NORMAL FLORA
Is the microbial inhabitants of the human body , mostly bacteria, but also fungi
and other microorganisms, which in a healthy individual are harmless and beneficial .
The internal organs and systems are sterile (the spleen, pancreas, liver, bladder,
central nervous system, and blood). A healthy newborn is sterile, but, after birth, it
rapidly acquires normal flora from food and the environment.
The normal flora species vary from individual to individual, as a result of:
1- Physiologic differences.
2- Diet.
3- Age.
4- Geographic habitat.
It is useful to know the normal types and distribution of resident flora, because
such knowledge provides an understanding of the possible infections that result from
injury to a particular body site and places in perspective the possible sources and
significance of microorganisms isolated from the site of an infection.
DISTRIBUTION OF NORMAL FLORA IN THE BODY
The most common sites of the body inhabited by normal flora are, those in contact
or communication with the outside world as, the skin, eye, mouth, upper respiratory
tract, gastrointestinal tract, and urogenital tract.
A. Skin
Skin can acquire any bacteria (transient or
resident flora) that happen to be in the immediate
environment. (Figure 1)
The transient flora (the skin supports a
removable bacterial population) either dies or is
removable by :
1- washing.
2- It's dryness.
3- It's slightly acidic pH.
4- The sweat glands produce a liquid containing a
high concentration of sodium chloride that causes
a hyperosmotic environment on the skin surface.
The resident flora (the skin supports a permanent bacterial population) residing in
multiple layers of the skin and regenerate even after vigorous scrubbing.
Most common among the skin colonizers (Figure 1) are :
A- 90 % aerobic organisms : Staphylococcus epidermidis and other coagulase-
negative Staphylococci that reside in the outer layers of the skin.
B- Anaerobic organisms: Propionibacterium acnes, reside in deeper skin layers, hair
follicles, and sweat and sebaceous glands.
C- Candida albicans is a yeast-like fungus.
B. Eye
The conjunctiva of the eye is colonized primarily
by
Staphylococcus
epidermidis,
followed
by
Staphylococcus
aureus,
aerobic
Corynebacteria
diphtheria, and Streptococcus pneumoniae. Other
organisms that normally inhabit the skin are also
present, but at a lower frequency (Figure 2). Tears,
which contain the antimicrobial enzyme lysozyme, help
limit the bacterial population of the conjunctiva.
C. Mouth and nose
The mouth and nose harbor many microorganisms, both aerobic and anaerobic
(Figure 3). Among the most common are aerobic Corynebacteria diphtheria,
S. aureus, and S. epidermidis.
The
teeth
and
surrounding
gingival
tissue
are
colonized
by
Streptococcus mutans. [S. mutans can
enter the bloodstream following dental
surgery, and colonize damaged heart
valves,
leading
to
fatal
infective
endocarditis].
Some normal residents of the
nasopharynx can also cause disease; for
example, Streptococcus pneumoniae, can
cause
acute
bacterial
pneumonia,
especially in the aged persons( those whose resistance is impaired) preceded by
respiratory viral infection.
D. Intestinal tract
In an adult, the density of microorganisms in
the stomach is low due to gastric enzymes and acidic
pH. The density of organisms increases along the
alimentary canal, in the ileum, and in the large
intestine. Some 20 percent of the fecal mass consists
of many different species of bacteria, more than 99
percent of which are anaerobes (Figure 4).
Bacterioides species are a significant percentage of
bacteria in the large intestine. Escherichia coli, a
facultatively anaerobic organism find in the intestinal
tract, this endogenous E. coli is a major cause of
urinary tract infections.
E. Urogenital tract
The low pH of the adult vagina is
maintained
by
the
presence
of
Lactobacillus species, which are the
primary components of normal flora. If
the Lactobacillus population in the vagina
is decreased by antibiotic therapy, the pH
rises
and
potential
pathogens
can
overgrow like the yeast-like fungus,
Candida albicans , which itself is a minor
member of the normal flora of the
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 (Figure 5).
BENEFICIAL FUNCTIONS OF NORMAL FLORA
1- The number of harmless bacteria in the lower bowel and mouth make it unlikely
that(in a healthy person) an invading pathogen by compete for nutrients and
receptor sites.
2- Some bacteria of the bowel produce antimicrobial substances to which the
producers themselves are immune.
3- Bacterial colonization of a newborn infant acts as a powerful stimulus for the
development of the immune system.
4- Bacteria of the gut provide important nutrients, such as vitamin K, and aid in
digestion and absorption of nutrients.
HARMFUL EFFECTS OF NORMAL FLORA
Clinical problems caused by normal flora arise under the following conditions:
1- When the organisms are displaced from their normal site in the body to an
abnormal site: e. g. the normal skin bacterium, Staphylococcus epidermidis,
introduce into the bloodstream where it can colonize catheters and heart valves,
resulting in bacterial endocarditis.
2- When pathogens gain a competitive advantage due to minimized normal
flora populations : e. g. when normal bowel flora is depleted by antibiotic
therapy leading to overgrowth by the resistant Clostridium difficile, which can
cause severe colitis.
3- When ingested food substances are converted into carcinogenic derivatives
by bacteria in the colon: e. g. bacteria sulfatases of the sweetener cyclamate into
the bladder carcinogen cyclohexamine.
4- When individuals are immunocompromised, normal flora can overgrow and
become pathogenic. [normal flora is carried by a healthy individual and passed
to other individuals which causes disease. Typhoid fever is an example of a
disease that can be acquired from a carrier .