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You are watching: Normal microbiota may cause disease if they are introduced into an unusual site in the body.

Baron S, editor. Clinical Microbiology. Fourth edition. Galveston (TX): college of Texas medical Branch in ~ Galveston; 1996.


Significance of the normal Flora

The regular flora influences the anatomy, physiology, susceptibility come pathogens,and morbidity of the host.

Skin Flora

The varied atmosphere of the skin results in locally thick or sparsepopulations, v Gram-positive biology (e.g., staphylococci, micrococci,diphtheroids) generally predominating.

Oral and also Upper respiratory Tract Flora

A differed microbial flora is found in the oral cavity, and also streptococcal anaerobesinhabit the gingival crevice. The pharynx deserve to be a suggest of entry and also initialcolonization for Neisseria, Bordetella,Corynebacterium, and Streptococcusspp.

Gastrointestinal street Flora

Organisms in the stomach room usually transient, and also their populaces are keptlow (103 come 106/g the contents) by acidity.Helicobacter pylori is a potential stomach pathogen thatapparently theatre a role in the formation of details ulcer types. In typical hoststhe duodenal flora is sporadic (0 come 103/g the contents). The ileumcontains a moderately blended flora (106 come 108/g ofcontents). The flora the the huge bowel is thick (109 to1011/g of contents) and is composed mainly of anaerobes.These organisms participate in bile acid conversion and in vitamin K and ammoniaproduction in the big bowel. Lock can additionally cause intestinal abscesses andperitonitis.

Urogenital Flora

The vaginal flora transforms with the age of the individual, the vaginal pH, andhormone levels. Transient biology (e.g., Candida spp.)frequently cause vaginitis. The distal urethra has a sparse mixed flora;these biology are current in urine specimens (104/ml) uneven aclean-catch, midstream specimen is obtained.

Conjunctival Flora

The conjunctiva harbors couple of or no organisms. Haemophilus andStaphylococcus are among the genera many oftendetected.

Host Infection

Many aspects of the normal flora might act as opportunistic pathogens, especiallyin hosts rendered at risk by rheumatic love disease, immunosuppression,radiation therapy, chemotherapy, perforated mucous membranes, etc. The flora ofthe gingival crevice causes dental caries in around 80 percent that thepopulation.


A diverse microbial flora is linked with the skin and also mucous membrane of everyhuman gift from soon after birth till death. The person body, i beg your pardon containsabout 1013 cells, regularly harbors around 1014 bacteria (Fig. 6-1). This bacterial populationconstitutes the regular microbial flora . The normal microbial florais reasonably stable, with specific genera populating assorted body areas duringparticular periods in an individual"s life. Microbe of the normal flora mayaid the organize (by competing for microenvironments an ext effectively than suchpathogens as Salmonella spp or by creating nutrients the host canuse), may damage the hold (by causing dental caries, abscesses, or various other infectiousdiseases), or may exist as commensals (inhabiting the organize for lengthy periods withoutcausing detectable damage or benefit). Even though most aspects of the normalmicrobial flora inhabiting the person skin, nails, eyes, oropharynx, genitalia, andgastrointestinal tract room harmless in healthy individuals, these organismsfrequently cause an illness in compromised hosts. Viruses and also parasites are notconsidered members that the typical microbial flora by many investigators because theyare no commensals and do not assist the host.


Figure 6-1

Numbers the bacteria the colonize various parts the the body. Numbers represent the variety of organisms every gram the homogenized tissueor liquid or per square centimeter the skin surface.

Significance the the regular Flora

The fact that the regular flora substantially impacts the health of the hostwas not well interpreted until germ-free animals became available. Germ-free animalswere acquired by cesarean section and also maintained in special isolators; this allowedthe investigator to raise them in an environment complimentary from detectable viruses,bacteria, and also other organisms. Two exciting observations were made around animalsraised under germ-free conditions. First, the germ-free animals lived practically twiceas long as their traditionally maintained counterparts, and also second, the majorcauses of death were various in the two groups. Infection often caused death inconventional animals, but intestinal atonia commonly killed germ-free animals.Other investigations proved that germ-free animals have anatomic, physiologic, andimmunologic functions not mutual with traditional animals. Because that example, in germ-freeanimals, the alimentary lamina propria is underdeveloped, tiny or noimmunoglobulin is existing in sera or secretions, intestinal motility is reduced, andthe minister epithelial cell renewal price is approximately one-half the of normalanimals (4 rather than 2 days).

