This process results in a vaginal ph of 4 to 5 which

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by lactobacilli. This process results in a vaginal pH of 4 to 5, which is optimal for growth and survival of the lactobacilli, but inhibits many other organisms. Roles in Health and Disease Opportunistic Infection Many species among the normal flora are opportunists in that they can cause infection when they reach protected areas of the body in sufficient numbers. For example, certain strains of E coli can reach the urinary bladder by ascending the urethra and cause acute urinary tract infection. Perforation of the colon from a ruptured diverticulum or a penetrating abdominal wound releases feces into the peritoneal cavity; this contamination may be followed by peritonitis or intra-abdominal abscesses caused by the more opportunistic members of the flora. Reduced innate defenses or immunologic responses can result in local invasion and disease by normal floral organisms. Caries and periodontal disease are caused by organisms that are members of the normal oral flora (see Chapter 60). Exclusionary Effect Balancing the prospect of opportunistic infection is the tendency of the normal flora to produce conditions that compete with extraneous pathogens and thus reduce their ability to establish a niche in the host. The flora in the colon of the breastfed infant produces an environment inimical to colonization by enteric pathogens, as does a vaginal flora dominated by lactobacilli. The benefit of this exclusionary effect has been demonstrated by what happens when it is removed. Antibiotic therapy, particularly with broad- spectrum agents, may so alter the normal flora of the gastrointestinal tract that antibiotic-resistant organisms multiply in the ecologic vacuum. Under these conditions, the spore-forming Clostridium difficile has a selective advantage that allows it to survive, proliferate, and produce a toxic colitis. 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 confer a degree of protection against pathogens. There is evidence of immunologic differences between children who are raised under usual conditions and those whose exposure to diverse flora is minimized. Some studies have found a higher incidence of asthma in the more isolated children. Promoting "Good" Flora
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The field of probiotics promotes colonization with "good" flora such as with lactobacilli in the intestinal tract. Elie Metchnikoff originally suggested that the longevity of Bulgarian peasants was attributable to their consumption of large amounts of yogurt; the live lactobacilli in the yogurt presumably replaced the colonic flora to the general benefit of their health. This notion persists today in the alleged benefit of natural (unpasteurized) yogurt, which contains live lactobacilli. Although we now know that
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