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.