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MEDICAL MICROBIOLOGY
By Anumein M.
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Aims for Learning Medical Microbiology
• What is medical microbiology?
• Why is it relevant?
• Some important concepts.
• Basic classification of organisms.
• Classifying bacteria.
• Benefits and harms
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• Medical Microbiology: - it involves the study of microbial pathogens,
the disease caused by them, and the body’s defenses against these
diseases.
• It is also concerned with epidemiology, transmission of pathogens,
disease prevention measures, and aseptic techniques, treatment of
infectious diseases, immunology, and the production of vaccines to
protect against infectious disease.
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Relative Sizes of Pathogenic microorganisms
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Distribution of microorganisms in nature
• Microorganisms can be found nearly everywhere as normal inhabitants of
the earth (biosphere).
• They exist in soil, water, air, in our food, in our clothing, in our body etc.
• Microorganisms can also survive in most unlikely environment like in cold
air, in hot springs at temperatures of 900C.
• Microorganisms inhabit the surface of living human and animal bodies and
grow abundantly in the mouth and intestinal tract.
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• Actually only a small percentage microbes are pathogenic, few are
able to cause disease.
• The others are considered beneficial or harmless, or they cause
disease only if they accidentally invade the wrong place at the
right time such as when the host immunity is low.
• These microbes are considered opportunistic.
• live on the human body with out causing disease; normal flora.
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HISTORICAL BACKGROUND
• Man kind has always been affected by diseases which were originally
believed to be visitations by the gods and meant to punish evil doers.
• Hippocrates, father of medicine, observed that ill health resulted due
to changes in air, winds, water, climate, food, nature of soil and habits
of people.
• Fracastorius (1500 G.C.) proposed living germs, that could be
transmitted but no proof because of lacking experimental evidence.
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• Antony Van Leeuwenhoek (1632-1723 G.C.), father of Microbiology,
observed “animalcules” using his simple microscope with one lens.
• He was the first who properly described the different shapes of bacteria.
• Question raised - where did they originate?
• Leeuwenhoek was not concerned about the origin
• many other scientists were searching
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• On the bases of this observation, two major theories were formulated.
1. Theory of Abiogenesis
2. Theory of Biogenesis
• Theory of Abiogenesis; stating that living things originated
“spontaneously” from non-living things.
• Aristotle (384-322 BC):
He observed spontaneous existence of fishes from dried ponds, when the
pond was filled with rain.
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• Biogenesis: - States that life comes from pre existing life
• Francesco Redi (1626-1697): He is the scientist who first tried to set
an experiment to disprove spontaneous generation.
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Francisco Redi
• Introduced and Performed experimental procedure to
disproof spontaneous generation
• Utilized jars containing meat. Some were covered, some were
not.
• Maggots appeared in uncovered jars. And conclude that
maggots did not emerge spontaneously but from the eggs laid
on the meat by the fly.
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John Needham (1749)
• Performed experiments similar to Redi’s
• Introduced the first culture medium for microbial growth.
• Utilized infusion broth prepared by boiling meat, grain,
• Broth put in flasks, some were sealed with corks, and some were
not.
• All flasks became cloudy, result different from Redi’s experiment.
• He suggested that life originate spontaneously from nonliving
matters
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Louis pasture (1822- 1895) was the scientist who disproved the theory of
abiogenesis once and for all.
- Performed experiment to disprove Theory of spontaneous
generation.
- He designed a large curved flask/swan-necked (pasture
goose neck flask) and placed a sterile infusion broths.
- Flasks remained sterile unless tilted or neck broken.
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* In ‘A’ air freely moved through the tube, but dust particles were
trapped in the curved portion of the flask. And no microbial growth was
observed.
.
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Therefore, Pasteur proved that microorganisms entered to
the broth with the air and micro organisms did not evolve
spontaneously
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Other major contribution of Louis Pasteur.
 Microbial theory of fermentation
• Principles and practices of sterilization and pasteurization
• Control of disease of silk worm
• Development of vaccines against anthrax and rabies
• Discovery of streptococci.
