Probiotics and health

Lactic acid bacteria are adapted to growing and surviving in milk or milk-derived environments.  But in order to survive the acidity of the finished product, they need to adapt their metabolism.
In the digestive tract, environmental conditions are very different from the dairy product.  In the stomach, acidity may be found at very high levels, but a very large percentage of lactic acid bacteria are evacuated during initial gastric emptying and rapidly attain the ileum (the terminal segment of the small intestine), one to two hours after a meal.  In the small intestine, bile salts, pancreatic enzymes and defensins (small proteins or peptides which adhere to the bacterial surface and kill them by damaging their cell wall) produced by cells in this organ can reduce the viability of lactic acid bacteria.  In the colon, lactic acid bacteria are confronted by microbiota which are between 1000 and 10,000 times more in number and may affect bacterial viability and physiology.
In order to understand how lactic acid bacteria react to this complex digestive environment, it is necessary to develop animal models which are similar to man, and molecular biology tools which can be used to study bacterial physiology.
The dominant microbiota in man differs considerably from that in the major domesticated animal species.  For example, the dominant microbiota in rodents contains high levels of lactobacilli, while only small numbers of these are detected in man. For this reason, rodent models (initially germ-free and then colonised with human microbiota) are being developed by INRA at Jouy-en-Josas.  In animals kept in a "bubble" without any contact with external contaminants, the equilibrium between microbial genera is similar to that in man.  Germ-free animals can also be used to study the adaptation of a strain in the absence of any other micro-organism, as well as the effects it induces on host physiology.
Thanks to these animal models, INRA researchers have been able to study the survival of a Lactobacillus casei strain in the intestinal tract.  They showed that not only was this probiotic able to survive, but it was also capable of modulating its metabolism in order to adapt to the human digestive environment.
Clinical trials in man have recently provided clear arguments which suggest that probiotics may, in the near future, have a role to play in the treatment of certain clinical types of chronic inflammatory intestinal diseases, notably to prevent recurrences.  Numerous clinical trials are currently under way and data are accumulating which will enable a clearer understanding of the mechanisms of action of probiotics and the interface between bacteria and human cells which underlies the pathophysiology of these diseases.
However, a distinction must be made between what can be expected of a medicinal product and a food.  Thus a food which slightly reduces a risk must not be confused with a medicinal product which treats a dysfunction.