The boy in the bubble

In general, most controlled studies have been carried out with GF and GN rats and mice. The GF animal technique has the potential for selective association of the originally GF animal with a specific microflora element, including parasites. Mentioned earlier is the potential of bringing its distended cecum back within acceptable limits by association with a well-defined cocktail of microbial species. These are introduced by bringing monocultures grown in test tube into the main unit after chemical exterior sterilization in the entry compartment. In this way, studies have become possible of what is called the gnotobiote, the animal that harbors a desired but defined combination of microflora elements. However, the term gnotobiote per se also includes the GF animal.

Once the GF animal had been produced, the logical extension was the study of function and metabolism of this gnotobiote. Apart from anomalies mentioned earlier, the absence of a metabolizing microflora resulted in the absence of certain nutrients normally produced by the flora (e.g., vitamin K) and certain flora-produced stimuli (e.g., LPS). Because of this, diet had to be adjusted, apart from adjustment necessitated by heat or radiation sterilization. The effects of the GF state on immune function are substantial, although all GF species proved to be eventually immunologically competent. Upon antigenic stimulation, the GF animal generally shows a delayed, but eventually adequate, immune response. However, antibacterial response may be inadequate, due to slow response of macrophages, which originally are defective in chemotaxis and destructive and lytic capacity, leading to a delayed presentation of antigenic material to other elements of the immune system (Wostmann, 1996).

The possibility of the production of gnotobiotes with a stable and defined microflora opened many possibilities and can be regarded as the originally GF animal's major potential. It soon became obvious that GF rats did not develop that scourge of our society, dental caries, thereby indicating its microbial origin. GF rats were then associated with a number of different bacteria. This resulted in the recognition of Streptococcus mutans as the major originator of caries and put the acid-producing lactobacilli in second place (Orland et al., 1955). Similarly, it was shown that GF guinea pigs inoculated with the parasite Entomoebia histolytica, the cause of a potentially lethal intestinal infection, do not develop any symptoms. The animals retained the amoeba for only a few days. A microflora is obviously needed to change intestinal conditions to the point where the infection could take hold (Phillips, 1964). On the other hand, it was found that a CV microflora, by its stimulation of the immune system, may affect a certain amount of protection against Schistosomiasis (Bezerra et al., 1985). Studies of the various factors involved in the establishment of the nematode Trichinella spiralis were carried out by Przyjalkowski (Przyjalkowski et al., 1983) and Despommier (Despommier, 1984). In the pork industry, GF pigs have been used to solve problems of bacterial and viral infection.

Gnotobiotes are important to the study of colonization resistance, which seeks to determine which microflora elements may be important for the flora's stability and which for its potential to resist pathogens. In a similar way, gnotobiotes enable the study of microbial translation to determine what microflora composition will enhance or inhibit certain of its members to pass the intestinal barrier and possibly cause disease.

Recently, it has become possible to establish a "normal" human microflora in originally GF mice. After the actual composition of this flora has been established, and its stability ascertained, this could open the door to a multitude of studies pertaining to human health and disease.

The plastic isolator, already in general use to protect premature infants, found its culmination in its use to protect David, the "Boy in the Bubble." Before birth diagnosed as having SCID (severe combined immune deficiency), he was born via Cesarean section and placed in an isolator, which eventually grew to a four-room apartment. Over the years, he accumulated a number of non-life-threatening organisms, but death came at the age of about 12-g years, apparently caused by the sequela of an unsuccessful bone marrow transplant (Bealmear et al., 1985).

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