Page 39 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com NORMAL FLORA OF THE mebooksfree.com mebooksfree.com
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28
PART I Basic Bacteriology
bacteria metabolize more of the input food making more
calories available to the mice. In other experiments, fecal
RESPIRATORY TRACT
transplants from identical (monozygotic) human twins,
A wide spectrum of organisms colonize the nose, throat,
one obese and the other not obese, were transplanted into
and mouth, but the lower bronchi and alveoli typically con-
germ-free mice. The mice that received the fecal transplant
from the obese twin gained significantly more weight than
ety of streptococcal and staphylococcal species, the most
the mice that received the fecal transplant from the non-
significant of which is the pathogen S. aureus. Occasional
obese twin.
outbreaks of disease due to this organism, particularly in
The intestinal microbiota also plays a role in certain tain few, if any, organisms. The nose is colonized by a vari-
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the newborn nursery, can be traced to nasal, skin, or peri-
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autoimmune diseases such as the inflammatory bowel dis-
mebooksfree.com mebooksfree.com mebooksfree.com Neisseria species, and S. epidermidis (see Table 6–2). These mebooksfree.com
anal carriage by health care personnel.
eases, Crohn’s disease and ulcerative colitis. In the stool of
The throat contains a mixture of viridans streptococci,
patients with these diseases, bacteria of the Proteobacteria
and Actinobacteria genera were found in much greater
numbers than Firmicutes and Bacteroidetes whereas in
nonpathogens occupy attachment sites on the pharyngeal
those who do not have those diseases the opposite was
mucosa and inhibit the growth of the pathogens Streptococcus
found, namely, the Firmicutes and Bacteroidetes greatly
pyogenes, Neisseria meningitidis, and S. aureus, respectively.
In the mouth, viridans streptococci make up about half
outnumbered the Proteobacteria and Actinobacteria.
In addition, the gut microbiota influences the matura-
of the bacteria. Streptococcus mutans, a member of the viri-
tion and function of the immune response. Germ-free mice
10
numbers (10 /g) in dental plaque, the precursor of caries.
do not produce lymphoid follicles in the small intestine, do
The plaque on the enamel surface is composed of gelati-
not produce secretory IgA, and do not produce CD8
nous, high-molecular-weight glucans secreted by the bacte-
intraepithelial lymphocytes. Further, the presence of cer- dans group, is of special interest since it is found in large
ria. The entrapped bacteria produce a large amount of acid,
tain members of the microbiota influences the proportion
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mebooksfree.com mebooksfree.com mebooksfree.com ing cause of subacute bacterial (infective) endocarditis. mebooksfree.com
which demineralizes the enamel and initiates caries. The
of Th1, Th2, and Th17 T cells.
Colonization resistance is another function of the
viridans streptococci, such as S. sanguinis, are also the lead-
microbiota. Within the colon, members of the intestinal
These organisms can enter the bloodstream at the time of
flora prevent the overgrowth of Clostridium difficile, the
dental surgery and attach to damaged heart valves.
cause of pseudomembranous colitis. When antibiotics kill
members of the colonic flora, colonization resistance is lost,
Eikenella corrodens, also part of the normal oral flora,
C. difficile overgrows, and colitis results. In some patients
causes skin and soft tissue infections associated with
human bites and “clenched-fist” injuries (i.e., injuries to the
with intractable colitis, fecal transplants from unaffected
hand that occur during fist fights).
individuals cures the colitis.
Anaerobic bacteria, such as species of Bacteroides, Pre-
NORMAL FLORA OF THE SKIN votella, Fusobacterium, Clostridium, and Peptostreptococcus,
are found in the gingival crevices, where the oxygen concen-
tration is very low. If aspirated, these organisms can cause
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mebooksfree.com mebooksfree.com mebooksfree.com ral habitat of Actinomyces israelii—an anaerobic actinomycete mebooksfree.com
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The predominant organism is Staphylococcus epidermidis,
lung abscesses, especially in debilitated patients with poor
which is a nonpathogen on the skin but can cause disease
dental hygiene. In addition, the gingival crevices are the natu-
when it reaches certain sites, such as artificial heart valves
and prosthetic joints. It is found on the skin much more
that can cause abscesses of the jaw, lungs, or abdomen.
frequently than its pathogenic relative Staphylococcus
3
4
aureus (see Table 6–2). There are about 10 –10 organisms/
NORMAL FLORA OF THE
2
cm of skin. Most of them are located superficially in the
INTESTINAL TRACT
stratum corneum, but some are found in the hair follicles
and act as a reservoir to replenish the superficial flora after
In normal fasting people, the stomach contains few organ-
hand washing. Anaerobic organisms, such as Propionibac-
terium and Peptococcus, are situated in the deeper follicles
usually contains small numbers of streptococci, lactobacilli,
in the dermis, where oxygen tension is low. Propionibacte-
and yeasts, particularly C. albicans. Larger numbers of
these organisms are found in the terminal ileum.
rium acnes is a common skin anaerobe that is implicated in isms, primarily because of its low pH. The small intestine
The colon is the major location of bacteria in the body.
the pathogenesis of acne.
mebooksfree.com mebooksfree.com mebooksfree.com listed in Table 6–3. Note that more than 90% of the fecal flora mebooksfree.com
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Roughly 20% of the feces consists of bacteria, approximately
The yeast Candida albicans is also a member of the nor-
11
10 organisms/g. The major bacteria found in the colon are
mal flora of the skin. It can enter a person’s bloodstream
when needles pierce the skin (e.g., in patients with intrave-
are anaerobes, the most important of which is Bacteroides
nous catheters or in those who use intravenous drugs). It is
fragilis. The most abundant facultative bacteria are the coli-
an important cause of systemic infections in patients with
reduced cell-mediated immunity.
forms, of which Escherichia coli is the most important.
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