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<meta name="robots" content="INDEX,NOFOLLOW,NOARCHIVE,NOIMAGEINDEX" /><meta name="citation_inbook_title" content="Medical Microbiology. 4th edition" /><meta name="citation_title" content="Normal Flora" /><meta name="citation_publisher" content="University of Texas Medical Branch at Galveston" /><meta name="citation_date" content="1996" /><meta name="citation_author" content="Charles Patrick Davis" /><meta name="citation_pmid" content="21413249" /><meta name="citation_fulltext_html_url" content="https://www.ncbi.nlm.nih.gov/books/NBK7617/" /><link rel="schema.DC" href="http://purl.org/DC/elements/1.0/" /><meta name="DC.Title" content="Normal Flora" /><meta name="DC.Type" content="Text" /><meta name="DC.Publisher" content="University of Texas Medical Branch at Galveston" /><meta name="DC.Contributor" content="Charles Patrick Davis" /><meta name="DC.Date" content="1996" /><meta name="DC.Identifier" content="https://www.ncbi.nlm.nih.gov/books/NBK7617/" /><meta name="DC.Language" content="en" /><meta name="description" content="A diverse microbial flora is associated with the skin and mucous membranes of every human being from shortly after birth until death. The human body, which contains about 1013 cells, routinely harbors about 1014 bacteria (Fig. 6-1). This bacterial population constitutes the normal microbial flora . The normal microbial flora is relatively stable, with specific genera populating various body regions during particular periods in an individual's life. Microorganisms of the normal flora may aid the host (by competing for microenvironments more effectively than such pathogens as Salmonella spp or by producing nutrients the host can use), may harm the host (by causing dental caries, abscesses, or other infectious diseases), or may exist as commensals (inhabiting the host for long periods without causing detectable harm or benefit). Even though most elements of the normal microbial flora inhabiting the human skin, nails, eyes, oropharynx, genitalia, and gastrointestinal tract are harmless in healthy individuals, these organisms frequently cause disease in compromised hosts. Viruses and parasites are not considered members of the normal microbial flora by most investigators because they are not commensals and do not aid the host.Figure 6-1Numbers of bacteria that colonize different parts of the bodyNumbers represent the number of organisms per gram of homogenized tissue or fluid or per square centimeter of skin surface." /><meta name="og:title" content="Normal Flora" /><meta name="og:type" content="book" /><meta name="og:description" content="A diverse microbial flora is associated with the skin and mucous membranes of every human being from shortly after birth until death. The human body, which contains about 1013 cells, routinely harbors about 1014 bacteria (Fig. 6-1). This bacterial population constitutes the normal microbial flora . The normal microbial flora is relatively stable, with specific genera populating various body regions during particular periods in an individual's life. Microorganisms of the normal flora may aid the host (by competing for microenvironments more effectively than such pathogens as Salmonella spp or by producing nutrients the host can use), may harm the host (by causing dental caries, abscesses, or other infectious diseases), or may exist as commensals (inhabiting the host for long periods without causing detectable harm or benefit). Even though most elements of the normal microbial flora inhabiting the human skin, nails, eyes, oropharynx, genitalia, and gastrointestinal tract are harmless in healthy individuals, these organisms frequently cause disease in compromised hosts. Viruses and parasites are not considered members of the normal microbial flora by most investigators because they are not commensals and do not aid the host.Figure 6-1Numbers of bacteria that colonize different parts of the bodyNumbers represent the number of organisms per gram of homogenized tissue or fluid or per square centimeter of skin surface." /><meta name="og:url" content="https://www.ncbi.nlm.nih.gov/books/NBK7617/" /><meta name="og:site_name" content="NCBI Bookshelf" /><meta name="og:image" content="https://www.ncbi.nlm.nih.gov/corehtml/pmc/pmcgifs/bookshelf/thumbs/th-mmed-lrg.png" /><meta name="twitter:card" content="summary" /><meta name="twitter:site" content="@ncbibooks" /><meta name="bk-non-canon-loc" content="/books/n/mmed/A500/" /><link rel="canonical" href="https://www.ncbi.nlm.nih.gov/books/NBK7617/" /><link rel="stylesheet" href="/corehtml/pmc/css/figpopup.css" type="text/css" media="screen" /><link rel="stylesheet" href="/corehtml/pmc/css/bookshelf/2.26/css/books.min.css" type="text/css" /><link rel="stylesheet" href="/corehtml/pmc/css/bookshelf/2.26/css/books_print.min.css" type="text/css" media="print" /><style type="text/css">p a.figpopup{display:inline !important} .bk_tt {font-family: monospace} .first-line-outdent .bk_ref {display: inline} .body-content h2, .body-content .h2 {border-bottom: 1px solid #97B0C8} .body-content h2.inline {border-bottom: none} a.page-toc-label , .jig-ncbismoothscroll a {text-decoration:none;border:0 !important} .temp-labeled-list .graphic {display:inline-block !important} .temp-labeled-list img{width:100%}</style><script type="text/javascript" src="/corehtml/pmc/js/jquery.hoverIntent.min.js"> </script><script type="text/javascript" src="/corehtml/pmc/js/common.min.js?_=3.18"> </script><script type="text/javascript" src="/corehtml/pmc/js/large-obj-scrollbars.min.js"> </script><script type="text/javascript">window.name="mainwindow";</script><script type="text/javascript" src="/corehtml/pmc/js/bookshelf/2.26/book-toc.min.js"> </script><script type="text/javascript" src="/corehtml/pmc/js/bookshelf/2.26/books.min.js"> </script><meta name="book-collection" content="NONE" />
