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On the basis of life habitat, microorganisms
can be classified into:
1- Saprophytes
2- Parasites
3- Commensals
Definition of opportunistic pathogens
-Pathogenic organisms: they are primary
pathogens capable of establishing and
causing disease in healthy person with intact
immunological defenses.
Virulence:
is a measure for the degree of pathogenicity.
Virulence depends on:
a) Ability of entrance -colonize-invadeovercome host defenses- multiply and
disseminate widely.
b) toxigenicity: ability to produce toxins.
Stages of bacterial pathogenesis:
1- transmission from an external source
in to the portal of entry.( modes of
transmission).
2- evasion of primary host defenses
such as skin or stomach acid.
3- adherence to mucous membrane.
4- Colonization by growth the bacteria on
the site of infection.
5- Disease symptoms caused by toxin
production or extracellular enzymes.
6- Host responses, both specific and
nonspecific (immunity).
7- Progression or resolution of the
disease.
Differences between Exotoxins and
Endotoxins
Properties
Exotoxin
Endotoxin
Source
Some +ve and
–ve bacteria
Part of cell wall of
most –ve bacteria
Chemical nature
polypeptide
L.P.S
Location of genes
plasmid
Bacterial
chromosome
Toxicity
high
low
Clinical effects
various
Fever,
hypotension,shock
Vaccines
Toxoids
No vaccine
Heat stability
thermolabile
thermostable
Extracellular enzymes:
Many bacteria secrete a range of enzymes
that involved in the pathogenic process e.g
-Coagulase: it clots plasma and produce
fibrin coat protects microorganisms from
phagocytosis. Staphylococcus aureus
- Collagenase: It breaks down collagen
(which is the constituent of muscle, bone and
cartilage) thus allow bacteria to spread.
Clostridium perfringens
-Lecithinase: It breaks down lecithin
(main constituent of cell membranes)
causes hemolysis, necrosis.Cl.perfringens
-Leukocidin: it destroys host leukocytes
Streptococci
- Haemolysin: cause lysis of RBC.
- Deoxyribonuclease: It breaks down
host DNA
Immunity
Protection /defense
. . . against whatever doesn’t belong
Other
pathogens
Cancer
Chemicals
Damage/
injury
Concept, Functions and Types
of Immunity
Concept and functions of Immunity
Types of Immunity
I- Concept of Immunity
1)Traditional concept: Immunity refers to
protection against infectious diseases.
2) Modern concept-: Immunity is a function of
which an individual recognizes and excludes
antigenic foreign substances.
It is normally beneficial, but sometimes, it is
injurious.
II- Concept of Immunology
1) Traditional concept:
anti-infection immunity to different types of
pathogenic microorganisms.
2) Modern concept:
Immunology is an independent subject about
- composition,
- functions of immune system;
- mechanism of immune response and the
disease associated with immunity.
III Functions of Immunity
Immune defense:
Immune homeostasis: eliminate injured
and senile cells . Tolerate to self
components .
Immune surveillance: destroy transformed
cells (anti-tumor ) .
Types of Immunity
I. Innate Immunity
(or native immunity/ non-specific immunity
/congenital immunity)
II. Adaptive Immunity
(or acquired immunity/specific immunity)
Body defense against
microbes
Non-specific
immunity
First line
(immediate)
- Skin
-Mucous membranes
-- Chemicals
Second
line
-Phagocytosis
-Complement
-Interferons
-Inflamation
-fever
Specific
immunity
(delayed)
-Lymphocytes
-Antibodies
 Non- specific immunity
Natural (innate) immunity
- It is v. effective mechanisms present from
birth which do not depend upon host having
previous exposure to any M.O.
- Acts immediately as the first line of defense
or within several hours after exposure to
antigen.
-Innate immunity does not recognize every
possible antigen.
Innate immunity
1. Characteristics
 Exists naturally
 Non–specific
 No immune memory (innate immunity can’t be
enhanced by the second stimulation of the same
antigen).
Immune memory: Exposure of the immune
system to a foreign antigen enhances its ability
to respond again to that antigen.
2. Composition
(1) Barriers
 Physical barrier : skin and mucosa
 Chemical barrier: antimicrobial substances in
secretion of skin and mucosa
 Biotic barrier: normal flora existing on the surface of
skin and mucosa
 Anatomic barrier
. blood- brain barrier
. blood- placental barrier
Physical Barriers
- Skin
- Mucous membranes
- Hairs and mucus of respiratory system
-Reflex flushing through coughing, sneezing, vomiting,,
-Urinary flow
-Blinking reflex of eyes.
- digestive tract peristalsis.
Chemical Defenses
- Acids produced by sweat, sebaceous glands, and stomach
- Lysozyme: in tears, saliva, mucus; cleaves peptidoglycan
(G+)
- Transferrin: in blood, binds free iron, thus inhibiting bacterial
growth
- Defensins: peptides disrupting cell membranes.
(2) Humoral factors
 Complement system:
 Lysozyme: Basic protein found in microphage
(PMN) as well as most tissue fluids except CSF,
sweet and urine. It destroys the cell wall of bacteria.
 Interferons(IFN):
 C-reactive protein (CRP): Measure of inflammation
produced in liver in response to inflammatory
chemicals.
3) Cellular factors:
- Normal flora ( Microbiota)
-Natural killer cells: (NK), these are non-T, non-B
lymphocytes, which have natural ability to kill foreign cell as tumor,
viral cells and activated by interferon and interleukin 2.
-Phagocytosis:
- mononuclear phagocytes (Monocytes,Macrophages)
- Professional phagocytes: microphages
- Dendritic cells(DC)
Macrophages
NK cells
Neutrophils
Macrophages
Macrophages excluding the pathogen
Complement system
-Is formed of heat labile proteins components
present normally in an inactive form in serum and
other body fluids except urine and CSF.
-There are 9 major components termed C1-C9.
-There activation proceed through:
1- Classical pathway or
2- Alternative pathway.
3-Lectin binding pathway.
Role of skin in nonspecific defense:
(first line of defense)
Intact skin and mucous membranes of the body
afford effective protection against M.O.
Skin contains
- Collagen: gives the skin strength and pliability to
prevent skin scraps and introduce M.O
- Dendritic cells .
- The sweat and sebaceous secretions contain
antimicrobial substances as F.A and lysozyme.
Two major layers of skin:
Epidermis
-Outer layer composed of multiple layers of tightly
packed cells.
-Few pathogens can penetrate these layers.
- Epidermal dendritic cells phagocytiz pathogens.
Dermis:
- Contains protein fibers called collagen
- Give skin strength and pliability to resist abrasions
that could introduce microorganisms.
Mucous, secreted by the membranes
lining the inner surfaces of the body
acts as a protective barrier by
trapping M.O.
-Lysozyme
-S IgA
-Lactoferrin
-Lactoperoxidase
Role of normal microbiota in nonspecific
microbial antagonism
Many bacteria and other M.O reside in
the human body. These normal body
flora
keep
potentially
harmful
opportunistic pathogens and inhibit the
colonization of M.O by:
A- producing metabolic products( fatty
acids, bacteriocins) .
B- Can change the pH.
C- adhering to target host cells thus
covering
them
and
preventing
pathogens from colonizing.
D- depleting nutrients essential for
growth of pathogens.
E- nonspecifically stimulating the
immune system.
For example:
When protective commensals are disturbed by
antibiotics, susceptibility to opportunistic
infections such as Candida albicans and
Clostridium difficile is increased.
pathogen invasion of the vagina is limited by
lactic acid produced by commensal organisms
which metabolize glycogen secreted by the
vaginal epithelium