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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. 4th edition. Galveston (TX): university of Texas medical Branch at Galveston; 1996.


Significance of the regular Flora

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

Skin Flora

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

Oral and also Upper respiratory tract Tract Flora

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

Gastrointestinal tract Flora

Organisms in the stomach room usually transient, and their populaces are keptlow (103 come 106/g of contents) by acidity.Helicobacter pylori is a potential stomach microorganism thatapparently plays a function in the development of details ulcer types. In normal hoststhe duodenal flora is sparse (0 come 103/g the contents). The ileumcontains a moderately combined flora (106 come 108/g ofcontents). The flora the the big bowel is thick (109 to1011/g the contents) and also is composed predominantly of anaerobes.These organisms participate in bile acid conversion and also in vitamin K and ammoniaproduction in the huge bowel. They can likewise cause minister abscesses andperitonitis.

Urogenital Flora

The quality flora transforms with the period of the individual, the quality pH, andhormone levels. Transient organisms (e.g., Candida spp.)frequently reason vaginitis. The distal urethra consists of a sparse combined flora;these organisms are present in urine specimens (104/ml) unless aclean-catch, midstream specimen is obtained.

Conjunctival Flora

The conjunctiva harbors few or no organisms. Haemophilus andStaphylococcus are among the genera most oftendetected.

Host Infection

Many elements of the normal flora may act as opportunistic pathogens, especiallyin hosts rendered prone by rheumatic heart disease, immunosuppression,radiation therapy, chemotherapy, perforated mucous membranes, etc. The flora ofthe gingival crevice reasons dental caries in around 80 percent of thepopulation.


A diverse microbial flora is linked with the skin and mucous membranes of everyhuman being from shortly after birth until death. The human being body, which containsabout 1013 cells, on regular basis harbors about 1014 bacteria (Fig. 6-1). This bacterial populationconstitutes the regular microbial flora . The regular microbial florais relatively stable, with certain genera populating miscellaneous body areas duringparticular durations in one individual"s life. Microorganisms of the regular flora mayaid the hold (by competing for microenvironments much more effectively 보다 suchpathogens together Salmonella spp or by developing nutrients the organize canuse), may damage the hold (by leading to dental caries, abscesses, or various other infectiousdiseases), or may exist together commensals (inhabiting the hold for lengthy periods withoutcausing detectable injury or benefit). Also though most elements of the normalmicrobial flora inhabiting the human being skin, nails, eyes, oropharynx, genitalia, andgastrointestinal tract space harmless in healthy individuals, these organismsfrequently cause disease in jeopardized hosts. Viruses and also parasites are notconsidered members of the typical microbial flora by most investigators because theyare no commensals and do not aid the host.


Figure 6-1

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

Significance that the regular Flora

The fact that the normal flora substantially impacts the well-being of the hostwas no well taken until germ-free animals became available. Germ-free animalswere derived by cesarean section and also maintained in unique isolators; this allowedthe investigator come raise them in an environment totally free from detectable viruses,bacteria, and other organisms. Two interesting observations to be made around animalsraised under germ-free conditions. First, the germ-free pets lived almost twiceas long as their traditionally maintained counterparts, and second, the majorcauses of death were various in the two groups. Infection frequently caused fatality inconventional animals, yet intestinal atonia generally killed germ-free animals.Other investigations proved that germ-free pets have anatomic, physiologic, andimmunologic attributes not mutual with conventional animals. For example, in germ-freeanimals, the alimentary lamina propria is underdeveloped, little or noimmunoglobulin is current in sera or secretions, minister motility is reduced, andthe intestinal epithelial cabinet renewal rate is around one-half that of normalanimals (4 fairly than 2 days).

Although the foregoing shows that bacterial flora might be undesirable, studieswith antibiotic treated animals suggest the the flora protects individuals frompathogens. Investigators have actually used streptomycin to mitigate the normal flora and havethen infected animals with streptomycin-resistant Salmonella.Normally, about 106 biology are required to establish a gastrointestinalinfection, however in streptomycin-treated pets whose flora is altered, fewer 보다 10organisms were needed to cause infectious disease. More studies argued thatfermentation assets (acetic and butyric acids) created by the typical florainhibited Salmonella development in the cradle tract. Figure 6-2 shows some the the determinants that areimportant in the competition in between the regular flora and bacterial pathogens.


Figure 6-2

Mechanisms by which the typical flora competes v invadingpathogens. Compare this schematic with Figure6-3.