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<div class="pre-content"><div><div class="bk_prnt"><p class="small">NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.</p><p>Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. </p></div><div class="iconblock clearfix whole_rhythm no_top_margin bk_noprnt"><a class="img_link icnblk_img" title="Table of Contents Page" href="/books/n/mmed/"><img class="source-thumb" src="/corehtml/pmc/pmcgifs/bookshelf/thumbs/th-mmed-lrg.png" alt="Cover of Medical Microbiology" height="100px" width="80px" /></a><div class="icnblk_cntnt eight_col"><h2>Medical Microbiology. 4th edition.</h2><a data-jig="ncbitoggler" href="#__NBK7617_dtls__">Show details</a><div style="display:none" class="ui-widget" id="__NBK7617_dtls__"><div>Baron S, editor.</div><div>Galveston (TX): <a href="http://www.utmb.edu/" ref="pagearea=page-banner&targetsite=external&targetcat=link&targettype=publisher">University of Texas Medical Branch at Galveston</a>; 1996.</div></div><div class="half_rhythm"><ul class="inline_list"><li style="margin-right:1em"><a class="bk_cntns" href="/books/n/mmed/">Contents</a></li></ul></div><div class="bk_noprnt"><form method="get" action="/books/n/mmed/" id="bk_srch"><div class="bk_search"><label for="bk_term" class="offscreen_noflow">Search term</label><input type="text" title="Search this book" id="bk_term" name="term" value="" data-jig="ncbiclearbutton" /> <input type="submit" class="jig-ncbibutton" value="Search this book" submit="false" style="padding: 0.1em 0.4em;" /></div></form></div></div><div class="icnblk_cntnt two_col"><div class="pagination bk_noprnt"><a class="active page_link prev" href="/books/n/mmed/A438/" title="Previous page in this title">< Prev</a><a class="active page_link next" href="/books/n/mmed/A537/" title="Next page in this title">Next ></a></div></div></div></div></div>
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<div class="main-content lit-style" itemscope="itemscope" itemtype="http://schema.org/CreativeWork"><div class="meta-content fm-sec"><h1 id="_NBK7617_"><span class="label">Chapter 6</span><span class="title" itemprop="name">Normal Flora</span></h1><p class="contrib-group"><span itemprop="author">Charles Patrick Davis</span>.</p></div><div class="jig-ncbiinpagenav body-content whole_rhythm" data-jigconfig="allHeadingLevels: ['h2'],smoothScroll: false" itemprop="text"><div id="mmed_ch6"><h2 id="_mmed_ch6_">General Concepts</h2><div id="A501"><h3>Significance of the Normal Flora</h3><p>The normal flora influences the anatomy, physiology, susceptibility to pathogens,
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and morbidity of the host.</p></div><div id="A502"><h3>Skin Flora</h3><p>The varied environment of the skin results in locally dense or sparse
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populations, with Gram-positive organisms (e.g., staphylococci, micrococci,
|
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diphtheroids) usually predominating.</p></div><div id="A503"><h3>Oral and Upper Respiratory Tract Flora</h3><p>A varied microbial flora is found in the oral cavity, and streptococcal anaerobes
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inhabit the gingival crevice. The pharynx can be a point of entry and initial
|
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colonization for <i>Neisseria</i>, <i>Bordetella</i>,
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<i>Corynebacterium</i>, and <i>Streptococcus</i>
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spp.</p></div><div id="A504"><h3>Gastrointestinal Tract Flora</h3><p>Organisms in the stomach are usually transient, and their populations are kept
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low (10<sup>3</sup> to 10<sup>6</sup>/g of contents) by acidity.
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<i>Helicobacter pylori</i> is a potential stomach pathogen that
|
|
apparently plays a role in the formation of certain ulcer types. In normal hosts
|
|
the duodenal flora is sparse (0 to 10<sup>3</sup>/g of contents). The ileum
|
|
contains a moderately mixed flora (10<sup>6</sup> to 10<sup>8</sup>/g of
|
|
contents). The flora of the large bowel is dense (10<sup>9</sup> to
|
|
10<sup>11</sup>/g of contents) and is composed predominantly of anaerobes.
|
|
These organisms participate in bile acid conversion and in vitamin K and ammonia
|
|
production in the large bowel. They can also cause intestinal abscesses and
|
|
peritonitis.</p></div><div id="A505"><h3>Urogenital Flora</h3><p>The vaginal flora changes with the age of the individual, the vaginal pH, and
|
|
hormone levels. Transient organisms (e.g., <i>Candida</i> spp.)
