CHAPTER 1 INTRODUCTION
CONTENT
1)
2)
3)
A brief history of medical microbiology
Host – parasite relationships
Mechanism of pathogenesis
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4)
Pathogenic properties of pathogens
Transmission of diseases
A brief history of medical
microbiology
Antony van Leeuwenhoek
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Antony van Leeuwenhoek (1632-1723) designed
a single lens microscope and demonstrated the
little
agents of disease, which he designated as
animalcules.
These animalcules are now well established
entities
belonging to bacteria, viruses and several other
pathogens. The organisms being invisible to naked
eye
are known as microorganisms.
Louis Pasteur
 Louis
Pasteur (1822-1895), a French chemist
generated strong evidence to show that the
microorganisms did not rise de novo or
spontaneously in the media but were
introduced from without. Pasteur showed that
these organisms were maximum in the dusty
air of towns and minimum in air of hilly areas
where human habitation did not exist.
Robert Koch
 Robert
Koch's first contribution to science
was demonstration of the character and
mode of growth of causative bacillus of
anthrax.
 In 1882, Koch discovered tubercle bacillus
 1883 the cholera vibrio. For his manifold
discoveries in bacteriology, Koch is
considered as father of bacteriology.
Host – parasite
relationships
Terminologies
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Pathogen – a parasites capable to cause
disease in a host
Symbiosis – associate between two species
(normal flora)*
Parasitism – 1 org is parasite (gain benefit) 1
org is harmed by it
Contamination – the microorganisms are
present
Infection – multiplication of any parasitic org
within or
cont
 Pathogenicity
– the capacity to produce
disease
 Example – Mycobacterium tuberculosis
frequently cause disease upon
susceptible host
 - S. epidermidis only in rare instance and
poor immunodeficiency host
Mechanism of pathogenesis
- Pathogenic properties of
pathogens
How microbes cause disease?
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They act certain ways to harm / destruct the
host
Actions – gain access to the host, adhering to
and colonizing cell surface, invading tissues,
producing toxins and other harmful metabolic
products.
Virulence factors – structural or physiological
characteristics that help organisms cause
infection and disease (pili, enzymes and toxin)
Direct action of bacteria
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4.
Adherence using (adhesin) protein found on
attachment pili and capsule.
Colonization is growth of m/org on epit
surfaces or other host tissues
Invasiveness is to invade and grow in host
hyaluronidase (spreading enzyme)- hold the
cell together
Damage by coagulase enzyme – clotting of
blood or streptokinase – dissolve blood clot
Bacterial toxin
 Substance
that poisonous to other
organism
 Examples – hemolysin, leukocidins,
leukostatin, toxemia, neurotoxin,
enterotoxin and toxoid
1. Invasiveness
 is
the ability to invade tissues. It
encompasses mechanisms for
colonization (adherence and initial
multiplication), production of extracellular
substances which facilitate invasion
(invasins) and ability to bypass or
overcome host defense mechanisms.
2. Toxigenesis
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Toxigenesis is the ability to produce toxins.
Bacteria may produce two types of toxins
called exotoxins and endotoxins. Exotoxins
are released from bacterial cells and may act
at tissue sites removed from the site of
bacterial growth. Endotoxins are cellassociated substance. (In a classic sense, the
term endotoxin refers to the
lipopolysaccharide component of the outer
membrane of Gram-negative bacteria).
Cont..
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However, endotoxins may be released from
growing bacterial cells and cells that are lysed as
a result of effective host defense (e.g. lysozyme) or
the activities of certain antibiotics (e.g. penicillins
and cephalosporins).
Hence, bacterial toxins, both soluble and cellassociated, may be transported by blood and
lymph and cause cytotoxic effects at tissue sites
remote from the original point of invasion or
growth. Some bacterial toxins may also act at the
site of colonization and play a role in invasion.
Transmission of diseases
Types of bourne
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Air borne transmission - a transmission mechanism
in the which the infectious agent is spread as an
aerosol and usually enters a person through the
respiratory tract
direct transmission - a transmission mechanism in
which the infectious agent is transferred directly
into the body via touching or biting or kissing or
sexual intercourse or by droplets entering the eye
or nose or mouth
indirect transmission - a transmission mechanism in
which the infectious agent is transferred to the
person by a fomite of vector
Airbourne transmission
Examples
Direct transmission
Contaminated water
Indirect transmission