 Dr. Rashad Shawgi Babiker
M.Sc.
(Microbiology) (UMST)
M.Sc. Stud. (Immunology )
Institute of Tropical Medicine.(SAS)
Normal Flora
 Definition
Normal flora is the
mixture of
microorganisms
(bacteria and fungi) that
are regularly found at
any anatomical site of
human body.
Symbiotic Relationship
 1. Mutualistic
 Both organisms benefit – “mutually benefical”
 2. Commensalistic
 One organism benefits, the other is neither helped nor
harmed
 3. Opportunistic
 Under normal conditions, microbe does not cause
disease, but if conditions become conducive , it can
cause disease
Mutualistic
 Escherichia coli
 Synthesizes Vitamin K & B complex Vitamins
 In return, we provide a warm, moist nutrient rich
environment for E. coli
Commensalistic
 We have no Commensalistic relationships with Bacteria
 If Bacteria are in or on our body, they are either helping
us (Microbial Antagonism) or harming us.
Opportunistic
 Escherichia coli - normally in our digestive tract where
it causes no problems, but if it gets into the urinary
tract it can become pathogenic.
 Staphylococcus aureus – commonly found in the upper
respiratory tract, but if it gets into a wound or a burn it
can become pathogenic
The Three Types of Symbiotic
Relationships
Table 14.1
Normal Microbial Flora
 1. Resident Flora
 Microbes that are always present
 2. Transient Flora
 Microbes that live in or on your body for a period of time
(hours, days, weeks, months) then move on or die off
Transient Microbiota
 Remain in the body for only hours to
months before disappearing
 Found in the same regions as resident
microbiota
 Cannot persist in the body…because…
 Competition from other microorganisms
 Elimination by the body’s defenses cells
 Chemical or physical changes in the body
Normal Flora
Sterile tissues
In a healthy human, the
internal tissues such as:
 blood
 brain
 muscle
 cerbrospinal fluid (csf.)
are normally free of
microorganisms.
Normal Flora







Skin
Eyes (i.e.Cunjunctiva)
Nose (i.e. Respiratory
tract)
Mouth (i.e Human Oral
Cavity)
Ears
Urogenetal tract
Elementry tract
Normal Flora
 Resident flora
 Acquired rapidly during
& after birth
Normal Flora
 Resident flora
 Reflects age of person
Normal Flora
Resident flora
Changes continuously through
out life .
Normal Flora
 Resident flora
 Reflects nutrition of
person
Normal Flora
 Resident flora
 Reflects genetics of
person
Normal Flora
Resident flora
Reflects environment of
person
Normal Flora
 Resident flora
 Reflects sex of
person
Importance of The
Normal Flora
(Advantages)
1. They constitute a
protective host
defense mechanism
by occupying
ecological niches.
(Advantages)
2. They produce vitamin B
and vitamin K in
intestine.
Importance of The Normal Flora
(Advantages)
3. The oral flora
contribute to
immunity by inducing
low levels of
circulating and
secretory antibodies
that may cross react
with pathogens.
Importance of The Normal Flora
(Advantages)
4. The oral bacteria flora
exert microbial
antagonism against
nonindigenous species
by production of
inhibitory fatty acids,
peroxides, bacteriocins,
etc.
Importance of The Normal Flora
(Advantages)
5. The normal flora may
antagonize other
bacteria through the
production of
substances which inhibit
or kill nonindigenous
species.
Importance of The Normal Flora
(Disadvantages)
1. They can cause disease in
the following:
a) When individuals
become
immunocompromised.
b) When they change their
usual anatomic location.
Importance of The Normal Flora
2. The oral flora of humans
may harm their host since
some of these bacteria are
pathogens or opportunistic
pathogens
Contamination vs. Infection
 Contamination – the mere presence of microbes in or on
the body
 Infection – results when the organism has evaded the
body’s external defenses, multiplied, and become
established in the body
Portals of Entry
 Sites through which pathogens enter the body
 Four major types
 Skin
 Mucous membranes
 Placenta
 Parenteral route
Skin
 Outer layer of packed, dead, skin cells
usually acts as a barrier to pathogens
 Some pathogens can enter through
openings or cuts
 Others enter by burrowing into or
digesting the outer layers of skin…flesh-
eating bacteria
Mucous Membranes
 Line the body cavities that are open to
the environment
 Provides a moist, warm environment
that is hospitable to pathogens
 Respiratory tract is the most commonly
used site of entry – entry is through the
nose, mouth or eyes
 Pathogens able to survive the acidic pH
of the stomach may use the
gastrointestinal tract as a route of entry
Some Pathogens that Cross the
Placenta
Table 14.3
Parenteral Route
 Pathogens deposited directly into tissues beneath the
skin or mucous membranes
Infection vs. Disease
 Infection is the invasion of the host by a pathogen
 Disease results only if the invading pathogen alters the
normal functions of the body
 Disease is also referred to as morbidity
Manifestations of Disease
 Symptoms – subjective characteristics of disease felt
only by the patient
 Signs – objective manifestations of disease that can be
observed or measured by others
 Syndrome – group of symptoms and signs that
characterize a disease or abnormal condition
 Asymptomatic, or subclinical, infections lack
symptoms but may still have signs of infection
Etiology
 Study of the cause of disease
 Germ theory of disease – disease caused by infections of
pathogenic microorganisms
 Robert Koch developed a set of postulates one must
satisfy to prove a particular pathogen causes a particular
disease
Koch’s Postulates
Figure 14.7