|
|
frequently cause vaginitis. The distal urethra contains a sparse mixed flora;
|
|
these organisms are present in urine specimens (10<sup>4</sup>/ml) unless a
|
|
clean-catch, midstream specimen is obtained.</p></div><div id="A506"><h3>Conjunctival Flora</h3><p>The conjunctiva harbors few or no organisms. <i>Haemophilus</i> and
|
|
<i>Staphylococcus</i> are among the genera most often
|
|
detected.</p></div><div id="A507"><h3>Host Infection</h3><p>Many elements of the normal flora may act as opportunistic pathogens, especially
|
|
in hosts rendered susceptible by rheumatic heart disease, immunosuppression,
|
|
radiation therapy, chemotherapy, perforated mucous membranes, etc. The flora of
|
|
the gingival crevice causes dental caries in about 80 percent of the
|
|
population.</p></div></div><div id="A508"><h2 id="_A508_">Introduction</h2><p>A diverse microbial flora is associated with the skin and mucous membranes of every
|
|
human being from shortly after birth until death. The human body, which contains
|
|
about 10<sup>13</sup> cells, routinely harbors about 10<sup>14</sup> bacteria (<a class="figpopup" href="/books/NBK7617/figure/A509/?report=objectonly" target="object" rid-figpopup="figA509" rid-ob="figobA509">Fig. 6-1</a>). This bacterial population
|
|
constitutes the <i>normal microbial flora</i> . The normal microbial flora
|
|
is relatively stable, with specific genera populating various body regions during
|
|
particular periods in an individual's life. Microorganisms of the normal flora may
|
|
aid the host (by competing for microenvironments more effectively than such
|
|
pathogens as <i>Salmonella</i> spp or by producing nutrients the host can
|
|
use), may harm the host (by causing dental caries, abscesses, or other infectious
|
|
diseases), or may exist as commensals (inhabiting the host for long periods without
|
|
causing detectable harm or benefit). Even though most elements of the normal
|
|
microbial flora inhabiting the human skin, nails, eyes, oropharynx, genitalia, and
|
|
gastrointestinal tract are harmless in healthy individuals, these organisms
|
|
frequently cause disease in compromised hosts. Viruses and parasites are not
|
|
considered members of the normal microbial flora by most investigators because they
|
|
are not commensals and do not aid the host.</p><div class="iconblock whole_rhythm clearfix ten_col fig" id="figA509" co-legend-rid="figlgndA509"><a href="/books/NBK7617/figure/A509/?report=objectonly" target="object" title="Figure 6-1" class="img_link icnblk_img figpopup" rid-figpopup="figA509" rid-ob="figobA509"><img class="small-thumb" src="/books/NBK7617/bin/ch6f1.gif" src-large="/books/NBK7617/bin/ch6f1.jpg" alt="Figure 6-1. Numbers of bacteria that colonize different parts of the body." /></a><div class="icnblk_cntnt" id="figlgndA509"><h4 id="A509"><a href="/books/NBK7617/figure/A509/?report=objectonly" target="object" rid-ob="figobA509">Figure 6-1</a></h4><p class="float-caption no_bottom_margin">Numbers of bacteria that colonize different parts of the body. Numbers represent the number of organisms per gram of homogenized tissue
|
|
or fluid or per square centimeter of skin surface. </p></div></div></div><div id="A510"><h2 id="_A510_">Significance of the Normal Flora</h2><p>The fact that the normal flora substantially influences the well-being of the host
|
|
was not well understood until germ-free animals became available. Germ-free animals
|
|
were obtained by cesarean section and maintained in special isolators; this allowed
|
|
the investigator to raise them in an environment free from detectable viruses,
|
|
bacteria, and other organisms. Two interesting observations were made about animals
|
|
raised under germ-free conditions. First, the germ-free animals lived almost twice
|
|
as long as their conventionally maintained counterparts, and second, the major
|
|
causes of death were different in the two groups. Infection often caused death in
|
|
conventional animals, but intestinal atonia frequently killed germ-free animals.
|
|
Other investigations showed that germ-free animals have anatomic, physiologic, and
|
|
immunologic features not shared with conventional animals. For example, in germ-free
|
|
animals, the alimentary lamina propria is underdeveloped, little or no
|
|
immunoglobulin is present in sera or secretions, intestinal motility is reduced, and
|
|
the intestinal epithelial cell renewal rate is approximately one-half that of normal
|
|
animals (4 rather than 2 days).</p><p>Although the foregoing indicates that bacterial flora may be undesirable, studies
|
|
with antibiotic treated animals suggest that the flora protects individuals from
|
|
pathogens. Investigators have used streptomycin to reduce the normal flora and have
|
|
then infected animals with streptomycin-resistant <i>Salmonella</i>.
|
|
Normally, about 10<sup>6</sup> organisms are needed to establish a gastrointestinal
|
|
infection, but in streptomycin-treated animals whose flora is altered, fewer than 10
|
|
organisms were needed to cause infectious disease. Further studies suggested that
|
|
fermentation products (acetic and butyric acids) produced by the normal flora
|
|
inhibited <i>Salmonella</i> growth in the gastrointestinal tract. <a class="figpopup" href="/books/NBK7617/figure/A511/?report=objectonly" target="object" rid-figpopup="figA511" rid-ob="figobA511">Figure 6-2</a> shows some of the factors that are
|
|
important in the competition between the normal flora and bacterial pathogens.</p><div class="iconblock whole_rhythm clearfix ten_col fig" id="figA511" co-legend-rid="figlgndA511"><a href="/books/NBK7617/figure/A511/?report=objectonly" target="object" title="Figure 6-2" class="img_link icnblk_img figpopup" rid-figpopup="figA511" rid-ob="figobA511"><img class="small-thumb" src="/books/NBK7617/bin/ch6f2.gif" src-large="/books/NBK7617/bin/ch6f2.jpg" alt="Figure 6-2. Mechanisms by which the normal flora competes with invading pathogens." /></a><div class="icnblk_cntnt" id="figlgndA511"><h4 id="A511"><a href="/books/NBK7617/figure/A511/?report=objectonly" target="object" rid-ob="figobA511">Figure 6-2</a></h4><p class="float-caption no_bottom_margin">Mechanisms by which the normal flora competes with invading
|
|
pathogens. Compare this schematic with Figure
|
|
6-3. </p></div></div><p>The normal flora in humans usually develops in an orderly sequence, or succession,
|
|
after birth, leading to the stable populations of bacteria that make up the normal
|
|
adult flora. The main factor determining the composition of the normal flora in a
|
|
body region is the nature of the local environment, which is determined by pH,
|
|
temperature, redox potential, and oxygen, water, and nutrient levels. Other factors
|
|
such as peristalsis, saliva, lysozyme secretion, and secretion of immunoglobulins
|
|
also play roles in flora control. The local environment is like a concerto in which
|
|
one principal instrument usually dominates. For example, an infant begins to contact
|
|
organisms as it moves through the birth canal. A Gram-positive population
|
|
(bifidobacteria arid lactobacilli) predominates in the gastrointestinal tract early
|
|
in life if the infant is breast-fed. This bacterial population is reduced and
|
|
displaced somewhat by a Gram-negative flora (Enterobacteriaceae) when the baby is
|
|
bottle-fed. The type of liquid diet provided to the infant is the principal
|
|
instrument of this flora control; immunoglobulins and, perhaps, other elements in
|
|
breast milk may also be important.</p><p>What, then, is the significance of the normal flora? Animal and some human studies
|
|
suggest that the flora influences human anatomy, physiology, lifespan, and,
|
|
ultimately, cause of death. Although the causal relationship of flora to death and
|
|
disease in humans is accepted, of her roles of the human microflora need further
|
|
study.</p></div><div id="A512"><h2 id="_A512_">Normal Flora of Skin</h2><p>Skin provides good examples of various microenvironments. Skin regions have been
|
|
compared to geographic regions of Earth: the desert of the forearm, the cool woods
|
|
of the scalp, and the tropical forest of the armpit. The composition of the dermal
|
|
microflora varies from site to site according to the character of the
|
|
microenvironment. A different bacterial flora characterizes each of three regions of
|
|
skin: (1) axilla, perineum, and toe webs; (2) hand, face and trunk; and (3) upper
|
|
arms and legs. Skin sites with partial occlusion (axilla, perineum, and toe webs)
|
|
harbor more microorganisms than do less occluded areas (legs, arms, and trunk).
