Gut microbiota is the largest organ of the human body. Although growth of bacteria more than 10(5) colony forming unit (CFU) per milliliter in culture of upper gut aspirate is used to diagnosis small intestinal bacterial over-growth (SIBO), 10(3) CFU or more is being considered to suggest the diag-nosis, particularly if colonic type bacteria are present in the upper gut. Although neither very sensitive nor specific, hydrogen breath tests are widely used to diagnose SIBO. Rifaximin is the best treatment for SIBO due to its broad spectrum, lack of systemic absorption, and safety profile.