This is a pretty good video for the layman. “Probiotics – A quick trip inside our guts” explains the role of probiotics in simple, easy to understand terms.
The only problem I have is the inaccuracy of the comments on Helicobacter pylori, a bacterium living in the human stomach since time immemorial.
An excellent article in the Scientific American, February 2005, entitled Endangered Species In The Stomach, explains the true role of H.pylori. In brief, it regulates the level of acidity in the stomach and the amount of food we eat. When the acid levels get too high, it releases a protein which feeds back into the stomach and reduces the production of acid. This protects against gastric reflux and a particularly deadly esophageal cancer. Both of these have increased dramatically over the last 100 years.
The amount of food is also regulated by the H.pylori releasing proteins that make us feel full and therefore stop eating. This reduction in the H.pylori is considered to be one of the contributing factors towards the current obesity epidemic.
The reduction in H.pylori in the developed world is due to excessive cleanliness and the use of antibiotics. Never use an antibiotic soap, unless you are doing surgery. Do not be afraid of a little dirt.