|
|
These quantitative differences may relate to increased amount of moisture, higher
|
|
body temperature, and greater concentrations of skin surface lipids. The axilla,
|
|
perineum, and toe webs are more frequently colonized by Gram-negative bacilli than
|
|
are drier areas of the skin.</p><p>The number of bacteria on an individual's skin remains relatively constant; bacterial
|
|
survival and the extent of colonization probably depend partly on the exposure of
|
|
skin to a particular environment and partly on the innate and species-specific
|
|
bactericidal activity in skin. Also, a high degree of specificity is involved in the
|
|
adherence of bacteria to epithelial surfaces. Not all bacteria attach to skin;
|
|
staphylococci, which are the major element of the nasal flora, possess a distinct
|
|
advantage over viridans streptococci in colonizing the nasal mucosa. Conversely,
|
|
viridans streptococci are not seen in large numbers on the skin or in the nose but
|
|
dominate the oral flora.</p><p>The microbiology literature is inconsistent about the density of bacteria on the
|
|
skin; one reason for this is the variety of methods used to collect skin bacteria.
|
|
The scrub method yields the highest and most accurate counts for a given skin area.
|
|
Most microorganisms live in the superficial layers of the stratum corneum and in the
|
|
upper parts of the hair follicles. Some bacteria, however, reside in the deeper
|
|
areas of the hair follicles and are beyond the reach of ordinary disinfection
|
|
procedures. These bacteria are a reservoir for recolonization after the surface
|
|
bacteria are removed.</p><div id="A513"><h3>Staphylococcus epidermidis</h3><p><i>S. epidermidis</i> is a major inhabitant of the skin, and in some
|
|
areas it makes up more than 90 percent of the resident aerobic flora.</p></div><div id="A514"><h3>Staphylococcus aureus</h3><p>The nose and perineum are the most common sites for <i>S. aureus</i>
|
|
colonization, which is present in 10 percent to more than 40 percent of normal
|
|
adults. <i>S. aureus</i> is prevalent (67 percent) on vulvar skin. Its
|
|
occurrence in the nasal passages varies with age, being greater in the newborn,
|
|
less in adults. <i>S. aureus</i> is extremely common (80 to 100
|
|
percent) on the skin of patients with certain dermatologic diseases such as
|
|
atopic dermatitis, but the reason for this finding is unclear.</p></div><div id="A515"><h3>Micrococci</h3><p>Micrococci are not as common as staphylococci and diphtheroids; however, they are
|
|
frequently present on normal skin. <i>Micrococcus luteus</i>, the
|
|
predominant species, usually accounts for 20 to 80 percent of the micrococci
|
|
isolated from the skin.</p></div><div id="A516"><h3>Diphtheroids (Coryneforms)</h3><p>The term diphtheroid denotes a wide range of bacteria belonging to the genus
|
|
Corynebacterium. Classification of diphtheroids remains unsatisfactory; for
|
|
convenience, cutaneous diphtheroids have been categorized into the following
|
|
four groups: lipophilic or nonlipophilic diphtheroids; anaerobic diphtheroids;
|
|
diphtheroids producing porphyrins (coral red fluorescence when viewed under
|
|
ultraviolet light); and those that possess some keratinolytic enzymes and are
|
|
associated with trichomycosis axillaris (infection of axillary hair). Lipophilic
|
|
diphtheroids are extremely common in the axilla, whereas nonlipophilic strains
|
|
are found more commonly on glabrous skin.</p><p>Anaerobic diphtheroids are most common in areas rich in sebaceous glands.
|
|
Although the name <i>Corynebacterium</i> acnes was originally used to
|
|
describe skin anaerobic diphtheroids, these are now classified as
|
|
<i>Propionibacterium acnes</i> and as <i>P.
|
|
granulosum</i>. <i>P. acnes</i> is seen eight times more
|
|
frequently than <i>P. granulosum</i> in acne lesions and is probably
|
|
involved in acne pathogenesis. Children younger than 10 years are rarely
|
|
colonized with <i>P. acnes</i>. The appearance of this organism on the
|
|
skin is probably related to the onset of secretion of sebum (a semi-fluid
|
|
substance composed of fatty acids and epithelial debris secreted from sebaceous
|
|
glands) at puberty. <i>P. avidum</i>, the third species of cutaneous
|
|
anaerobic diphtheroids, is rare in acne lesions and is more often isolated from
|
|
the axilla.</p></div><div id="A517"><h3>Streptococci</h3><p>Streptococci, especially β-hemolytic streptococci, are rarely seen on
|
|
normal skin. The paucity of β-hemolytic streptococci on the skin is
|
|
attributed at least in part to the presence of lipids on the skin, as these
|
|
lipids are lethal to streptococci. Other groups of streptococci, such as
|
|
α-hemolytic streptococci, exist primarily in the mouth, from where
|
|
they may, in rare instances, spread to the skin.</p></div><div id="A518"><h3>Gram-Negative Bacilli</h3><p>Gram-negative bacteria make up a small proportion of the skin flora. In view of
|
|
their extraordinary numbers in the gut and in the natural environment, their
|
|
scarcity on skin is striking. They are seen in moist intertriginous areas, such
|
|
as the toe webs and axilla, and not on dry skin. Desiccation is the major factor
|
|
preventing the multiplication of Gram-negative bacteria on intact skin.
