally published and Jenner coined the word vaccine from the Latin 'vaccia' for cow.

Who was Louis Pastuer?

Louis Pasteur was a world renowned French chemist and biologist. He was born on December 27 1822 in the town of Dole in Eastern France. Pasteur's parents were peasants; his father was a tanner by trade. He spent the early days of his life in the small town of Arbois where he attended school and where it seems that Pasteur did not do very well, preferring instead to go fishing. His headmaster, however, spotted potential in Pasteur and encouraged him to go to Paris to study. So, aged fifteen Pasteur set off for Paris hoping to study for his entrance exams. Unfortunately, the young Pasteur was so homesick that his father had to travel to Paris to bring him home. He then continued to study locally at Besancon, until he decided to try again in Paris. This time he succeeded and went on to study at the Ecole Normale Superieure. Curiously, although the young Pasteur worked hard during his student days he was not considered to be exceptional in any way at chemistry.

In 1847 Pasteur was awarded his doctorate and then took up a post as assistant to one of his teachers. He spent several years teaching and carrying out research at Dijon and Strasbourg and in 1854 moved to the University of Lille where he became professor of chemistry. Here he continued the work on fermentation he had already started at Strasbourg. By 1857 Pasteur had become world famous and took up a post at the Ecole Normale Superieure in Paris. In 1863 he became dean of the new science faculty at Lille University. While there, he started evening classes for workers. In 1867 a laboratory was established for his discovery of the rabies vaccine, using public funds. It became known as the Pasteur Institute and was headed by Pasteur until his death in 1888.

What he did?
Pasteur founded the science of microbiology and proved that most infectious diseases are caused by micro-organisms. This became known as the "germ theory" of disease. He was the inventor of the process of pasteurisation and also developed vaccines for several diseases including rabies. The discovery of the vaccine for rabies led to the founding of the Pasteur Institute in Paris in 1888.

Who is Robert Koch?

Robert Koch was born in 1843. Koch worked on anthrax and tuberculosis (TB) and he further developed the work of Louis Pasteur. Koch’s fame, alongside that of Alexander Fleming, Edward Jenner, Joseph Lister and Pasteur himself, is firmly cemented in medical history.

What did he discover?
Pasteur was convinced that microbes caused diseases in humans but his work on cholera had failed. He was never able to directly link one microbe with a disease. Koch succeeded in doing this.

The first disease that Koch investigated was anthrax. This was a disease that could seriously affect herds of farm animals and farmers were rightly in fear of it. Other scientists had also been working on anthrax. In 1868, a French scientist called Davaine had proved that a healthy animal that did not have anthrax could get the disease if it was injected with blood containing anthrax. Koch developed this work further and for three years he spent all his spare time finding out what he could about the disease, including its life cycle.

Koch found out that the anthrax microbe produced spores that lived for a long time after an animal had died. He also proved that these spores could then develop into the anthrax germ and could infect other animals.

After this, Koch moved onto germs that specifically affected humans. In 1878, he identified the germ that caused blood poisoning and septicaemia. He also developed new techniques for conducting experiments that influenced the way many other scientists carried out their experiments. He knew that infected blood contained the septicaemia germ but he could not see these germs under a microscope, and therefore, other scientists were unlikely to believe what he thought to be true without the evidence.

Koch discovered that methyl violet dye showed up the septicaemia germ under a microscope by staining it. He also photographed the germs so that people outside of his laboratory could see them.

Koch also devised a method of proving which germ caused an infection. His work was rewarded in 1880 when he was appointed to a post at the Imperial Health Office in Berlin. Here, Koch perfected the technique of growing pure cultures of germs using a mix of potatoes and gelatine. This was a solid enough substance to allow for the germs to be studied better. Koch gathered round him a team of researchers in Berlin in 1881 and began to work on one of the worst diseases of the nineteenth century – tuberculosis (TB).

The TB germ was much smaller than the anthrax germ so the search for it was difficult. Using a more specialised version of his dye technique, Koch and his team searched for the TB germ. In May 1882, Koch announced that his team had found the germ. His announcement caused great excitement. It also generated what became known as ‘microbe hunters’ – a new generation of young scientists who were inspired by the work of both Koch and Pasteur. One of those who was inspired by Koch was Paul Ehrlich.

