ACCESS Human Biology.
Disease Causing Micro
organisms.
Vera Harding.
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Aims. The student will be able to:
• Define the meaning of infection.
• Define communicable disease.
• Differentiate between bacteria, viruses,
fungi, protoctista and parasites.
• Understand how infection is spread.
• Discussion.
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Infection:
• ‘The process by which organisms capable of
causing disease gain entry to the body and
establish colonies’ (Youngson 1992).
• ‘The successful invasion, establishment and
growth of micro-organisms in the tissues of the
host. May be acute or chronic’ (Roper 1989).
• ‘Invasion of the body by harmful organisms
(pathogens) such as bacteria, fungi or viruses’
(Roper 2003).
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Infectious (Communicable)
Disease:
• ‘Any disease that can be transmitted from
one person to another’ (Martin 2003).
• ‘Diseases caused by organisms that can
spread directly from person to person’
(Younson 1992).
• ‘Disease caused by a specific, pathogenic
organism and capable of being transmitted
to another individual by direct or indirect
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contact’ (Roper 1989).
Reason for health care workers
to be concerned? YES!!
• Infection poses a serious and continuing
threat to the public with 1 in 10 (10%) NHS
hospital patients affected each year.
• This problem costs an estimated £1billion
per annum.
• Hand hygiene is one of the most important
indicators in reducing the risk of infection.
• (Pearce 2004).
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Bacteria.
• A group of simple organisms.
• Many bacteria are parasites.
• Bacteria can survive for a long time as a
spore that protects them in a latent
(inactive) state.
• Antibiotics cause damage to the spore wall.
• Only bacteria is denatured (killed) by
antibiotics, therefore they should not be
used to treat fungi or viruses.
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Bacteria - not always bad!
• Commensal: Bacterium thrives - host not
affected, eg:
– Staphylococcus aureus lives naturally on skin,
only causing problems if it gains entry into the
body through broken skin, when it causes
abscesses to form.
– Methicillin Resistant Staphylococcus Aureus
lives in the noses and throats of general
public. Dangerous only if entry is gained
through broken or punctured skin.
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Bacteria - not always bad!
• Symbiotic: Bacteria is beneficial to both
bacterium and host, eg:
– Intestinal flora, (flora - colonisation of microorganisms) produces Vitamins B12 and K.
• Decomposition: Bacteria are involved in
important processes of, eg:
– The ‘Nitrogen Cycle’.
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Fungi.
• Simple organisms that include yeasts,
moulds and mushrooms.
• Some fungi can cause disease in humans.
• Some yeasts are a good source of Vit. B.
• Many antibiotics are obtained from moulds.
• Fungi can cause rapid deterioration of food,
which can lead to disease.
• Human skin (intact) prevents invasion.
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Viruses.
• Minute particles that can only replicate
inside living cells.
• Viruses have specific targets, eg the cold
virus targets the mucous membrane that
lines the human nose.
• Viruses are specific to one host, eg,
myxomatosis is fatal to rabbits, (it causes
the mucous membranes to swell), but not
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harmful to humans.
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Protoctista.
•
•
•
•
•
Single and multiple eukaryotic, (animal), cells.
Most are microscopic.
Protozoan and worms.
Amoeba ingest small particles of food.
Amoebic dysentery via contaminated water. Eats
intestinal flora.
• Malaria via mosquito. Absorbs nutrients through cell
membranes.
• Trichomoas vaginalis. Causes trichomoniasis usually in
vagina, sometimes in urethra or prostate. STI.
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Parasites.
• Parasites include any living thing that lives
in or off another organism.
• The organism that the parasite survives on
is called the ‘host’.
• Human parasites include;
– Bacteria.
– Fungi.
– Viruses.
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– Worms.
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To cause disease pathogens:
•
•
•
•
•
Must be in sufficient numbers.
Must be virulent (powerful enough).
Must have a susceptible host.
Must enter body by appropriate means.
Must have appropriate environment:
– Food.
– Moisture.
– Warmth.
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The pathogens enter the body by:
• Inhalation: airborne spread (eg colds,
measles, sore throats, tuberculosis).
• Ingestion: through mouth via
contaminated hands, infected food (eg
salmonella).
• Inoculation: through the skin (eg MRSA).
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Infection is spread by:
• Airborne route: coughing and sneezing etc.
• Direct contact: hands, sex, blood etc .
• Indirect contact: equipment, bedding, food,
(fomites) etc.
• Vectors: flies, mosquitoes, rats, etc.
• Waterborne.
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References.
• Martin, E. (2003) Minidictionary for Nurses.
RCN: London. P135, 301, 320.
• Pearce, L. (2004) Beating the Bugs. Nursing
standard. V.18.No.31.pp17-18.
• Roper, N. (1989) Medical Dictionary.
Churchill Livingstone. London. P133, 143.
• Thompson, D. (1996)
• Youngson, R. (1992) Dictionary of Medicine.
Collins: Great Britain. P296, 313.
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