The common flora in human beings usually develops in an orderly sequence, or succession,after birth, leading to the stable populaces of bacteria that consist of the normaladult flora. The main variable determining the ingredient of the regular flora in abody region is the nature that the regional environment, i m sorry is determined by pH,temperature, oxidation potential, and also oxygen, water, and nutrient levels. Other factorssuch as peristalsis, saliva, lysozyme secretion, and secretion that immunoglobulinsalso play roles in flora control. The local atmosphere is choose a concerto in whichone major instrument usually dominates. For example, an infant starts to contactorganisms together it moves through the birth canal. A Gram-positive population(bifidobacteria arid lactobacilli) predominates in the gastrointestinal street earlyin life if the infant is breast-fed. This bacterial populace is diminished anddisplaced somewhat by a Gram-negative flora (Enterobacteriaceae) once the baby isbottle-fed. The type of fluid diet provided to the infant is the principalinstrument of this flora control; immunoglobulins and, perhaps, other aspects inbreast milk may likewise be important.

What, then, is the meaning of the common flora? Animal and some human studiessuggest the the flora influences person anatomy, physiology, lifespan, and,ultimately, cause of death. Although the causal connection of flora to death anddisease in human beings is accepted, the her duties of the human being microflora need furtherstudy.

Normal Flora the Skin

Skin provides good examples of miscellaneous microenvironments. Skin regions have beencompared to geographical regions the Earth: the desert the the forearm, the cool woodsof the scalp, and also the tropical forest of the armpit. The ingredient of the dermalmicroflora different from site to website according come the personality of themicroenvironment. A various bacterial flora characterizes each of three regions ofskin: (1) axilla, perineum, and also toe webs; (2) hand, face and trunk; and also (3) upperarms and legs. Skin sites with partial occlusion (axilla, perineum, and toe webs)harbor much more microorganisms 보다 do much less occluded areas (legs, arms, and also trunk).These quantitative differences may relate to enhanced amount that moisture, higherbody temperature, and greater concentrations of skin surface ar lipids. The axilla,perineum, and toe network are an ext frequently conquered by Gram-negative bacilli thanare drier locations of the skin.

The number of bacteria on one individual"s skin remains reasonably constant; bacterialsurvival and also the level of colonization probably depend partly on the exposure ofskin come a specific environment and partly ~ above the innate and species-specificbactericidal task in skin. Also, a high level of specificity is affiliated in theadherence the bacteria to epithelial surfaces. Not all bacteria connect to skin;staphylococci, which room the major element of the nasal flora, own a distinctadvantage over viridans streptococci in colonizing the sleep mucosa. Conversely,viridans streptococci are not viewed in huge numbers top top the skin or in the nose butdominate the dental flora.

The microbiology literature is inconsistent about the density of bacteria ~ above theskin; one factor for this is the range of techniques used to collect skin bacteria.The scrub an approach yields the highest and also most precise counts for a provided skin area.Most microorganisms live in the superficial class of the stratum corneum and in theupper components of the hair follicles. Some bacteria, however, reside in the deeperareas the the hair follicles and also are beyond the reach of plain disinfectionprocedures. This bacteria space a reservoir because that recolonization ~ the surfacebacteria room removed.

Staphylococcus epidermidis

S. Epidermidis is a major inhabitant of the skin, and also in someareas it provides up an ext than 90 percent of the resident aerobic flora.

Staphylococcus aureus

The nose and perineum room the most usual sites because that S. Aureuscolonization, i m sorry is present in 10 percent to an ext than 40 percent the normaladults. S. Aureus is common (67 percent) on vulvar skin. Itsoccurrence in the sleep passages varies v age, being better in the newborn,less in adults. S. Aureus is extremely common (80 come 100percent) on the skin of patients with details dermatologic illness such asatopic dermatitis, yet the reason for this recognize is unclear.


Micrococci space not as typical as staphylococci and also diphtheroids; however, they arefrequently existing on common skin. Micrococcus luteus, thepredominant species, normally accounts for 20 come 80 percent that the micrococciisolated native the skin.

Diphtheroids (Coryneforms)

The hatchet diphtheroid denotes a wide variety of bacteria belonging to the genusCorynebacterium. Category of diphtheroids continues to be unsatisfactory; forconvenience, cutaneous diphtheroids have actually been categorized right 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 v trichomycosis axillaris (infection of axillary hair). Lipophilicdiphtheroids are extremely typical in the axilla, vice versa, nonlipophilic strainsare found more commonly top top glabrous skin.

