2. Objectives
• Describe normal flora
• Describe different locations harbouring
normal flora on the body
• Describe the importance of normal flora
3. • Normal flora is the term used to describe the
various bacteria and fungi that are permanent
residents of certain body sites, especially the
skin, oropharynx, colon, and vagina.
• Viruses and parasites (protozoa and helminths),
which are the other major groups of
microorganisms, are usually not considered
members of the normal flora, although they can
be present in asymptomatic individuals.
• The normal flora organisms are often referred to
as commensals.
4. • Commensals are organisms that derive benefit
from another host but do not damage that
host.
• The term human microbiome is often used to
describe the normal flora.
• The members of the normal flora vary in both
number and kind from one site to another.
5. • Although the normal flora extensively
populates many areas of the body, the
internal organs usually are sterile. Areas such
as the central nervous system, blood, lower
bronchi and alveoli, liver, spleen, kidneys, and
bladder are free of all but the occasional
transient organism.
• There is a distinction between the presence
of these organisms and the carrier state.
6. • In a sense, we all are carriers of microorganisms.
• The term carrier implies that an individual harbors a
potential pathogen and therefore can be a source of
infection of others.
• It is most frequently used in reference to a person with
an asymptomatic infection or to someone who has
recovered from a disease but continues to carry the
organism and may shed it for a long period.
• There is also a distinction to be made between
members of the normal flora, which are the permanent
residents, and the colonization of the individual with a
new organism.
7. • In a sense, we are all colonized by the normal flora
organisms, but the term colonization typically refers to
the acquisition of a new organism.
• After the new organism colonizes (i.e.,attaches and
grows, usually on a mucosal membrane), it may cause
an infectious disease, or it may be eliminated by our
host defenses.
• Furthermore, the person colonized by a new organism
can transmit that organism to others (i.e., act as a
reservoir of infection for others).
8. • The members of the normal flora play a role
both in the maintenance of health and in the
causation of disease in three significant ways :
• (1) They can cause disease, especially in
immunocompromised and debilitated
individuals. Although these organisms are non
pathogens in their usual anatomic location,
they can be pathogens in other parts of the
body.
9. • (2) They constitute a protective host defense
mechanism. The non-pathogenic resident bacteria
occupy attachment sites on the skin and mucosa that
can interfere with colonization by pathogenic bacteria.
The ability of members of the normal flora to limit the
growth of pathogens is called colonization resistance.
If the normal flora is suppressed, pathogens may grow
and cause disease. For example, antibiotics can reduce
the normal colonic flora that allows Clostridium
difficile, which is resistant to the antibiotics, to
overgrow and cause pseudomembranous colitis.
10. • (3) They may serve a nutritional function. The
intestinal bacteria produce several B vitamins
and vitamin K. Poorly nourished people who
are treated with oral antibiotics can have
vitamin deficiencies as a result of the
reduction in the normal flora. However, since
germ-free animals are well-nourished, the
normal flora is not essential for proper
nutrition.
12. • The human microbiome is the term used to
describe the thousands of microbes
(“microbiota”) located on the skin,on mucosal
surfaces, and within the lumen of the
gastrointestinal tract.
• The majority of these microbes are bacteria,
but yeasts and protozoa are also found in
large numbers.
13. • The largest and most complex microbial population resides
in the colon. Within the colon, the two largest phyla of
bacteria are the Firmicutes(64%) and the Bacteroidetes
(23%).
• The Firmicutes are gram-positive rods and members of the
genera Clostridium and Faecal bacterium are prominent
organisms.
• The Bacteroidetes are gram-negative rods and the genera,
Bacteroides and Prevotella are important members. Species
of Proteobacteria (gram-negative rods such as Escherichia
and Salmonella) and Actinobacteria (gram-positive rods
such as Actinomyces) make up a large percentage of the
remainder.
14. • There is mounting evidence that the
organisms in the microbiome play an
important role in several body functions and
diseases, such as weight control (obesity),
inflammatory bowel disease, the immune
response in general, and resistance to
infectious disease.
15. • The effect on obesity is revealed by studies involving the
transfer of fecal bacteria between strains of inbred
mice.For example, fecal bacteria from obese mice
transplanted into germ-free strains of non obese mice
resulted in the non obese mice becoming obese.
• It appears that the fecal bacteria metabolize more of the
input food making more calories available to the mice. In
other experiments, fecal transplants from identical
(monozygotic) human twins,one obese and the other not
obese, were transplanted into germ-free mice.
• The mice that received the fecal transplant from the obese
twin gained significantly more weight than the mice that
received the fecal transplant from the non obese twin.
16. • The intestinal microbiota also plays a role in certain
autoimmune diseases such as the inflammatory bowel
diseases, Crohn’s disease and ulcerative colitis.
