Microbiota

Microbiota (human)

Microbial flora harbored by normal, healthy individuals. A number of microorganisms have become adapted to a particular site or ecologic niche in or on their host. Some are normal residents that are regularly found, and if disturbed will rapidly reestablish themselves; others are transient microorganisms that may colonize the host for short periods but are unable to permanently colonize. The normal fetus is sterile, but during and after birth the infant is exposed to an increasing number of microorganisms. Subsequently, those organisms best adapted to survive and colonize particular sites establish themselves and become predominant. Physiologic factors such as the availability of nutrients, temperature, moisture, pH, oxidation-reduction potential, and resistance to local antibacterial substances play an important role in determining the ability of a microorganism to become established at a particular site. The normal indigenous microbial flora is exceedingly complex, consisting of many different species of bacteria, fungi, viruses, and protozoa. The great majority of these commensal and symbiotic organisms are bacteria and fungi.

The indigenous microorganisms play an important role by protecting the normal host from invasion by microorganisms with a greater potential for causing disease. They compete with the pathogens for essential nutrients and for receptors on host cells by producing bacteriocins and other inhibitory substances, making the environment inimical to colonization by pathogens.

In the healthy individual the morphologic integrity of the body surface provides a very effective first line of defense. The intact skin is an efficient physical barrier that can be penetrated by very few microorganisms. The secretion of specific antimicrobial substances and bactericidal fatty acids by the sebaceous glands also retards microbial invasion.

Mucosal surfaces also provide a mechanical barrier in the respiratory, gastrointestinal, and genitourinary tracts. These surfaces are bathed in secretions with antimicrobial activity. In the respiratory tract, mechanical cleansing is accomplished by the cough and mucociliary action. Recurrent infections of the sinuses, middle ear, bronchial tract, and lungs occur in individuals who have an impairment of ciliary activity. These infections are usually caused by Staphylococcus pneumoniae and Haemophilus influenzae, the more virulent pus-forming organisms found in the nasopharynx. Defects in ciliary activity also cause bacterial respiratory infections in cigarette smokers and heavy alcohol drinkers. See Staphylococcus

Once the natural barriers of the skin and mucous membranes are breached, the next major line of defense is the polymorphonuclear leukocytes. Individuals with disorders of these phagocytic cells have an increased incidence of serious infections with their indigenous microflora.

The complement system is another nonspecific mechanism of the body for the elimination of invading microorganisms. Complement proteins in conjunction with organs of the reticuloendothelial system (spleen, liver, and bone marrow) play a key role in the removal of encapsulated bacteria from the bloodstream. Splenectomized individuals and those with a nonfunctioning spleen because of sickle cell disease have an increased incidence of fulminating infections caused by S. pneumoniae, H. influenzae, Neisseria meningitidis, and recently recognized unusual organisms. See Complement