Anaerobic diphtheroids space most common in locations rich in sebaceous glands.Although the name Corynebacterium acnes to be originally used todescribe skin anaerobic diphtheroids, this are currently classified asPropionibacterium acnes and as P.granulosum. P. Acnes is seen eight time morefrequently 보다 P. Granulosum in acne lesions and is probablyinvolved in acne pathogenesis. Youngsters younger than 10 years space rarelycolonized with P. Acnes. The figure of this organism on theskin is probably related to the beginning of cheap of sebum (a semi-fluidsubstance composed of fat acids and also epithelial debris secreted from sebaceousglands) in ~ puberty. P. Avidum, the third types of cutaneousanaerobic diphtheroids, is rarely in acne lesions and also is more often isolated fromthe axilla.


Streptococci, specifically β-hemolytic streptococci, are rarely seen onnormal skin. The paucity that β-hemolytic streptococci ~ above the skin isattributed at least in part to the visibility of lipids on the skin, together theselipids room lethal come streptococci. Other groups of streptococci, such asα-hemolytic streptococci, exist generally in the mouth, indigenous wherethey may, in rarely instances, spread to the skin.

Gram-Negative Bacilli

Gram-negative bacteria consist of a small proportion the the skin flora. In view oftheir extraordinary number in the gut and also in the natural environment, theirscarcity ~ above skin is striking. They are seen in moist intertriginous areas, suchas the toe webs and also axilla, and also not on dried skin. Desiccation is the major factorpreventing the multiplication that Gram-negative bacteria on undamaged skin.Enterobacter, Klebsiella,Escherichia coli, and also Proteus spp. Are thepredominant Gram-negative organisms uncovered on the skin.Acinetobacter spp also occurs on the skin of normalindividuals and, like other Gram-negative bacteria, is more common in the moistintertriginous areas.

Nail Flora

The microbiology of a regular nail is generally similar to that of the skin. Dustparticles and other extraneous products may get trapped under the nail,depending ~ above what the pond contacts. In enhancement to resides skin flora, thesedust particles may lug fungi and bacilli. Aspergillus,Penicillium, Cladosporium, andMucor room the major types of fungi uncovered under thenails.

Oral and Upper respiratory Tract Flora

The oral flora is involved in dentist caries and periodontal disease, i beg your pardon affectabout 80 percent. That the population in the west world. The oral flora, itsinteractions v the host, and its an answer to environmental factors arethoroughly questioned in another Chapter. Anaerobes in the dental flora areresponsible for many of the brain, face, and also lung infections that space frequentlymanifested by abscess formation.

The pharynx and trachea contain mainly those bacter genera discovered in thenormal dental 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 also be foundin the pharynx. Anaerobic organisms also are reported frequently. The upperrespiratory tract is so often the website of initial emigration by pathogens(Neisseria meningitides, C. Diphtheriae,Bordetella pertussis, and many others) and could beconsidered the an initial region of attack for such organisms. In contrast, the lowerrespiratory tract (small bronchi and also alveoli) is typically sterile, becauseparticles the size of bacteria carry out not readily reach it. If bacteria perform reachthese regions, lock encounter hold defense mechanisms, such as alveolarmacrophages, that are not present in the pharynx.

Gastrointestinal tract Flora

The stomach is a fairly hostile setting for bacteria. The containsbacteria swallowed with the food and those dislodged from the mouth. Aciditylowers the bacterial count, which is highest possible (approximately 103 to106 organisms/g the contents) after meals and lowest (frequentlyundetectable) after ~ digestion. Part Helicobacter varieties cancolonize the stomach and are connected with form B gastritis and also peptic ulcerdisease. Aspirates of duodenal or jejunal liquid contain approximately103 organisms/ml in many individuals. Many of the bacteriacultured (streptococci, lactobacilli, Bacteroides) are thoughtto be transients. Level of 105 to about 107 bacteria/mlin together aspirates usually suggest an abnormality in the digestive system (forexample, achlorhydria or malabsorption syndrome). Rapid peristalsis and thepresence of bile may define in part the paucity of biology in the uppergastrointestinal tract. Additional along the jejunum and into the ileum, bacterialpopulations begin to increase, and at the ileocecal junction they reach levelsof 106 come 108 organisms/ml, with streptococci,lactobacilli, Bacteroides, and also bifidobacteriapredominating.

