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].

Several biological barriers are met by bacteria during residence in and travel through the different parts of the host's GI tract, such as the gastric acidity encountered in the stomach, the presence of bile salts in the duodenum and stress conditions associated with oxygen gradients that are steep at the mucosal surface, while the colon lumen is virtually anoxic. Moreover, considerable bacterial competition is encountered throughout the intestinal tract and is most prominent in the colon where bacterial density is highest. There are many functions that can be ascribed to the bacterial Gl-tract communities, including the processing of undigested food, the stimulation of the host's immune system, and providing colonization resistance to pathogens (9). However, it seems that we are only beginning to understand the dimensions of these interactions. This is evident from the major impact that bacterial colonization seems to have on the host and the presently known response of intestinal bacteria that are reviewed below.

Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

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