Scientific findings in recent decades have demonstrated that the commensal intestinal microbiota has profound effects on the physiology and diseases of the host. It is estimated that the human microbiota is composed of 10(14) bacterial cells, a number 10 times greater than the total number of human cells. The variety and the complex interactions of the intestinal microbiota are associated with physiological details that remain largely unknown. Germfree hosts, especially murine (rat or mouse) animals that have been maintained free from demonstrable microbial associates such as bacteria, viruses, fungi, and parasites throughout life, have become a powerful tool for exploring the interplay between the host and microorganisms inhabiting the human intestine. This review and survey of recent findings will argue that the germfree mouse model can produce its greatest potential benefits in the study of the metabolism and immunity of the host.
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