Koch’s Postulates

• The organism must be regularly associated with the disease and its characteristic lesions.
• The organism must be isolated from the diseased hosts and grown in culture.
• The disease must be reproduced when a pure culture of the organism is introduced into a healthy susceptible host.
• The same organism must again be re-isolated from the experimentally infected host.

Discovery of Viruses

On 12th February 1892, Dmitri Iwanowski, a Russian botanist, presented a paper to the St. Petersburg Academy of Science which showed that extracts from diseased tobacco plants could transmit disease to other plants after passage through ceramic filters fine enough to retain the smallest known bacteria. This is generally recognised as the beginning of Virology. Unfortunately, Iwanowski did not recognize that he had discovered a new type of infectious agent.

Six years later, in 1898, a Dutch scientist, Martinus Beijerinick confirmed & extended Iwanowski's results on tobacco mosaic virus & was the first to develop the modern idea of the virus, which he referred to as contagium vivum fluidum ('soluble living germ'). He discovered that the infectious agent which passed through the filter could reproduce but would not grow on Petri dishes used to cultivate bacteria. He further realized that these agents required the presence of a host cell to reproduce. He named the agent responsible for tobacco mosaic disease a virus, after the Latin term for poison. He thought that this agent must be much smaller and simpler than bacteria. The subsequent crystallization and electron microscope images obtained by the American scientist, Wendell Stanley, in 1935 confirmed this, and the agent was named tobacco mosaic virus.

Tobacco mosaic virus causes stunted plant growth and mottled, discoloured plant leaves, especially in tobacco and other members of the tomato family (Solanceae).

In 1898, Freidrich Loeffler & Paul Frosch showed that a similar agent was responsible for foot-and-mouth disease in cattle. Thus these new agents caused disease in animals as well as plants. In spite of these findings, there was resistance to the idea that these mysterious agents might have anything to do with human diseases.

References
http://kidshealth.org/parent/infections/bacterial_viral/rabies.html
http://www.mayoclinic.com/health/smallpox/DS00424
http://www.zephyrus.co.uk/louispasteur.html
http://www.zephyrus.co.uk/edwardjenner.html
http://www.bioedonline.org/slides/slide01.cfm?q=dimitri+ivanowsky&dpg=1
http://www.historylearningsite.co.uk/robert_koch.htm
http://www.umm.edu/ency/article/001402.htm
www.raw-milk-facts.com/tuberculosis.html
library.thinkquest.org/26644/us/pasteur.htm
http://www.bbc.co.uk/
http://www.microbiologybytes.com/introduction/introduction.html
http://sg.wrs.yahoo.com/_ylt=A0S0zu11c2NJNV4ABlUu4gt./SIG=12ic5snmm/EXP=1231340789/**http%3A/www.cbe21.com/subject/biology/photo.php%3Fphoto_id=1343
www.inra.fr/hyp3/pathogene/3tomovi.htm
http://www.medizin.uni-greifswald.de/mikrobio/forschung/virologie.html
http://www.nlm.nih.gov/
www.news.bbc.co.uk
http://www.who.int/
www.encarta.msn.com
www.viewzone.com/smallpox.html

Nature of Viruses

Viruses are:

• Sub-microscopic, obligate intracellular parasites.
• Viruses are not made up of cells. Compared to animal and plant cells, viruses are structurally simple. Viruses are even simpler than bacteria cells. The basic of structure of all viruses is similar. Viruses have a core of nucleic acid surrounded by a protein coat. At their core, all viruses contain either DNA or RNA.
• They are so simple that they are technically not even considered "alive." There are six characteristics of all living things:
  - Adaptation to the environment
  - Cellular makeup
  - Metabolic processes that obtain and use energy
  - Movement response to the environment
  - Growth and development
  - Reproduction
• A virus is not able to metabolize, grow, or reproduce on its own, but must take over a host cell that provides these functions; therefore a virus is not considered "living." The structure of a virus is extremely simple and is not sufficient for an independent life.
• Viruses are structurally simpler than regular cells. Outside of the host cells, viruses are inactive. However, inside living cells, viruses show some of the characteristics of living things.
  Virus particles are produced from the assembly of pre-formed components, whereas other agents 'grow' from an increase in the integrated sum of their components & reproduce by division.
• Virus particles (virions) themselves do not 'grow' or undergo division.Viruses lack the genetic information which encodes apparatus necessary for the generation of metabolic energy or for protein synthesis (ribosomes).

