For years, questions lingered about how essential the microbial flora is to normal life and what functions various members of the flora might serves. the need for animal models to further investigate these questions led eventually to development of laboratory strains of Germ-free, or axenic, mammals and birds. The techniques and facilities required for producing and maintaining a germ-free colony are exceptionally rigorous. After the young mammals are taken from the mother aseptically by cesarian section, they immediately transferred to a sterile isolator or incubator. The newborns must be fed by hand through gloved ports in the isolator until they can eat on their own, and all materials entering their chamber must be sterile. Rats, mice, rabbits, guinea pigs, monkeys, dogs, hamsters, and cats are some of the mammals raised in the germ-free state.
Experiments with germ-free animals are of two basic varieties: 1. general studies on how the lack of animal microbial flora influences the nutrition, metabolism, and anatomy of the animal, and 2. gnotobiotic studies, in which the germ-free subject is inoculated either with a single type of microbe to determine its individual effect or with several known microbes to determine interrelationships. Results are validated by comparing the germ-free group with a conventional, normal control group.
A dramatic characteristic of germ-free animals is that they live longer and have fewer diseases than normal controls, as long as they remain in a sterile environment. From this standpoint, it is clear that the flora is not needed for survival and may even be the sources of infectious agents. At the same time, it is also clear that axenic life is highly impractical. Additional studies have revealed important facts about the effect of the flora on various organs and systems. For examples, the flora contributes significantly to the development of immune system. When germ-free animals are placed in contact with normal control animals, they gradually develop a flora similar to that of the control. However, germ-free subjects are less tolerant of microorganisms and can die from infections by relatively harmless species. This susceptibility is due to the immature character of the immune systems of germ-free animals. These animals have a reduced number of certain types of white blood cells and slower antibody response.
Gnotobiotic (known the organisms of a region) experiments have clarified the dynamics of several infectious diseases. Perhaps the most striking discoveries were made in the case of oral diseases. For years, the precise involvement of microbes in dental caries had been ambiguous. Studies with germ-free rats, hamsters, and beagles confirmed that caries development its influenced by heredity, a diet high in sugars, and poor oral hygiene. Even when all these predisposing factors are present, however, germ-free animals still remain free of caries unless they have been inoculated with specific bacteria.
The ability of known pathogens to cause infection can also be influenced by normal flora, sometimes in opposing ways. Studies have indicated that germ-free animals are highly susceptible to experimental infection by enteric pathogens Shigella and vibrio, whereas normal animals are less susceptible, presumably because of their protective flora. In marked contrast, Entamoeba histolytic ( the agent of amebic dysentery) is more pathogenic in the normal animal that in the germ-fre animal. One explanation for this phenomenon is the E.histolytica must feed on intestinal bacteria to complete its life cycle.
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