The human body harbors a variety of microorganisms that can be arranged into two groups: (1) the resident microbiota consists of relatively fixed types of microorganisms regularly found in a given area at a given age; if disturbed, it promptly reestablishes itself; and (2) the transient microbiota consists of nonpathogenic or potentially pathogenic microorganisms that inhabit body sites for hours, days, or weeks. The transient microbiota is derived from the environment, does not produce disease, and does not establish itself permanently. Members of the transient microbiota are generally of little significance so long as the normal resident flora remains intact. However, if the resident microbiota is disturbed, transient microorganisms may colonize, proliferate, and produce disease.

Organisms frequently encountered in specimens obtained from various areas of the human body—and considered nor mal microbiota—are listed in Table 1. The classification of anaerobic normal bacterial flora is discussed in Chapter 21.

Table1. Normal Bacterial Microbiota

It is likely that microorganisms that can be cultured in the laboratory represent only a fraction of those that are part of the normal resident or transient microbiota. When the broad range polymerase chain reaction (PCR) is used to amplify bac terial 16S rDNA, many previously unidentified bacteria can be detected, as in secretions from patients with bacterial vaginosis. The number of species that make up the normal micro biota has been shown to be much greater than previously recognized. Thus, the understanding of normal microbiota is in transition. As already mentioned, the relationship of previously unidentified microorganisms, which are potentially part of the normal microbiota, to disease is likely to change.

The microorganisms that are constantly present on body surfaces are frequently described as commensals (ie, one partner benefits, while the other seems unaffected). However, in some sites (eg, gut), mutualistic (ie, both parties derive benefit) may be a better description of this relationship. Their flourishing in a given area depends on physiologic factors of temperature, moisture, and the presence of certain nutrients and inhibitory substances. Their presence is not essential to life because “germ-free” animals can be reared in the complete absence of a normal microbiota. Yet the resident flora of certain areas plays a definite role in maintaining health and normal function. Members of the resident microbiota in the intestinal tract synthesize vitamin K, bioactive compounds such as 3-indolepropionic acid (IPA), and aid in the absorption of nutrients. IPA is a potent neuroprotective antioxidant that scavenges hydroxyl radicals. IPA binds to the pregnane X receptor on intestinal epithelial cells. After absorption from the intestine and distribution to the brain, IPA is thought to exert a neuroprotective effect against cerebral ischemia and Alzheimer’s disease. On mucous membranes and skin, the res ident microbiota may prevent colonization by pathogens and possible disease through “bacterial interference.” The mechanism of bacterial interference may involve competition for receptors or binding sites on host cells, competition for nutrients, mutual inhibition by metabolic or toxic products, mutual inhibition by antibiotic materials or bacteriocins, or other mechanisms. Suppression of the normal microbiota clearly creates a partial local void that tends to be filled by organisms from the environment or from other parts of the body. Such organisms behave as opportunists and may become pathogens.

On the other hand, members of the normal microbiota may themselves produce disease under certain circumstances. These organisms are adapted to a noninvasive mode of life defined by the limitations of the environment. If forcefully removed from the restrictions of that environment and introduced into the bloodstream or tissues, these organisms may become pathogenic. For example, streptococci of the viridans group are the most common resident organisms of the upper respiratory tract. If large numbers of them are introduced into the bloodstream (eg, after tooth extraction or oral surgery), they may settle on deformed or prosthetic heart valves and produce infective endocarditis. Small numbers occur transiently in the bloodstream with minor trauma (eg, dental scaling or vigorous brushing). Bacteroides species are the most common resident bacteria of the large intestine and are quite harmless in that location. However, if introduced into the peritoneal cavity or into pelvic tissues along with other bacteria as a result of trauma, they cause suppuration and bacteremia. There are many other examples, but the important point is that the normal resident microbiota is harmless and may be beneficial in their normal location in the host and in the absence of coincident abnormalities. They may produce disease if introduced into foreign locations in large numbers and if predisposing factors are present.

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مواضيع ذات صلة

The Role of the Normal Mouth Microbiota in Dental Plaque and Caries

Normal Microbiota of the Mouth and Upper Respiratory Tract

Normal Microbiota of the Skin

Actinomyces israelii

Mycobacterium leprae

Mycobacterium tuberculosis

Bacterial Virulence Factors : Toxins

Laboratory Diagnosis of Vibrio, Aeromonas, Chromobacterium, and Related Organisms

Pathogenesis and Spectrum of Disease of Vibrio, Aeromonas, Chromobacterium, and Related Organisms

Laboratory Diagnosis of Alcaligenes, Bordetella (Non-pertussis), Comamonas, and Similar Organisms

Antimicrobial Susceptibility Testing and Therapy of Rhizobium, Ochrobactrum, and Similar Organisms

Laboratory Diagnosis of Rhizobium, Ochrobactrum, and Similar Organisms