Normal Flora
 Human
body is continuously inhabited
by many different microorganisms.
 Mostly bacteria, some fungi and other
microorganisms.
 They are harmless for healthy people
and may even be beneficial.
 These organisms are termed normal
flora or commensals.
A
healthy newborn enters the world
in sterile condition, after birth it
acquires normal flora from food and
environment, including other humans.
 Internal organs and systems are
sterile, including spleen, pancreas,
liver, bladder, central nervous system
and blood.
Distribution of normal flora in the
body
 The
most common sites of the
body inhabited by the normal
flora are those in contact with the
outside world, namely the skin,
eye, mouth, upper respiratory
tract, gastrointestinal tract, and
urogenital tract.
Skin
 It
can acquire any bacteria that
happen to be in the immediate
environment, but this transiet flora
either die or remove by washing.
 The skin surface does not provide a
favorable environment for
colonization by microorganisms (dry,
acidic PH, and hyperosmotic
environment).
 Nevertheless,
the skin supports
a permanent bacterial population
(resident flora).
 Most common among skin colonizers
are:
- aerobic staphylococcus epidermidis &
other coagulase-negative
staphylococci.
- anaerobic Propionibacterium acnes &
Corynebacterium species.
 The
skin inhabitants are generally
harmless, although s.epidermidis can
attach to and colonize plastic
catheters and medical devices that
penetrate the skin, some times
resulting in serious blood stream
infections.
Eye
 The
conjunctiva of the eye is
colonized primarily by s.epidermidis,
followed by S.aureus, aerobic
Corynbacteria (diphtheroids) &
S.pneumoniae.
 Tears, which contain the antimicrobial
enzyme lysozyme, help limit the
bacterial population of the
conjunctiva.
 Most
Mouth & nose
common among mouth & nose
colonizers are aerobic Corynbacteria
(diphtheroids), S.aureus &
S.epidermidis.
 The teeth are colonized by S.mutans,
that can enter blood stream following
dental surgery, and colonize damaged
or prosthetic heart valves, leading to
potentially fatal infective endocarditis.
 Some
normal residents of the
nasopharynx can also cause disease
For example: S. pneumoniae can
cause acute bacterial pneumoniae in
the aged and those whose resistance
is impaired.
Intestinal tract
 The
density of microorganisms in the
stomach is relatively low and
increased along the alimentary canal.
 Some twenty percent of fecal mass
consists of many different species of
bacteria, 99% of which are
anaerobes.
 Examples: Bacteroids species & E.coli
which is the major cause of urinary
tract infection.
Urogenital tract
 The
low PH of the adult vagina is
maintained by lactobacillus species
(normal flora of the vagina), which
are decreased by antibiotic therapy,
PH rises & potential pathogens can
overgrow as candida albicans which it
self is a normal flora of vagina,
mouth, and small intestine.
 The
urine in the kidney and bladder is
sterile, but can become contaminated
in the lower urethra by the same
organisms that inhabit the outer layer
of the skin.
Benificial functions of normal flora
 Inhibit
-
colonization and infection
by pathogenic bacteria by:
Interfere with bacterial adherence.
Maintain inhibitory PH in vagina and
skin.
Compete for essential nutrients.
Produce inhibitory substances.
 They
perform important
metabolic functions Such as:
- Synthesis of vitamin K.
- Absorption of nutrients from the
intestine.
 Stimulate
the development of
the immune system of the new
born infant.
Harmful effect of normal flora
 When
the organisms are diplaced from
their normal site in the body to an
abnormal site.
 When potential pathogens gain a
competitive advantage due to
diminished populations of harmless
competitor.
 When individuals are
immunocompromised, normal flora can
over grow and become pathogenic.