The gastrointestinal (GI) tract is one of the most complex ecosystems consisting of microbial and host cells. It is suggested that the host genotype, the physiology of the host and environmental factors affect the composition and function of the bacterial community in the intestine. However, the relative impact of these factors is unknown. In this study, we used a culture-independent approach to analyze the bacterial composition in the GI tract. Denaturing gradient gel electrophoresis (DGGE)

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... les of fecal bacterial 16S rDNA amplicons from adult humans with varying degrees of genetic relatedness were compared by determining the similarity indices of the pro les compared. The similarity between fecal DGGE pro les of monozygotic twins were signi cantly higher than those for unrelated individuals (t s ¾2.73, p 1 -t a i l ¾0.0063, df¾21). In addition, a positive relationship (F 1 , 3 0 ¾ 8.63, p ¾ 0.0063) between the similarity indices and the genetic relatedness of the hosts was observed. In contrast, fecal DGGE pro les of marital partners, which are living in the same environment and which have comparable feeding habits, showed low similarity which was not signi cantly different from that of unrelated individuals (t s ¾1.03, p 1 -ta i l ¾0.1561, df ¾27). Our data indicate that factors related to the host genotype have an important effect on determining the bacterial composition in the GI tract.