|
|
<i>Enterobacter</i>, <i>Klebsiella</i>,
|
|
<i>Escherichia coli</i>, and <i>Proteus</i> spp. are the
|
|
predominant Gram-negative organisms found on the skin.
|
|
<i>Acinetobacter</i> spp also occurs on the skin of normal
|
|
individuals and, like other Gram-negative bacteria, is more common in the moist
|
|
intertriginous areas.</p></div><div id="A519"><h3>Nail Flora</h3><p>The microbiology of a normal nail is generally similar to that of the skin. Dust
|
|
particles and other extraneous materials may get trapped under the nail,
|
|
depending on what the nail contacts. In addition to resident skin flora, these
|
|
dust particles may carry fungi and bacilli. <i>Aspergillus</i>,
|
|
<i>Penicillium</i>, <i>Cladosporium</i>, and
|
|
<i>Mucor</i> are the major types of fungi found under the
|
|
nails.</p></div></div><div id="A520"><h2 id="_A520_">Oral and Upper Respiratory Tract Flora</h2><p>The oral flora is involved in dental caries and periodontal disease, which affect
|
|
about 80 percent. of the population in the Western world. The oral flora, its
|
|
interactions with the host, and its response to environmental factors are
|
|
thoroughly discussed in another Chapter. Anaerobes in the oral flora are
|
|
responsible for many of the brain, face, and lung infections that are frequently
|
|
manifested by abscess formation.</p><p>The pharynx and trachea contain primarily those bacterial genera found in the
|
|
normal oral cavity (for example, α-and β-hemolytic
|
|
streptococci); however, anaerobes, staphylococci, neisseriae, diphtheroids, and
|
|
others are also present. Potentially pathogenic organisms such as
|
|
<i>Haemophilus</i>, mycoplasmas, and pneumococci may also be found
|
|
in the pharynx. Anaerobic organisms also are reported frequently. The upper
|
|
respiratory tract is so often the site of initial colonization by pathogens
|
|
(<i>Neisseria meningitides</i>, <i>C. diphtheriae</i>,
|
|
<i>Bordetella pertussis</i>, and many others) and could be
|
|
considered the first region of attack for such organisms. In contrast, the lower
|
|
respiratory tract (small bronchi and alveoli) is usually sterile, because
|
|
particles the size of bacteria do not readily reach it. If bacteria do reach
|
|
these regions, they encounter host defense mechanisms, such as alveolar
|
|
macrophages, that are not present in the pharynx.</p></div><div id="A521"><h2 id="_A521_">Gastrointestinal Tract Flora</h2><p>The stomach is a relatively hostile environment for bacteria. It contains
|
|
bacteria swallowed with the food and those dislodged from the mouth. Acidity
|
|
lowers the bacterial count, which is highest (approximately 10<sup>3</sup> to
|
|
10<sup>6</sup> organisms/g of contents) after meals and lowest (frequently
|
|
undetectable) after digestion. Some <i>Helicobacter</i> species can
|
|
colonize the stomach and are associated with type B gastritis and peptic ulcer
|
|
disease. Aspirates of duodenal or jejunal fluid contain approximately
|
|
10<sup>3</sup> organisms/ml in most individuals. Most of the bacteria
|
|
cultured (streptococci, lactobacilli, <i>Bacteroides</i>) are thought
|
|
to be transients. Levels of 10<sup>5</sup> to about 10<sup>7</sup> bacteria/ml
|
|
in such aspirates usually indicate an abnormality in the digestive system (for
|
|
example, achlorhydria or malabsorption syndrome). Rapid peristalsis and the
|
|
presence of bile may explain in part the paucity of organisms in the upper
|
|
gastrointestinal tract. Further along the jejunum and into the ileum, bacterial
|
|
populations begin to increase, and at the ileocecal junction they reach levels
|
|
of 10<sup>6</sup> to 10<sup>8</sup> organisms/ml, with streptococci,
|
|
lactobacilli, <i>Bacteroides</i>, and bifidobacteria
|
|
predominating.</p><p>Concentrations of 10<sup>9</sup> to 10<sup>11</sup> bacteria/g of contents are
|
|
frequently found in human colon and feces. This flora includes a bewildering
|
|
array of bacteria (more than 400 species have been identified); nonetheless, 95
|
|
to 99 percent belong to anaerobic genera such as <i>Bacteroides</i>,
|
|
<i>Bifidobacterium</i>, <i>Eubacterium</i>,
|
|
<i>Peptostreptococcus</i>, and <i>Clostridium</i>. In
|
|
this highly anaerobic region of the intestine, these genera proliferate, occupy
|
|
most available niches, and produce metabolic waste products such as acetic,
|
|
butyric, and lactic acids. The strict anaerobic conditions, physical exclusion
|
|
(as is shown in many animal studies), and bacterial waste products are factors
|
|
that inhibit the growth of other bacteria in the large bowel.</p><p>Although the normal flora can inhibit pathogens, many of its members can produce
|
|
disease in humans. Anaerobes in the intestinal tract are the primary agents of
|
|
intra-abdominal abscesses and peritonitis. Bowel perforations produced by
|
|
appendicitis, cancer, infarction, surgery, or gunshot wounds almost always seed
|
|
the peritoneal cavity and adjacent organs with the normal flora. Anaerobes can
|
|
also cause problems within the gastrointestinal lumen. Treatment with
|
|
antibiotics may allow certain anaerobic species to become predominant and cause
|
|
disease. For example, <i>Clostridium difficile</i>, which can remain
|
|
viable in a patient undergoing antimicrobial therapy, may produce
|
|
pseudomembranous colitis. Other intestinal pathologic conditions or surgery can
|
|
cause bacterial overgrowth in the upper small intestine. Anaerobic bacteria can
|
|
then deconjugate bile acids in this region and bind available vitamin
|
|
B<sub>12</sub> so that the vitamin and fats are malabsorbed. In these
|
|
situations, the patient usually has been compromised in some way; therefore, the
|
|
infection caused by the normal intestinal flora is secondary to another
|
|
problem.</p><p>More information is available on the animal than the human microflora. Research
|
|
on animals has revealed that unusual filamentous microorganisms attach to ileal
|
|
epithelial cells and modify host membranes with few or no harmful effects.
