BIOLOGY(Micro Organisms)

8/12/2019 BIOLOGY(Micro Organisms)

1/6

Biology

S.I.S

Mishaal Ali alshidi

10/B


2/6

These are microscopic forms of life which are ubiquitous in the environment and

on the human body. They were first detected in 1675 by a Dutch draper, Anthony

van Leeuwenhoek, who noticed tiny animalcules in droplets of rainwater under

his microscope. He went on to discover that they were present in dental plaque,

faeces, and many other substances.

Most microorganisms are harmless or even beneficial to man. Only a minority

cause disease in healthy humans (although many more may do so in patients with

damaged immune systems). Thus it may not be so surprising that it was another

112 years after van Leeuwenhoek's discovery before it was shown that these

minute creatures were also under certain circumstances the agents of disease:

this was first demonstrated by Agostino Bassi in 1835 for a bacterial infection

of silkworms. The German Robert Koch was the first to prove that a bacterium

could cause a human disease, namely anthrax, in 1876. Naturally this discovery

aroused huge scientific and public interest, although of course there were those

in both the lay and the scientific communities who were opposed to the new

theory of infection. This is the subject of Ibsen's play An Enemy of the People,

in which the town doctor learns that the presence of bacteria in the water

supply might transmit diseases such as typhoid and cholera. He campaigns to have

the water cleaned, but the whole idea of tiny invisible animals is met with

ridicule by the community and his career is ruined.

The study of microorganisms is known as microbiology and not surprisingly muchof the study is directed at those organisms which do cause human disease. It is

now realized that not only are microorganisms responsible for what are

conventionally considered infectious diseases but they may also contribute

to such diverse illnesses as peptic ulcers, angina, and cervical cancer. There

is no doubt that in the future microorganisms will be found to be involved in

many more non-infectious diseases. However, they are also essential to human

life. Every square inch of our body surface is colonized by many thousands of

organisms which help to protect the body from invasion by other potentially

harmful organisms. If this normal flora is damaged, for instance by a course

of antibiotics, it leaves the way open for the harmful organisms to get a

foothold and establish themselves instead. Microorganisms are also employed in a

wide variety of home and industrial processes. For example, yeasts are essential

for making bread rise and for the process of alcohol fermentation; genetically

engineered bacteria are used to make insulin and in the production of

genetically modified foods.

Microorganisms are classified into bacteria, viruses, fungi, and parasites.

Prions, which are thought to be infective protein particles rather than live

organisms, are included also in the field of microbiology. These groups are


3/6

totally unrelated to one another, the only common factor being that they are all

microscopic.

acteriaThese are single-celled organisms, usually either rod-shaped or roughly

spherical. They are classified according to their reaction to Gram's stain:those that go blue with this stain are termed Gram positive, those staining red

are Gram negative. This reflects a fundamental difference in the structure of

their cell walls. They are responsible for diseases ranging from typhoid,

plague, cholera, meningococcal meningitis, tuberculosis, tetanus, gonorrhoea,

and syphilis, to the more mundane urinary tract infections, boils, and acne.

They are killed by antiseptics and by boiling, although they may produce toxins

which are not destroyed. Many were originally sensitive to antibiotics such as

penicillins, but overuse of these drugs has resulted in many multi-resistant

bacteria. The most notorious is multi-drug resistant tuberculosis. In terms of

numbers, however, the greatest problem in the UK has been methicillin resistant

Staphylococcus aureus, or MRSA. Although often it merely colonizes the skin and

causes no harm, it can cause a wide range of infections and is difficult to

treat. Great attempts have been made to limit its spread in hospitals, for

instance by placing patients carrying the organism in isolation rooms.

VirusesThese are even tinier than bacteria and cannot be seen through a light

microscope: electron microscopy is required. Their name comes from the Latin for

toxin. Their existence was first suspected when it was found that some

infectious agents could pass through a fine filter, unlike bacteria, which were

too large. They are very simple life forms, often consisting simply of a DNA or

RNA core and a protein coat. They cannot reproduce except inside another living

cell, which they must hijack in order to do so. They are susceptible to heat and

to some antiseptics.

The most high-profile virus of recent years has been the human immunodeficiency

virus HIV which causes AIDS. Other viral illnesses include influenza, thecommon cold, Lassa fever, and ebola. There have been few anti-viral drugs

available, and the main line of defence has been vaccination, which has

eradicated smallpox and may do the same for polio in the near future.

Unfortunately, many viruses, including those causing HIV and the common cold,

have a very high rate of genetic change so that they can evade the immune

system, which makes the development of vaccines extremely difficult. Recently

there have been great advances in the treatment of HIV with the development of

triple therapy, or highly active anti-retroviral therapy (HAART). This

consists of the use of three or sometimes more drugs together to combat the

infection, and has been highly successful in slowing the progression of the


4/6

disease to full-blown AIDS. The use of the combination of drugs prevents the

virus from becoming drug-resistant so rapidly.

FungiLike bacteria, microscopic fungi are everywhere. They may be yeasts or moulds.

Yeasts have been used for centuries by peoples worldwide to ferment sugar toalcohol; the drug penicillin was found in a mould. They have a very resilient

cell wall, which makes them difficult to treat. The commonest fungal infections

are vaginal thrush, which often occurs after a course of antibiotics has

destroyed the normal vaginal bacteria, and nail and skin infections such as

ringworm.

In the UK more serious fungal infections are generally only found in seriously

immune-deficient patients, such as leukaemia or AIDS patients. However, in otherparts of the world there are fungi which can cause severe disease in healthy

individuals.

ParasitesThese are organisms which live in or on the body of another known as the

host. The host mayprovide a source of nutrients or a safe haven in which to

reproduce. They range in size from single cells, like the malaria parasite, to

tapeworms which may reach thirty feet in length and hence are not microscopic at

all! Parasites are mainly a problem of developing countries, although some, suchas thread worm, are also common in the UK.

PrionsThese are the simplest infectious particles known, now widely accepted as the

cause of a group of degenerative diseases of the brain that includes Creutzfeld-

Jacob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle

and scrapie in sheep. Since the BSE epidemic in Britain in the 1980s there has

been concern that a new variant of CJD may be transmitted to humans who eat the

meat (specifically nervous tissue) of infected cattle. Prions are thought to

consist merely of a protein with no DNA or RNA so in this sense are not

live. They are nonetheless infectious because when the abnormal prion

protein comes into contact with a similar but normal protein it causes a change

in the way the normal protein molecule is folded up. This turns the normal

protein into another abnormal protein, which alters other nearby proteins in

turn, and so on until disease is evident. No treatment is available for prion

diseases; also prions can withstand heat, desiccaion and conventional

antiseptics, making safe disposal of infected material very difficult.


5/6

In the 1960s and 70s the rapid expansion in the number of antibiotics being

developed led man to believe that he had won the battle against infection with

microorganisms, but it is now evident that this is far from being the case. The

emergence of new viruses and prion diseases, together with ever more antibiotic-

resistant organisms, makes the study of microorganisms as urgent as ever.

Angharad Puw Davies


6/6

Documents

BIOLOGY(Micro Organisms)