Although the foregoing suggests that bacter flora may be undesirable, studieswith antibiotic treated animals suggest that the flora protects individuals frompathogens. Investigators have used streptomycin to minimize the typical flora and havethen infected animals with streptomycin-resistant Salmonella.Normally, around 106 organisms are required to create a gastrointestinalinfection, yet in streptomycin-treated pets whose flora is altered, fewer 보다 10organisms were needed to reason infectious disease. Additional studies said thatfermentation assets (acetic and also butyric acids) developed by the normal florainhibited Salmonella development in the cradle tract. Figure 6-2 shows some of the factors that areimportant in the competition between the normal flora and bacterial pathogens.


Figure 6-2

Mechanisms by which the common flora competes v invadingpathogens. To compare this schematic with Figure6-3.

The regular flora in human beings usually establishes in an orderly sequence, or succession,after birth, bring about the stable populaces of bacteria that make up the normaladult flora. The main variable determining the composition of the normal flora in abody region is the nature that the neighborhood environment, i beg your pardon is identified by pH,temperature, oxidation potential, and oxygen, water, and nutrient levels. Other factorssuch as peristalsis, saliva, lysozyme secretion, and also secretion the immunoglobulinsalso play duties in flora control. The local atmosphere is favor a concerto in whichone principal instrument usually dominates. Because that example, an infant begins to contactorganisms together it moves v the bear canal. A Gram-positive population(bifidobacteria arid lactobacilli) predominates in the gastrointestinal street earlyin life if the child is breast-fed. This bacterial population is lessened anddisplaced somewhat by a Gram-negative flora (Enterobacteriaceae) as soon as the baby isbottle-fed. The type of fluid diet detailed to the child is the principalinstrument that this flora control; immunoglobulins and, perhaps, other elements inbreast milk may additionally be important.

What, then, is the significance of the regular flora? Animal and some human studiessuggest that the flora influences human being anatomy, physiology, lifespan, and,ultimately, cause of death. Return the causal partnership of flora to death anddisease in human beings is accepted, of her duties of the person microflora need furtherstudy.

Normal Flora that Skin

Skin provides great examples of various microenvironments. Skin regions have beencompared to geographic regions that Earth: the desert the the forearm, the cool woodsof the scalp, and also the tropical forest of the armpit. The ingredient of the dermalmicroflora varies from website to website according to the personality of themicroenvironment. A various bacterial flora characterizes every of three areas ofskin: (1) axilla, perineum, and also toe webs; (2) hand, face and also trunk; and (3) upperarms and also legs. Skin sites through partial occlusion (axilla, perineum, and toe webs)harbor an ext microorganisms than do much less occluded areas (legs, arms, and trunk).These quantitative distinctions may called to increased amount that moisture, higherbody temperature, and greater concentration of skin surface ar lipids. The axilla,perineum, and also toe webs are more frequently conquered by Gram-negative bacilli thanare drier areas of the skin.

The number of bacteria on one individual"s skin remains fairly constant; bacterialsurvival and the extent of colonization probably depend partly on the exposure ofskin to a particular environment and partly on the innate and also species-specificbactericidal task in skin. Also, a high degree of specificity is affiliated in theadherence the bacteria come epithelial surfaces. No all bacteria attach to skin;staphylococci, which space the major element the the sleep flora, own a distinctadvantage end viridans streptococci in colonizing the nasal mucosa. Conversely,viridans streptococci room not seen in big numbers top top the skin or in the sleep butdominate the oral flora.

