ABSTRACT
From birth to death, the human gastrointestinal tract (GI tract) is colonized by a vast and
complex consortium of mainly bacterial cells that outnumbers our somatic and germ cells
(1). The microflora in this niche is estimated to be composed of at least 500 different
species. However, this number is likely to represent a large underestimate, since it has
been based on culturing studies that are known to be selective and notably underestimate
the large number of Gram-positive intestinal bacteria. Molecular approaches, such as
broad-range sequencing of 16S ribosomal RNA genes, have been used to monitor the
composition of the dominant GI-tract microbiota in different individuals at different points
in their lives (see chapter 1). These approaches revealed a relatively stable composition in
individual adults, but they appeared to be considerably variable when different individuals
were compared (2,3). Moreover, host development (4,5), host genotype (6), and
environmental factors (7) influence the composition of the microbiota, emphasizing
how challenging it is to define and compare bacterial communities within and between
specified intestinal niches of a given individual at a particular time point in his or her life.
The fact that we have not yet been able to culture the majority of the members of this
bacterial community further complicates studies on the activity of individual members of
the GI-tract consortium. An important development in this respect are the sophisticated
enrichment strategies that have led to the isolation of new bacterial species from fecal
samples [(8) and see chapter 1].
