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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Normal Microbial Flora

Beneficial Effects of the Normal Flora

Organisms of the normal flora play an important role in the development of immunologic competence.


Priming of Immune System

Organisms of the normal flora play an important role in the development of immunologic competence. Animals delivered and raised under completely aseptic conditions (“sterile” or gnotobiotic animals) have a poorly developed reticuloendothelial system, low serum levels of immunoglobulins, and none of the antibodies to normal floral antigens that often cross-react with those of pathogenic organisms and confer a degree of protection against them. There is evidence of immunologic differences between children who are raised un-der usual conditions and those that minimize the exposure to diverse flora. Some studies have found a higher incidence of asthma in the more isolated children.

Exclusionary Effect

The normal flora produces conditions that tend to block the establishment of extraneous pathogens and their ability to infect the host. The bifidobacteria in the colon of the breast-fed infant produce an environment inimical to colonization by enteric pathogens; this pro-tective effect is aided by ingested maternal IgA. Breastfeeding has clearly been shown to help protect the infant from enteric bacterial infection. The normal vaginal flora has a similar protective effect. Before the introduction of antibiotic therapy, researchers found that synthetic estrogen therapy controlled institutional outbreaks of fomite-transmitted gonococcal vulvovaginitis in prepubertal girls. This treatment led to glycogen deposition in the vaginal epithelium and establishment of a protective lactobacillary flora. The possi-ble hazard of such therapy in this population was not then recognized.

Antibiotic therapy, particularly with broad-spectrum agents, may so alter the normal flora of the gastrointestinal tract that antibiotic-resistant organisms multiply in the relative ecologic vacuum, sometimes causing significant infections, particularly in immunocom-promised patients. The pathogenic yeast Candida albicans, a minor component of the normal flora, may multiply dramatically and cause superficial fungal infections in the mouth, vagina, or anal area. Pseudomembranous colitis results from overproliferation of a toxin-producing anaerobe,Clostridium difficile, which has a selective advantage in the presence of antibiotic therapy. It may be resistant to several antibiotics that act on other members of the colonic flora, allowing C. difficile to increase from a minor to a major component. Its toxins cause diarrhea and direct damage to the colonic epithelium.

The exclusionary effect of the flora in health has been demonstrated in numerous ex-periments on gnotobiotic and antibiotic-treated animals. For example, C. albicans attaches to oral epithelial cells of germ-free rats; however, prior colonization with certain viridans streptococci that attach to similar epithelial cells prevents establishment of C. albicans. In another experiment, the infecting oral dose for mice of streptomycin-resistant Salmonella was approximately 105 organisms in untreated animals. Oral streptomycin treatment, which inhibits many members of the normal flora, reduced the infecting dose by approxi-mately 1000-fold.

Production of Essential Nutrients

In ruminants, the action of the extensive anaerobic flora in the rumen is essential to the nu-trition of the animal. The flora digests cellulose to usable form and provides many vitamins, including 70% of the animal’s vitamin B requirements. In humans, members of the vitamin B group and vitamin K are produced by the normal flora; however, except for vitamin K the amounts available or absorbed are small compared with those in a well-balanced diet. Bacterial vitamin production is reduced during broad-spectrum antibiotic therapy, and supplementation with vitamin B complex is indicated in malnourished individuals.

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