RESUMEN
Human body is one of the most complex and diverse microbial ecosystem in which various microbes are living together with their hosts. Starting with Louis Pasteur's postulation that human health is dependent on gut-resident microbiota, microbes in the gastrointestinal tract have been studied using culture-based techniques. Cultivation has the great advantage that isolates can be recovered and used to further studies for their ability to utilize different substrates and other physiological properties. However, cultivation method is very labor-intensive and can not reveal representative microbial diversity of human intestinal tract. Only small fraction of the microbes residing in human intestine can be cultured and majority of intestinal microbes (approximately 60~70% of intestinal microbes) can not be come into view with currently available cultivation techniques. To avoid reliance on cultivation, many culture-independent molecular methods have been developed to analysis environmental microbes and our understanding of complex microbial communities has been greatly increased by molecular methods in recent decades. These culture-independent methods are mainly based on the use of microbial DNA sequences. Among prokaryotic DNAs targeted by molecular analysis, approximately 1.5 kb long 16S ribosomal DNA gene that encodes part of the small subunit (SSU) of ribosome is often used for analysis of microbial diversity. Molecular techniques introduced in microbial ecology have made it possible to study the composition of intestinal flora in a culture-independent way based on the detection of SSU rDNA.