The mammalian stomach is quite acidic. These gastric acids are necessary for efficient digestion of feed stuffs. For years it was assumed that the mammalian stomach was a relatively sterile environment. This assumption was based on the low pH as a result of the high levels of gastric acids secreted into the stomach. Recently this dogma has been refuted as numerous genera /types of bacteria have been identified in the stomach. These bacteria are acidophilic (acid loving) and important microflora in maintaining the healthy stomach. Lactobacillus reuteri has been cultured from the horses stomach.
Therefore, the concern over survival of probiotics (beneficial bacteria) and passage through the stomach is dependent on the genera/type of bacteria being considered. Acidophilic bacteria , such as lactobacillus reuteri, can survive and thrive during the passage through the acidic stomach.
In addition to the type of bacteria used there are other important considerations when choosing a probiotic.
There needs to be a LARGE number of the right type of LIVE bacteria. The use of large numbers of bacteria will ensure that adequate numbers of bacteria reach sights of colonization such as the small intestine, large bowel and cecum. When the lactobacillus reuteri bacteria colonizes it provides a protective barrier for the GI tract secreting a reuterin that kills salmonella and other pathogens on contact. It is important for the bacteria to be alive when administered and when it reaches sites of colonization.
In addition to bacteria there are other organisms that can be helpful to the the digestive tract. Saccharomyces Boulardii is a yeast product that also serves as a probiotic. Saccharomyces Boulardii acts as a binding agent and binds to pathogens in the body preventing them from trying to attach to the mucosal wall and has been shown to have a positive effect on the immune system.
In summary, when choosing a probiotic there are a few key things to look for. Make sure that the bacteria can survive the acidity of the stomach. Make sure that the bacteria are normal inhabitants of the specific species gastrointestinal tract (not industrial cheese /yogurt culture organisms) are given in large numbers and alive when administered.