In the adult, the human intestine houses myriads of microorganisms, quantitatively up to 100 trillion and qualitatively over 500 species of bacteria, exceeding the number of host somatic cells by at least one order of magnitude. Actually, it remains a mystery as to how the intestine is able to contain such large quantities of bacteria without evident harm to the host. However, it is well known that a very complex symbiotic relationship between the intestinal microflora and the host insures mutual advantages for both partners. Despite the recent advances in immunology and microbiology, the possibility of studying human intestinal microflora is limited by great inter-individual variability and difficulties in creating standard conditions to uniform the samples. However, there are clinical conditions which are useful to explain the role of intestinal bacteria in the human gut. Small intestinal bacterial overgrowth (SIBO) is a good example, because this is a microbial alteration of intestinal microflora, in absence of pathogenic bacteria and severe dysregulation of the immune system. On the other hand, the pathogenesis and clinical aspects of SIBO could clarify the complex and bi-directional relationship between the microbiota and the host.
