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

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Manipulation of the Normal Flora

Attempts to manipulate the normal flora have often been fruitless and have sometimes been dangerous.

MANIPULATION OF THE NORMAL FLORA

Attempts to manipulate the normal flora have often been fruitless and have sometimes been dangerous. Exclusion of the normal flora has been effective in patients whose immunologic defenses are massively compromised (eg, following the whole-body irradiation used in bone marrow transplantation). Significant effects require the use of antimicrobics, sterilization of food and supplies, air filtration, and strict aseptic nursing procedures. These conditions substantially reduce the risk of infection during highly vulnerable periods.

Efforts to control which organisms make up the flora have been more problematic. During nursery outbreaks of S. aureus infections in the 1950s, deliberate colonization of an infant’s nares with S. aureus 502A, a strain of low virulence, was attempted as a con-trol measure. This approach was based on the hope that it would exclude more virulent strains of S. aureus. Unfortunately, some infections occurred with the 502A strain.

One area where there has been some success in promoting colonization with “good” flora is 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 lactobacillary replacement of the flora of the adult colon does not take place so easily, there have been some successes with capsules containing lyophilized bacteria. In some studies, administration of preparations containing a particular strain of Lactobacil-lus (L. rhamnosus strain GG, LGG) has reduced the duration of rotavirus diarrhea in chil-dren and prevented relapses of antibiotic-associated diarrhea caused by C. difficile. LGG suppositories have also been used to prevent recurrent vaginitis caused by the yeast C. al-bicans with mixed results. A better understanding of the relationship between virulenceand the extremely complex interactions of the normal flora is needed for the rational de-ployment of “good” flora to our benefit.


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