It's hard to imagine that we each host billions of live bacteria in our digestive tract that colonize by replicating quickly and massively, but it's true. What's even more fascinating is that different bacteria live in different places based on what they desire as a home environment. Think of a native Montanan versus a New York City resident -- some like crowds and feed off the energy of the crowd, the noise doesn't bother them, the pollution isn't a major deterrent, and they've adapted to the types of food available whereas others need wide open space. They preferentially choose pure air and hunting for their food, and they are okay if they only see a crowd on holidays or if they happen to travel. By the same token, a person who lives in Montana will have a different gut flora than someone who lives in New York -- or Tokyo -- for that matter.
There are many more bacteria in the large intestine versus the small intestine (100,000:1), as it is significantly less acidic; another way of saying this is to remember that your lower GI prefers to be more alkaline. The small intestine contains more digestive enzymes, has more movement (peristalsis), and generates more antimicrobial chemicals (for example probiotics acting on fiber can create an acidic byproduct that functions like an antibiotic). Thus, because bacteria have different preferences, what we eat and as a result the environment in each area of our digestive tract will either encourage or discourage the bacteria to inhabit and flourish in their respective geography.
More Answers from Ashley Koff, RD