Gnotobiotic animals in nutrition research

Rccognition that creatures in a conventional environment carry a large burden of metabolically active micro-organisms in their gastro-intestinal tracts makes untenable the classic concept of nutrition as a simple relationship between the diet and its consumer. Instead it must be regarded as a series of complicated interactions between the diet, its consumer and his resident microflora. T o understand the interrelationships between the three components it is necessary to study each one in isolation. The idea of a host devoid of its usual microbial associates, although forecast by Pasteur as long ago as 1895, remained largely hypothetical until recent advances in gnotobiotic technique made it a practical possibility. The technique depends upon the fact that an embryo developing inside a hen’s egg or a mammalian uterus is almost always microbiologically sterile so long as the dam is free from disease. If transferred aseptically to a sterile container and reared there on sterile food and water the bird or mammal will remain ‘germ-free’, that is free from any other detectable form of life. The precise meaning of the term depends upon the extent of the tests applied to detect contamination. In practice it usually implies freedom from bacteria, fungi, protozoa, and internal and external parasites. A gnotobiote is an animal in which all the life forms present are known, that is it may be germ-free or associated with any number of strains of organism of known identity. In the germ-free animal any nutritional process can be investigated without interference from the micro-organisms commonly inhabiting the gut in a conventional environment. The extent to which that process is modified by microbial action can then be examined by introducing known components of the gut microflora. However, the results of such studies cannot always be taken at their face value since, in a gnotobiotic system, the biological characteristics both of the animal and the associated microbes may deviate from the so-called ‘normal’ pattern displayed when the full conventional microflora is present.