The microbiology literature is inconsistent around the density of bacteria ~ above theskin; one reason for this is the range of techniques used to collect skin bacteria.The scrub method yields the highest and also most precise counts because that a provided skin area.Most microbe live in the superficial layers of the stratum corneum and also in theupper components of the hair follicles. Some bacteria, however, reside in the deeperareas the the hair follicles and also are beyond the with of plain disinfectionprocedures. These bacteria are a reservoir for recolonization ~ the surfacebacteria are removed.

Staphylococcus epidermidis

S. Epidermidis is a significant inhabitant that the skin, and in someareas it makes up more than 90 percent of the residents aerobic flora.

Staphylococcus aureus

The nose and also perineum are the most common sites for S. Aureuscolonization, i m sorry is current in 10 percent to an ext than 40 percent of normaladults. S. Aureus is widespread (67 percent) top top vulvar skin. Itsoccurrence in the sleep passages varies through age, being better in the newborn,less in adults. S. Aureus is extremely typical (80 to 100percent) ~ above the skin of patients with specific dermatologic illness such asatopic dermatitis, yet the reason for this recognize is unclear.


Micrococci space not as usual as staphylococci and diphtheroids; however, lock arefrequently present on regular skin. Micrococcus luteus, thepredominant species, commonly accounts because that 20 come 80 percent of the micrococciisolated indigenous the skin.

Diphtheroids (Coryneforms)

The hatchet diphtheroid denotes a wide range of bacteria belonging to the genusCorynebacterium. Category of diphtheroids continues to be unsatisfactory; forconvenience, cutaneous diphtheroids have been categorized into the followingfour groups: lipophilic or nonlipophilic diphtheroids; anaerobic diphtheroids;diphtheroids creating porphyrins (coral red fluorescence when viewed underultraviolet light); and also those the possess part keratinolytic enzymes and also areassociated through trichomycosis axillaris (infection of axillary hair). Lipophilicdiphtheroids room extremely usual in the axilla, conversely, nonlipophilic strainsare found an ext commonly top top glabrous skin.

Anaerobic diphtheroids are most typical in areas rich in sebaceous glands.Although the name Corynebacterium acnes was originally offered todescribe skin anaerobic diphtheroids, this are now classified asPropionibacterium acnes and as P.granulosum. P. Acnes is checked out eight time morefrequently than P. Granulosum in acne lesions and also is probablyinvolved in acne pathogenesis. Youngsters younger than 10 years are rarelycolonized through P. Acnes. The illustration of this organism on theskin is probably related come the onset of cheap of sebum (a semi-fluidsubstance written of fatty acids and also epithelial debris secreted from sebaceousglands) at puberty. P. Avidum, the third varieties of cutaneousanaerobic diphtheroids, is rarely in acne lesions and also is much more often isolated fromthe axilla.


Streptococci, specifically β-hemolytic streptococci, are seldom seen onnormal skin. The paucity the β-hemolytic streptococci top top the skin isattributed at least in part to the visibility of lipids on the skin, as theselipids are lethal to streptococci. Other groups of streptococci, such asα-hemolytic streptococci, exist mainly in the mouth, from wherethey may, in rarely instances, spread to the skin.

Gram-Negative Bacilli

Gram-negative bacteria consist of a tiny proportion the the skin flora. In see oftheir extraordinary number in the gut and also in the herbal environment, theirscarcity ~ above skin is striking. They room seen in moist intertriginous areas, suchas the toes webs and axilla, and not on dry skin. Desiccation is the significant factorpreventing the multiplication the Gram-negative bacteria on intact skin.Enterobacter, Klebsiella,Escherichia coli, and Proteus spp. Are thepredominant Gram-negative organisms discovered on the skin.Acinetobacter spp likewise occurs top top the skin that normalindividuals and, like other Gram-negative bacteria, is much more common in the moistintertriginous areas.

Nail Flora

The microbiology of a common nail is generally similar to that of the skin. Dustparticles and also other extraneous materials may acquire trapped under the nail,depending on what the nail contacts. In addition to residents skin flora, thesedust corpuscle may bring fungi and bacilli. Aspergillus,Penicillium, Cladosporium, andMucor room the significant types that fungi discovered under thenails.