|
|
Microorganisms have been observed in thick layers on gastrointestinal surfaces
|
|
(<a class="figpopup" href="/books/NBK7617/figure/A522/?report=objectonly" target="object" rid-figpopup="figA522" rid-ob="figobA522">Fig. 6-3</a>) and in the crypts of
|
|
Lieberkuhn. Other studies indicate that the immune response can be modulated by
|
|
the intestinal flora. Studies of the role of the intestinal flora in
|
|
biosynthesis of vitamin K and other host-utilizable products, conversion of bile
|
|
acids (perhaps to cocarcinogens), and ammonia production (which can play a role
|
|
in hepatic coma) show the dual role of the microbial flora in influencing the
|
|
health of the host. More basic studies of the human bowel flora are necessary to
|
|
define their effect on humans.</p><div class="iconblock whole_rhythm clearfix ten_col fig" id="figA522" co-legend-rid="figlgndA522"><a href="/books/NBK7617/figure/A522/?report=objectonly" target="object" title="Figure 6-3" class="img_link icnblk_img figpopup" rid-figpopup="figA522" rid-ob="figobA522"><img class="small-thumb" src="/books/NBK7617/bin/ch6f3.gif" src-large="/books/NBK7617/bin/ch6f3.jpg" alt="Figure 6-3. (A) Scanning electron micrograph of a cross-section of rat colonic mucosa." /></a><div class="icnblk_cntnt" id="figlgndA522"><h4 id="A522"><a href="/books/NBK7617/figure/A522/?report=objectonly" target="object" rid-ob="figobA522">Figure 6-3</a></h4><p class="float-caption no_bottom_margin">(A) Scanning electron micrograph of a cross-section of rat colonic
|
|
mucosa. The bar indicates the thick layer of bacteria between the
|
|
mucosal surface and the lumen (L) (X 262,) (B) Higher magnification
|
|
of the area indicated by the arrow in Fig. A, showing <a href="/books/NBK7617/figure/A522/?report=objectonly" target="object" rid-ob="figobA522">(more...)</a></p></div></div></div><div id="A523"><h2 id="_A523_">Urogenital Flora</h2><p>The type of bacterial flora found in the vagina depends on the age, pH, and hormonal
|
|
levels of the host. <i>Lactobacillus</i> spp. predominate in female
|
|
infants (vaginal pH, approximately 5) during the first month of life. Glycogen
|
|
secretion seems to cease from about I month of age to puberty. During this time,
|
|
diphtheroids, <i>S. epidermidis</i>, streptococci, and <i>E.
|
|
coli</i> predominate at a higher pH (approximately pH 7). At puberty,
|
|
glycogen secretion resumes, the pH drops, and women acquire an adult flora in which
|
|
<i>L. acidophilus,</i> corynebacteria, peptostreptococci,
|
|
staphylococci, streptococci, and Bacteroides predominate. After menopause, pH again
|
|
rises, less glycogen is secreted, and the flora returns to that found in
|
|
prepubescent females. Yeasts (<i>Torulopsis</i> and
|
|
<i>Candida</i>) are occasionally found in the vagina (10 to 30 percent
|
|
of women); these sometimes increase and cause vaginitis.</p><p>In the anterior urethra of humans, <i>S. epidermidis</i>, enterococci, and
|
|
diphtheroids are found frequently; <i>E. coli</i>,
|
|
<i>Proteus</i>, and <i>Neisseria</i> (nonpathogenic species)
|
|
are reported occasionally (10 to 30 percent). Because of the normal flora residing
|
|
in the urethra, care must be taken in clinically interpreting urine cultures; urine
|
|
samples may contain these organisms at a level of 10<sup>4</sup>/ml if a midstream
|
|
(clean-catch) specimen is not obtained.</p></div><div id="A524"><h2 id="_A524_">Conjunctival Flora</h2><p>The conjunctival flora is sparse. Approximately 17 to 49 percent of culture
|
|
samples are negative. Lysozyme, secreted in tears, may play a role in
|
|
controlling the bacteria by interfering with their cell wall formation. When
|
|
positive samples show bacteria, corynebacteria, Neisseriae, and Moraxellae are
|
|
cultured. Staphylococci and streptococci are also present, and recent reports
|
|
indicate that <i>Haemophilus parainfluenzae</i> is present in 25
|
|
percent of conjunctival samples.</p></div><div id="A525"><h2 id="_A525_">Host Infection by Elements of the Normal Flora</h2><p>This chapter has briefly described the normal human flora; however, the
|
|
pathogenic mechanisms of various genera or the clinical syndromes in which they
|
|
are involved was not discussed. Although such material is presented in other
|
|
chapters, note that a breach in mucosal surfaces often results in infection of
|
|
the host by members of the normal flora. Caries, periodontal disease, abscesses,
|
|
foul-smelling discharges, and endocarditis are hallmarks of infections with
|
|
members of the normal human flora (<a class="figpopup" href="/books/NBK7617/figure/A526/?report=objectonly" target="object" rid-figpopup="figA526" rid-ob="figobA526">Fig.