General Properties of Viruses

• Obligate parasites
• Sub-cellular size
  - Viruses are among the smallest infectious agents, and most of them can only be seen by electron microscopy and not light microscope. Their sizes range from 20 to 300 nm. They are so small that it would take 30,000 to 750,000 of them, side by side, to stretch to one cm.
• Structurally simple
  1. Nucleic acid - contains 3-400 genes
  Deoxyribonucleic Acid (DNA) -unique features
  - Single and/or double stranded
  - Glycosylated and/or methylated
  - Gaps present in double stranded molecule
  - Circular or linear
  - Bound protein molecules
  - Unique purine and/or pyrimidine bases present
  - Ribonucleotides present
  Ribonucleic Acid (RNA) - Unique features
  - Single or double stranded
  - Segmented or unsegmented
  - Bound protein molecules
  - Unique purine and/or pyrimidine bases present
  - Folding pattern
  2. Capsid - The capsid accounts for most of the virion mass. It is the protein coat of the virus. It is a complex and highly organized entity which gives form to the virus. Subunits called protomeres aggregate to form capsomeres which in turn aggregate to form the capsid.
  3. Envelope - this is an amorphous structure composed of lipid, protein and carbohydrate which lies to the outside of the capsid. It contains a mosaic of antigens from the host and the virus. A naked virus is one without an envelope.
  4. Spikes- These are glycoprotein projections which have enzymatic and/or adsorption and/or hemagglutinating activity. They arise from the envelope and are highly antigenic.

• Wide variety of host

Structure of viruses

Each virus is made up of two elementary components. The first is a strand of genetic material, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Unlike living cells, viruses will have either DNA or RNA, but not both. The genetic material is a blueprint for determining the structure and behavior of a cell. In a virus, a protein coat called a "capsid" surrounds the nucleic acid. This coat serves to protect the nucleic acid and aid in its transmission between host cells. The capsid is made of many small protein particles called "capsomeres," and can be formed in three general shapes – helical, icosahedral (a 20-sided figure with equilateral triangles on each side), and complex. Some of the more advanced viruses have a third structure that surrounds the capsid. This is called the "envelope" and is composed of a bilipid layer, like the membrane on a cell, and glycoproteins, which are protein and carbohydrate compounds. The envelope serves to disguise the virus to look like a 'real' cell, protecting it from appearing as a foreign substance to the immune system of the host. The structure of a virus is closely related to its mode of reproduction.

Reproduction
A virus's sole purpose is to reproduce, but it needs a host cell to do so. Once a suitable host cell has been located, the virus attaches to the surface of the cell or is ingested into the cell by a process called "phagocytosis." It then releases its genetic material into the cell, and essentially shuts down normal cell processes. The cell stops producing the proteins that it usually makes and uses the new blueprint provided by the virus to begin making viral proteins. The virus uses the cell's energy and materials to produce the nucleic acid and capsomeres to make numerous copies of the original virus. Once these 'clones' are assembled, the virus causes the host cell to rupture, releasing the viruses to infect neighbouring cells.

Reference
http://www.microbiologybytes.com/introduction/introduction.html
http://www.kcom.edu/faculty/chamberlain/Website/Lects/PROPERT.HTM
http://www.miamisci.org/youth/unity/Unity1/Lubens/pages/characteristic.html
http://en.wikipedia.org/wiki/Viruses
http://www.roshanpakistan.com/
http://www.pinkmonkey.com/studyguides/subjects/biology-edited/chap14/b1400001.asp