At any time we carry about seven pounds of bacteria, all busily making gas. It is doubtful we would survive well without our symbiotic guests. Additionally, significant amounts of vitamin B-12, thiamine and riboflavin are also produced. As an embarrassing byproduct, almost a gallon of methane gas is also produced per day. Bacterial flora varies not only from person to person, but also from region to region.
Some forms can kill or cause serious illness.
Likewise, if they get into other parts of the body, such as the urinary tract, they can cause infections. Food takes any where from 15 to 30 hours to pass through the entire system. This is called transit time and is strongly correlated to the amount of roughage in the diet. You can measure yours by simply eating a large serving of corn or red beets. Once swallowed, food is sloshed and churned in the stomach for three to five hours, before being sent to the 20-some feet of the small intestine where it spends another four to five hours.