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6-4</a>). In addition, impairment of the host (for example, those with
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heart failure or leukemia) or host defenses (due to immunosuppression,
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chemotherapy, or irradiation) may result in failure of the normal flora to
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suppress transient pathogens or may cause members of the normal flora to invade
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the host themselves. In either situation, the host may die.</p><div class="iconblock whole_rhythm clearfix ten_col fig" id="figA526" co-legend-rid="figlgndA526"><a href="/books/NBK7617/figure/A526/?report=objectonly" target="object" title="Figure 6-4" class="img_link icnblk_img figpopup" rid-figpopup="figA526" rid-ob="figobA526"><img class="small-thumb" src="/books/NBK7617/bin/ch6f4.gif" src-large="/books/NBK7617/bin/ch6f4.jpg" alt="Figure 6-4. Clinical conditions that may be caused by members of the normal flora." /></a><div class="icnblk_cntnt" id="figlgndA526"><h4 id="A526"><a href="/books/NBK7617/figure/A526/?report=objectonly" target="object" rid-ob="figobA526">Figure 6-4</a></h4><p class="float-caption no_bottom_margin">Clinical conditions that may be caused by members of the normal
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flora. </p></div></div></div><div id="A527"><h2 id="_A527_">References</h2><ol><li><div class="bk_ref" id="A528"> Bitton G, Marshall KC: Adsorption of Microorganisms
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to Surfaces. John Wiley & Sons, New York, 1980 .</div></li><li><div class="bk_ref" id="A529"> Draser BS, Hill MJ: Human Intestinal Flora. Academic
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Press, London, 1974. </div></li><li><div class="bk_ref" id="A530">Freter R, Brickner J, Botney M. et al. Survival and implantation of <em>Escherichia coli</em> in
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the intestinal tract. <span><span class="ref-journal">Infect Immun. </span>1983;<span class="ref-vol">39</span>:686.</span> [<a href="/pmc/articles/PMC348005/" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pmc">PMC free article<span class="bk_prnt">: PMC348005</span></a>] [<a href="https://pubmed.ncbi.nlm.nih.gov/6339389" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pubmed">PubMed<span class="bk_prnt">: 6339389</span></a>]</div></li><li><div class="bk_ref" id="A531">Hentges DJ, Stein AJ, Casey SW, Que JU. Protective role of intestinal flora against <em>Pseudomonas
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aeruginosa</em> in mice: influence of antibiotics on colonization
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resistance. <span><span class="ref-journal">Infect Immun. </span>1985;<span class="ref-vol">47</span>:118.</span> [<a href="/pmc/articles/PMC261485/" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pmc">PMC free article<span class="bk_prnt">: PMC261485</span></a>] [<a href="https://pubmed.ncbi.nlm.nih.gov/2856912" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pubmed">PubMed<span class="bk_prnt">: 2856912</span></a>]</div></li><li><div class="bk_ref" id="A532">Herthelius M, Gorbach SL, Mollby R. et al. Elimination of vaginal colonization with <em>Escherichia
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coli</em> by administration of indigenous flora. <span><span class="ref-journal">Infect Immun. </span>1989;<span class="ref-vol">57</span>:2447.</span> [<a href="/pmc/articles/PMC313468/" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pmc">PMC free article<span class="bk_prnt">: PMC313468</span></a>] [<a href="https://pubmed.ncbi.nlm.nih.gov/2663724" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pubmed">PubMed<span class="bk_prnt">: 2663724</span></a>]</div></li><li><div class="bk_ref" id="A533"> Maibach H, Aly R: Skin Microbiology: Relevance to
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Clinical Infection. Springer-Verlag, New York, 1981 .</div></li><li><div class="bk_ref" id="A534">Marples MJ. Life in the skin. <span><span class="ref-journal">Sci Am. </span>1969;<span class="ref-vol">220</span>:108.</span> [<a href="https://pubmed.ncbi.nlm.nih.gov/5761729" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pubmed">PubMed<span class="bk_prnt">: 5761729</span></a>]</div></li><li><div class="bk_ref" id="A535">Savage DC. Microbial ecology of the gastrointestinal tract. <span><span class="ref-journal">Annu Rev Microbiol. </span>1977;<span class="ref-vol">31</span>:107.</span> [<a href="https://pubmed.ncbi.nlm.nih.gov/334036" ref="pagearea=cite-ref&targetsite=entrez&targetcat=link&targettype=pubmed">PubMed<span class="bk_prnt">: 334036</span></a>]</div></li><li><div class="bk_ref" id="A536"> Tannock GW: Normal Microflora. Chapman and
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Hall,London, UK, 1995 .</div></li></ol></div><div id="bk_toc_contnr"></div></div></div>
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<div class="post-content"><div><div class="half_rhythm"><a href="/books/about/copyright/">Copyright</a> © 1996, The University of Texas Medical Branch
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at Galveston.</div><div class="small"><span class="label">Bookshelf ID: NBK7617</span><span class="label">PMID: <a href="https://pubmed.ncbi.nlm.nih.gov/21413249" title="PubMed record of this page" ref="pagearea=meta&targetsite=entrez&targetcat=link&targettype=pubmed">21413249</a></span></div><div style="margin-top:2em" class="bk_noprnt"><a class="bk_cntns" href="/books/n/mmed/">Contents</a><div class="pagination bk_noprnt"><a class="active page_link prev" href="/books/n/mmed/A438/" title="Previous page in this title">< Prev</a><a class="active page_link next" href="/books/n/mmed/A537/" title="Next page in this title">Next ></a></div></div></div></div>