Oral and Upper respiratory Tract Flora

The dental flora is connected in dental caries and periodontal disease, i m sorry affectabout 80 percent. Of the population in the western world. The dental flora, itsinteractions v the host, and its solution to environmental factors arethoroughly debated in another Chapter. Anaerobes in the dental flora areresponsible for countless of the brain, face, and also lung epidemic that are frequentlymanifested by abscess formation.

The pharynx and trachea contain primarily those bacter genera discovered in thenormal oral cavity (for example, α-and β-hemolyticstreptococci); however, anaerobes, staphylococci, neisseriae, diphtheroids, andothers are additionally present. Potentially pathogenic organisms such asHaemophilus, mycoplasmas, and pneumococci may likewise be foundin the pharynx. Anaerobic organisms likewise are report frequently. The upperrespiratory tract is so often the website of initial colonization by pathogens(Neisseria meningitides, C. Diphtheriae,Bordetella pertussis, and also many others) and could beconsidered the first region of assault for such organisms. In contrast, the lowerrespiratory street (small bronchi and alveoli) is normally sterile, becauseparticles the dimension of bacteria do not easily reach it. If bacteria do reachthese regions, they encounter host defense mechanisms, such together alveolarmacrophages, that are not existing in the pharynx.

Gastrointestinal street Flora

The stomach is a reasonably hostile atmosphere for bacteria. The containsbacteria swallowed v the food and also those dislodged from the mouth. Aciditylowers the bacterial count, i beg your pardon is highest possible (approximately 103 to106 organisms/g of contents) ~ meals and lowest (frequentlyundetectable) ~ digestion. Some Helicobacter species cancolonize the stomach and are connected with kind B gastritis and peptic ulcerdisease. Aspirates that duodenal or jejunal liquid contain approximately103 organisms/ml in most individuals. Many of the bacteriacultured (streptococci, lactobacilli, Bacteroides) are thoughtto be transients. Levels of 105 to around 107 bacteria/mlin together aspirates usually indicate an abnormality in the digestive mechanism (forexample, achlorhydria or malabsorption syndrome). Rapid peristalsis and thepresence of bile may explain in component the paucity of biology in the uppergastrointestinal tract. Additional along the jejunum and also into the ileum, bacterialpopulations begin to increase, and at the ileocecal junction they with levelsof 106 to 108 organisms/ml, v streptococci,lactobacilli, Bacteroides, and bifidobacteriapredominating.

Concentrations of 109 come 1011 bacteria/g of materials arefrequently uncovered in human being colon and feces. This flora includes a bewilderingarray that bacteria (more than 400 species have been identified); nonetheless, 95to 99 percent belong to anaerobic genera such as Bacteroides,Bifidobacterium, Eubacterium,Peptostreptococcus, and Clostridium. Inthis extremely anaerobic an ar of the intestine, these genera proliferate, occupymost available niches, and also produce metabolic waste products such as acetic,butyric, and also lactic acids. The strict anaerobic conditions, physics exclusion(as is shown in many pet studies), and bacterial waste commodities are factorsthat inhibit the growth of other bacteria in the big bowel.

Although the normal flora have the right to inhibit pathogens, numerous of that members deserve to producedisease in humans. Anaerobes in the minister tract room the primary agents ofintra-abdominal abscesses and also peritonitis. Bowel perforations developed byappendicitis, cancer, infarction, surgery, or gunshot wounds virtually always seedthe peritoneal cavity and nearby organs v the normal flora. Anaerobes canalso cause problems in ~ the gastrointestinal lumen. Therapy withantibiotics may allow certain anaerobic species to become predominant and also causedisease. For example, Clostridium difficile, which have the right to remainviable in a patient undergoing antimicrobial therapy, might producepseudomembranous colitis. Other intestinal pathologic conditions or surgical procedure cancause bacter overgrowth in the upper small intestine. Anaerobic bacteria canthen deconjugate bile mountain in this an ar and bind easily accessible vitaminB12 so that the vitamin and also fats are malabsorbed. In thesesituations, the patience usually has actually been endangered in part way; therefore, theinfection caused by the regular intestinal flora is an additional to anotherproblem.