• In the stool of patients with these diseases, bacteria of
the Proteobacteria and Actinobacteria genera were
found in much greater numbers than Firmicutes and
Bacteroidetes whereas in those who do not have those
diseases the opposite was found, namely, the
Firmicutes and Bacteroidetes greatly outnumbered the
Proteobacteria and Actinobacteria
17. • Colonization resistance is another function of the
microbiota.
• Within the colon, members of the intestinal flora
prevent the overgrowth of Clostridium difficile,
the cause of pseudomembranous colitis. When
antibiotics kill members of the colonic flora,
colonization resistance is lost,C. difficile
overgrows, and colitis results.
• In some patients with intractable colitis, fecal
transplants from unaffected individuals cures the
colitis.
18. NORMAL FLORA OF THE SKIN
• The predominant organism is Staphylococcus
epidermidis,which is a non pathogen on the skin but
can cause disease when it reaches certain sites, such as
artificial heart valves and prosthetic joints.
• It is found on the skin much more frequently than its
pathogenic relative Staphylococcus aureus . There are
about 103–104 organisms/cm2 of skin. Most of them
are located superficially in the stratum corneum, but
some are found in the hair follicles and act as a
reservoir to replenish the superficial flora after hand
washing.
19. • Anaerobic organisms, such as Propionibacteriumand
Peptococcus, are situated in the deeper follicles in the
dermis, where oxygen tension is low.
• Propionibacteriumacnes is a common skin anaerobe
that is implicated in the pathogenesis of acne. The
yeast Candida albicans is also a member of the normal
flora of the skin. It can enter a person’s bloodstream
when needles pierce the skin (e.g., in patients with
intravenous catheters or in those who use intravenous
drugs). It is an important cause of systemic infections in
patients with reduced cell-mediated immunity
20. NORMAL FLORA OF THE RESPIRATORY
TRACT
• A wide spectrum of organisms colonize the nose,
throat, and mouth, but the lower bronchi and
alveoli typically contain few, if any, organisms.
• The nose is colonized by a variety of streptococcal
and staphylococcal species, the most significant
of which is the pathogen S. aureus. Occasional
outbreaks of disease due to this organism,
particularly in the newborn nursery, can be
traced to nasal, skin, or perianal carriage by
health care personnel.
21. • The throat contains a mixture of viridans
streptococci,Neisseria species, and S. Epidermidis.
These nonpathogens occupy attachment sites on
the pharyngeal mucosa and inhibit the growth of
the pathogens Streptococcus pyogenes, Neisseria
meningitidis, and S. aureus, respectively.
• In the mouth, viridans streptococci make up
about halfof the bacteria. Streptococcus mutans,
a member of the viridansgroup, is of special
interest since it is found in large numbers
(1010/g) in dental plaque, the precursor of caries.
22. • The plaque on the enamel surface is
composed of gelatinous,high-molecular-
weight glucans secreted by the bacteria. The
entrapped bacteria produce a large amount of
acid,which demineralizes the enamel and
initiates caries. The viridans streptococci, such
as S. sanguinis, are also the leading cause of
subacute bacterial (infective) endocarditis.
23. • These organisms can enter the bloodstream at
the time of dental surgery and attach to
damaged heart valves. Eikenella corrodens,
also part of the normal oral flora,causes skin
and soft tissue infections associated with
human bites and “clenched-fist” injuries (i.e.,
injuries to the hand that occur during fist
fights).
24. • Anaerobic bacteria, such as species of
Bacteroides, Prevotella,Fusobacterium,
Clostridium, and Peptostreptococcus,are found in
the gingival crevices, where the oxygen
concentrationis very low. If aspirated, these
organisms can cause lung abscesses, especially in
debilitated patients with poor dental hygiene. In
addition, the gingival crevices are the natural
habitat of Actinomyces israelii—an anaerobic
actinomycete that can cause abscesses of the jaw,
lungs, or abdomen.
25. NORMAL FLORA OF THE INTESTINAL
TRACT
• In normal fasting people, the stomach contains
few organisms,primarily because of its low pH.
The small intestine usually contains small
numbers of streptococci, lactobacilli,and yeasts,
particularly C. albicans. Larger numbers of these
organisms are found in the terminal ileum.
• The colon is the major location of bacteria in the
body. Roughly 20% of the feces consists of
bacteria, approximately 1011 organisms/g. The
major bacteria found in the colon are listed in the
table.
26.
27. • Note that more than 90% of the fecal flora are
anaerobes, the most important of which is Bacteroides
fragilis.
• The most abundant facultative bacteria are the
coliforms,of which Escherichia coli is the most
important.
• The normal flora of the intestinal tract plays a
significant role in extraintestinal disease. For example,
E. coli is the leading cause of urinary tract infections,
and B. fragilisis an important cause of peritonitis
associated with perforation of the intestinal wall
following trauma, appendicitis, or diverticulitis.