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<div xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"></div><div class="portlet"><div class="portlet_head"><div class="portlet_title"><h3><span>Views</span></h3></div><a name="Shutter" sid="1" href="#" class="portlet_shutter" title="Show/hide content" remembercollapsed="true" pgsec_name="PDF_download" id="Shutter"></a></div><div class="portlet_content"><ul xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="simple-list"><li><a href="/books/NBK7617/?report=reader">PubReader</a></li><li><a href="/books/NBK7617/?report=printable">Print View</a></li><li><a data-jig="ncbidialog" href="#_ncbi_dlg_citbx_NBK7617" data-jigconfig="width:400,modal:true">Cite this Page</a><div id="_ncbi_dlg_citbx_NBK7617" style="display:none" title="Cite this Page"><div class="bk_tt">Davis CP. Normal Flora. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 6.<span class="bk_cite_avail"></span></div></div></li></ul></div></div><div class="portlet"><div class="portlet_head"><div class="portlet_title"><h3><span>In this Page</span></h3></div><a name="Shutter" sid="1" href="#" class="portlet_shutter" title="Show/hide content" remembercollapsed="true" pgsec_name="page-toc" id="Shutter"></a></div><div class="portlet_content"><ul xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="simple-list"><li><a href="#mmed_ch6" ref="log$=inpage&link_id=inpage">General Concepts</a></li><li><a href="#A508" ref="log$=inpage&link_id=inpage">Introduction</a></li><li><a href="#A510" ref="log$=inpage&link_id=inpage">Significance of the Normal Flora</a></li><li><a href="#A512" ref="log$=inpage&link_id=inpage">Normal Flora of Skin</a></li><li><a href="#A520" ref="log$=inpage&link_id=inpage">Oral and Upper Respiratory Tract Flora</a></li><li><a href="#A521" ref="log$=inpage&link_id=inpage">Gastrointestinal Tract Flora</a></li><li><a href="#A523" ref="log$=inpage&link_id=inpage">Urogenital Flora</a></li><li><a href="#A524" ref="log$=inpage&link_id=inpage">Conjunctival Flora</a></li><li><a href="#A525" ref="log$=inpage&link_id=inpage">Host Infection by Elements of the Normal Flora</a></li><li><a href="#A527" ref="log$=inpage&link_id=inpage">References</a></li></ul></div></div><div class="portlet"><div class="portlet_head"><div class="portlet_title"><h3><span>More on the Subject in Bookshelf</span></h3></div><a name="Shutter" sid="1" href="#" class="portlet_shutter" title="Show/hide content" remembercollapsed="true" pgsec_name="source-links" id="Shutter"></a></div><div class="portlet_content"><ul xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="simple-list"><li><a href="https://www.ncbi.nlm.nih.gov/books?term=%22Microbiology%20Resources%22%5BResource%20Type%5D" ref="pagearea=source-links&targetsite=external&targetcat=link&targettype=uri">All Microbiology Resources</a></li></ul></div></div><div class="portlet"><div class="portlet_head"><div class="portlet_title"><h3><span>Related Items in Bookshelf</span></h3></div><a name="Shutter" sid="1" href="#" class="portlet_shutter" title="Show/hide content" remembercollapsed="true" pgsec_name="source-links" id="Shutter"></a></div><div class="portlet_content"><ul xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="simple-list"><li><a href="https://www.ncbi.nlm.nih.gov/books?term="textbooks"%5BResource%20Type%5D" ref="pagearea=source-links&targetsite=external&targetcat=link&targettype=uri">All Textbooks</a></li></ul></div></div><div class="portlet"><div class="portlet_head"><div class="portlet_title"><h3><span>Related information</span></h3></div><a name="Shutter" sid="1" href="#" class="portlet_shutter" title="Show/hide content" remembercollapsed="true" pgsec_name="discovery_db_links" id="Shutter"></a></div><div class="portlet_content"><ul><li class="brieflinkpopper"><a class="brieflinkpopperctrl" href="/books/?Db=pmc&DbFrom=books&Cmd=Link&LinkName=books_pmc_refs&IdsFromResult=2316760" ref="log$=recordlinks">PMC</a><div class="brieflinkpop offscreen_noflow">PubMed Central citations</div></li><li class="brieflinkpopper"><a class="brieflinkpopperctrl" href="/books/?Db=pubmed&DbFrom=books&Cmd=Link&LinkName=books_pubmed_refs&IdsFromResult=2316760" ref="log$=recordlinks">PubMed</a><div class="brieflinkpop offscreen_noflow">Links to PubMed</div></li></ul></div></div><div class="portlet"><div class="portlet_head"><div class="portlet_title"><h3><span>Similar articles in PubMed</span></h3></div><a name="Shutter" sid="1" href="#" class="portlet_shutter" title="Show/hide content" remembercollapsed="true" pgsec_name="PBooksDiscovery_RA" id="Shutter"></a></div><div class="portlet_content"><ul><li class="brieflinkpopper two_line"><a class="brieflinkpopperctrl" href="/pubmed/21515397" ref="ordinalpos=1&linkpos=1&log$=relatedreviews&logdbfrom=pubmed"><span xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="invert">Review</span> Immunology and probiotic impact of the newborn and young children intestinal microflora.</a><span class="source">[Anaerobe. 2011]</span><div class="brieflinkpop offscreen_noflow"><span xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="invert">Review</span> Immunology and probiotic impact of the newborn and young children intestinal microflora.<div class="brieflinkpopdesc"><em xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="author">Bezirtzoglou E, Stavropoulou E. </em><em xmlns:np="http://ncbi.gov/portal/XSLT/namespace" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" class="cit">Anaerobe. 2011 Dec; 17(6):369-74. 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