More info is obtainable on the animal than the human microflora. Researchon animals has revealed that unusual filamentous microorganisms attach to ilealepithelial cells and also modify hold membranes with few or no harmful effects.Microorganisms have actually been observed in special layers on cradle surfaces(Fig. 6-3) and also in the crypts ofLieberkuhn. Various other studies suggest that the immune an answer can be modulated bythe intestinal flora. Researches of the function of the intestinal flora inbiosynthesis of vitamin K and other host-utilizable products, conversion of bileacids (perhaps to cocarcinogens), and also ammonia manufacturing (which have the right to play a rolein hepatic coma) present the dual role of the microbial flora in influencing thehealth the the host. More basic researches of the human being bowel flora are necessary todefine their result on humans.


Figure 6-3

(A) Scanning electron micrograph of a cross-section the rat colonicmucosa. The bar suggests the special layer that bacteria between themucosal surface and also the lumen (L) (X 262,) (B) higher magnificationof the area shown by the arrowhead in Fig. A, reflecting (more...)

Urogenital Flora

The kind of bacter flora uncovered in the vagina relies on the age, pH, and also hormonallevels of the host. Lactobacillus spp. Predominate in femaleinfants (vaginal pH, around 5) during the first month of life. Glycogensecretion appears to stop from about I month of age to puberty. During this time,diphtheroids, S. Epidermidis, streptococci, and E.coli predominate in ~ a greater pH (approximately pH 7). In ~ puberty,glycogen cheap resumes, the pH drops, and women obtain an adult flora in whichL. Acidophilus, corynebacteria, peptostreptococci,staphylococci, streptococci, and Bacteroides predominate. After ~ menopause, pH againrises, much less glycogen is secreted, and also the flora return to that found inprepubescent females. Yeasts (Torulopsis andCandida) are occasionally discovered in the vagina (10 to 30 percentof women); these periodically increase and also cause vaginitis.

In the anterior urethra the humans, S. Epidermidis, enterococci, anddiphtheroids are found frequently; E. Coli,Proteus, and Neisseria (nonpathogenic species)are reported periodically (10 come 30 percent). Because of the typical flora residingin the urethra, treatment must be taken in clinically interpreting to pee cultures; urinesamples might contain these organisms in ~ a level the 104/ml if a midstream(clean-catch) specimen is no obtained.

Conjunctival Flora

The conjunctival flora is sparse. Roughly 17 come 49 percent the culturesamples are negative. Lysozyme, secreted in tears, might play a role incontrolling the bacteria through interfering v their cell wall surface formation. Whenpositive samples present bacteria, corynebacteria, Neisseriae, and Moraxellae arecultured. Staphylococci and streptococci are likewise present, and recent reportsindicate that Haemophilus parainfluenzae is existing in 25percent the conjunctival samples.

Host infection by aspects of the normal Flora

This chapter has briefly explained the normal human flora; however, thepathogenic mechanisms of various genera or the clinical syndromes in i m sorry theyare involved was no discussed. Back such product is presented in otherchapters, note that a breach in mucosal surfaces often results in infection ofthe host by members the the common flora. Caries, periodontal disease, abscesses,foul-smelling discharges, and endocarditis room hallmarks of infections withmembers the the normal human flora (Fig.6-4). In addition, special needs of the host (for example, those withheart fail or leukemia) or organize defenses (due to immunosuppression,chemotherapy, or irradiation) may result in failure of the normal flora tosuppress transient pathogens or may reason members the the typical flora to invadethe hold themselves. In one of two people situation, the host may die.


Bitton G, Marshall KC: Adsorption the Microorganismsto Surfaces. Man Wiley & Sons, new York, 1980 .

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Maibach H, Aly R: Skin Microbiology: relationship toClinical Infection. Springer-Verlag, brand-new York, 1981 .