The protective friendly bacterial flora is mostly obligated anaerobic. It is the flora who lives closest to the gut lining covered by faculatative anaerobic and anaerobic bacteria. There is a dense mucus layer covering the intestinal walls. The small intestine is mostly sterile but the colon is home of a big microbial niche. Current pharmaceutical grade probiotics don't contain the main enterotypes that habits the human colon. They assist and help the host as long as you supplement them.