28. • Other important anaerobic pathogens include
Fusobacterium and Peptostreptococcus, and
other important facultative bacteria include
Enterococcus faecalis, which causes urinary tract
infections and endocarditis, and Pseudomonas
aeruginosa, which can cause various infections,
particularly in hospitalized patients with
decreased host defenses.
• P. aeruginosa is present in 10% of normal stools,
as well as in soil and water.
29. • Antibiotic therapy (e.g., with clindamycin) can
suppress the predominant normal flora, thereby
allowing a rare organism such as the toxin-
producing Clostridium difficile to overgrow and
cause severe colitis.
• Administration of certain antibiotics, such as
neomycin orally, prior to gastrointestinal surgery
to “sterilize” the gut leads to a significant
reduction of the normal flora for several days,
followed by a gradual return to normal levels.
30. NORMAL FLORA OF THE
GENITOURINARY TRACT
• The vaginal flora of adult women consists
primarily of Lactobacillus species. Lactobacilli are
responsible for producing the acid that keeps the
pH of the adult woman’s vagina low.
• Before puberty and after menopause, when
estrogen levels are low, lactobacilli are rare and
the vaginal pH is high. Lactobacilli appear to
prevent the growth of potential pathogens, since
their suppression by antibiotics can lead to
overgrowth by C. albicans. Overgrowth of this
yeast can result in Candida vaginitis.
31. • The vagina is located close to the anus and can be
colonized by members of the fecal flora. For example,
women who are prone to recurrent urinary tract
infections harbour organisms such as E. coli and
Enterobacter in the introitus.
• About 15% to 20% of women of childbearing age
carrygroup B streptococci in the vagina. This organism
is an important cause of sepsis and meningitis in the
newborn and is acquired during passage through the
birth canal. The vagina is colonized by S. aureus in
approximately 5% of women, which predisposes them
to toxic shock syndrome.
32. • Urine in the bladder is sterile in the healthy person, but
during passage through the outermost portions of the
urethra, it often becomes contaminated with S.
epidermidis,coliforms, diphtheroids, and nonhemolytic
streptococci.
• The area around the urethra of women and
uncircumcised men contains secretions that carry
Mycobacterium smegmatis, an acid-fast organism. The
skin surrounding the genitourinary tract is the site of
Staphylococcus saprophyticus, a cause of urinary tract
infections in women
34. • Normal flora are those microorganisms that are
the permanent residents of the body that
everyone has. Some people can be colonized,
either transiently or for long periods, with certain
organisms, but those are not considered
members of the normal flora.
• Carriers (also called chronic carriers) are those
individuals in whom pathogenic organisms are
present in significant numbers and therefore are
a source of infection for others.
35. • Normal flora organisms are either bacteria
or yeasts. Viruses,protozoa, and helminths are
not considered to be members ofthe normal
flora (but humans can be carriers of some of
these organisms).
• Normal flora organisms inhabit the body
surfaces exposed tothe environment, such as
the skin, oropharynx, intestinal tract, and
vagina.
36. • Members of the normal flora differ in number
and kind at various anatomic sites Members
of the normal flora are low-virulence
organisms.
– In their usual anatomic site, they are
nonpathogenic. However, if they leave their usual
anatomic site, especially in an
immunocompromised individual, they can cause
disease.
37. • Colonization resistance occurs when
members of the normal flora occupy receptor
sites on the skin and mucosal surfaces,
thereby preventing pathogens from binding to
those receptors.
39. • Skin.
– The predominant member of the normal flora of the
skin is S. epidermidis. It is an important cause of
infections of prosthetic heart valves and prosthetic
joints.
– C. albicans, a yeast also found on the skin, can enter
the bloodstream and cause disseminated infections,
such as endocarditis in intravenous drug users.
– S. aureus is also present on the skin, but its main site
is in the nose. It causes abscesses in the skin and in
many other organs.
40. • Oropharynx.
– The main members of the normal flora of
themouth and throat are the viridans
streptococci, such as S.sanguinis and S. mutans.
– Viridans streptococci are the most common cause
of subacute endocarditis.
41. • Gastrointestinal tract.
– The stomach contains very few organisms because of the
low pH. The colon contains the largest number of normal
flora and the most diverse species, including both
anaerobic and facultative bacteria.
– There are both gram-positive and gram-negative rods and
cocci.
– The members of the colonic normal flora are an important
cause of disease outside of the colon.
– The two most important members of the colonic flora that
cause disease are the anaerobe B. fragilis and the
facultative E. coli. E. faecalis, a facultative, is also a very
important pathogen.
42. • Vagina.
– Lactobacilli are the predominant normal flora
organisms in the vagina. They keep the pH of the
vagina low, which inhibits the growth of organisms
such as C. albicans, an important cause of vaginitis.
• Urethra.
– The outer third of the urethra contains a mixture of
bacteria, primarily S. epidermidis.
– The female urethra can become colonized with fecal
flora such as E. coli, which predisposes to urinary tract
infections.