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The Germ Theory of diseases
• Pasture has also developed the germ theory of diseases, which states that
“a specific disease is caused by a specific type of microorganism”.
• Robert Koch, in 1876 established an experimental procedure to prove the
germ theory of disease, which states that specific disease is caused by
specific pathogen. The scientific procedure is known as Koch’s Postulate.
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Koch’s Postulate: - proof of germ theory of disease
• A Micro-organism can be accepted as a causative agent of an infectious disease
only if the following conditions are satisfied.
1. The microorganism should be found in every case of the disease and under
conditions.
2. It should be possible to isolate the causative agent in pure culture from the lesion
3. When such pure culture is inoculated in to appropriate lab animal, the lesion of the
disease should be reproduced.
4. It should be possible to re-isolate the bacterium in pure culture from the lesion
produced in the experimental animal.
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Exceptions to Koch’s postulate
• Many healthy people carry pathogens but do not exhibit symptoms of the
disease.
• Some microbes are very difficult or impossible to grow in vitro (in the
laboratory) in artificial media. E.g.. Treponema pallidum.
• Many pathogens are species specific. E.g.. Brucella abortus cause abortion
in animals but not in humans.
• Certain diseases develop only when an opportunistic pathogen invades
immunocompromised host.
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CLASSIFICATION OF BACTERIA
• Bacteria are classified in to 19 different categories in Bergey’s manual of
determinative bacteriology, 8th (1974), and the classification is based on
1. Morphology 8. Amino acid sequencing of proteins
2. Staining 9. Genetic composition
3. Motility
4. Growth
5. Nutritional requirement
6. Bio chemical and metabolic activity
7. Pathogenecity
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Morphology: - Bacteria vary widely in size, ranging from 0.2 um
to 10um long
-There are three basic shapes
1. Spherical or coccoid/cocci- (singular –coccus)
2. Rods or bacilli (singular - bacillus)
3. Spirals or spirilla (Singular - Spirillum)
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Fig. different bacterial arrangements
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Terminology
Carrier- is an infected person or animal who don't have apparent clinical disease
but a potential source of infection to others.
Colonization- invasion of a certain part of the body by organisms
Health- a complete physical, mental, and social well-being and not merely the
absence of disease or infirmity.
Disease- pathological condition of the body that presents with group of clinical
symptoms and signs and abnormal laboratory findings.
Epidemic- a sudden increase in the incidence of a particular disease rapidly
reaching a peak and then declining to low levels.
Endemic- the presence of a disease more or less constantly in a particular locality
Pandemic- epidemic over a wide geographic area or even worldwide.
Causative agent- the organism that is responsible for causing an infection.
Reservoir- an animal that harbours a species of parasite that is also parasitic for
man and from which many may become infected.
Portal of exit- the route of an organism through which an infectious agent is
shed.
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Portal of entry- the route of an organism through which an infectious agent gets
access.
Route of transmission- it reefers to the route through which the disease causing
microorganism transmits from person to person or to animal.
Incubation period- the time from initial infection until the onset of clinical
symptoms of diseases.
Acute - Having rapid onset, sever symptoms and a short course
Chronic - of long duration, denoting a disease with slow progression.
Normal Flora- These are a normal inhabitant that are acquired early in life and
reside in the human intestinal track, skin, internal organ, body fluid with out
harming the patient in normal condition
Communicable versus non communicable disease
A disease is communicable (contagious) if it is spread from one individual to
another Eg. MTB
A disease is non communicable when it cannot be spread from one individual to
another.
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Transmission
Vertical transmission of a disease refers to the spread of the illness from
mother to child either in utero or during the birth process.
Eg. Trepanoma palladium and Neisseria gonorrhea respectively
Horizontal transmission: - refers to the disease spread from one person to
another by some means.
Nosocomial infection: - infection acquired in a hospital
Iatrogenic disease: - is caused by physician or medical treatment. For example, an
antibiotic used as therapy may destroy bacteria that are sensitive to the agents
but allow yeast that are resistant to the agent to
cause disease.
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