Concentrations that 109 come 1011 bacteria/g of components arefrequently uncovered in person colon and feces. This flora has a bewilderingarray that bacteria (more 보다 400 types have to be identified); nonetheless, 95to 99 percent belong to anaerobic genera such together Bacteroides,Bifidobacterium, Eubacterium,Peptostreptococcus, and Clostridium. Inthis extremely anaerobic region of the intestine, these genera proliferate, occupymost available niches, and produce metabolic waste products such as acetic,butyric, and lactic acids. The strictly anaerobic conditions, physical exclusion(as is displayed in many pet studies), and also bacterial waste products are factorsthat inhibit the expansion of other bacteria in the large bowel.

Although the normal flora have the right to inhibit pathogens, many of the members can producedisease in humans. Anaerobes in the minister tract are the major agents ofintra-abdominal abscesses and also peritonitis. Bowel perforations created byappendicitis, cancer, infarction, surgery, or gunshot wounds nearly always seedthe peritoneal cavity and nearby organs v the common flora. Anaerobes canalso reason problems in ~ the cradle lumen. Therapy withantibiotics might allow details anaerobic types to become predominant and also causedisease. For example, Clostridium difficile, which have the right to remainviable in a patience undergoing antimicrobial therapy, may producepseudomembranous colitis. Other intestinal pathologic problems or surgery cancause bacter overgrowth in the upper little intestine. Anaerobic bacteria canthen deconjugate bile acids in this region and bind available vitaminB12 so that the vitamin and fats are malabsorbed. In thesesituations, the patient usually has actually been compromised in some way; therefore, theinfection brought about by the common intestinal flora is second to anotherproblem.

More information is accessible on the animal than the person microflora. Researchon animals has revealed that inexplicable filamentous microorganisms affix to ilealepithelial cells and modify hold membranes with few or no harmful effects.Microorganisms have been it was observed in thick layers on gastrointestinal surfaces(Fig. 6-3) and also in the crypts ofLieberkuhn. Other studies suggest that the immune response can it is in modulated bythe intestinal flora. Researches of the function of the minister flora inbiosynthesis the vitamin K and also other host-utilizable products, counter of bileacids (perhaps come cocarcinogens), and also ammonia production (which deserve to play a rolein hepatic coma) present the double role the the microbial flora in affecting thehealth of the host. More basic researches of the person bowel flora are necessary todefine their effect on humans.


Figure 6-3

(A) Scanning electron micrograph the a cross-section the rat colonicmucosa. The bar shows the thick layer of bacteria in between themucosal surface and the lumen (L) (X 262,) (B) higher magnificationof the area suggested by the arrow in Fig. A, mirroring (more...)

Urogenital Flora

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

In the anterior urethra of humans, S. Epidermidis, enterococci, anddiphtheroids are found frequently; E. Coli,Proteus, and Neisseria (nonpathogenic species)are reported periodically (10 to 30 percent). Since of the typical flora residingin the urethra, treatment must be taken in clinically interpreting urine cultures; urinesamples might contain this organisms in ~ a level that 104/ml if a midstream(clean-catch) specimen is not obtained.

Conjunctival Flora

The conjunctival flora is sparse. About 17 to 49 percent the culturesamples space negative. Lysozyme, secreted in tears, may play a function incontrolling the bacteria by interfering v their cell wall formation. Whenpositive samples present bacteria, corynebacteria, Neisseriae, and Moraxellae arecultured. Staphylococci and streptococci are also present, and recent reportsindicate that Haemophilus parainfluenzae is present in 25percent the conjunctival samples.

Host infection by facets of the normal Flora

This chapter has actually briefly described the normal human flora; however, thepathogenic mechanisms of assorted genera or the clinical syndromes in which theyare affiliated was not discussed. Although such material is gift in otherchapters, note that a breach in mucosal surfaces regularly results in epidemic ofthe host by members of the regular flora. Caries, periodontal disease, abscesses,foul-smelling discharges, and also endocarditis are hallmarks of infections withmembers that the normal human being flora (Fig.6-4). In addition, special needs of the host (for example, those withheart failure or leukemia) or hold defenses (due to immunosuppression,chemotherapy, or irradiation) may an outcome in failure of the normal flora tosuppress transient pathogens or may cause members of the typical flora come invadethe host themselves. In one of two people situation, the hold may die.


Bitton G, Marshall KC: Adsorption that Microorganismsto Surfaces. John Wiley & Sons, brand-new York, 1980 .

See more: Gems Of War Ascension Explanation, Orb Of Ascension

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