Several "friendly" intestinal bacteria perform many important bodily functions. There are actually a great many lactobacillus and other bacteria that can inhabit the human colon, but I will mention the three that seem to be most important. These are Lactobacillus acidophilus (the most famous), Lactobacillus bifidus (more common to the baby colon), and Streptococcus faecium (not S. faecalis, a possible pathogenic bacteria).
Various cultures of acidophilus are available in many stores, particularly health food stores, as powders, capsules, tablets, and liquids and measured by the amount of viable bacteria per dosage. There are many claims for the use of acidophilus, though it is best known for reimplanting friendly bacteria into the colon to assure return of bodily functions after a course of antibiotic drugs. Actually, acidophilus itself acts as a mild antibiotic—that is, it has antibacterial activity. With regular use, it may even replace harmful bacteria in the colon or vaginal tract of women, where acidophilus is also commonly used to treat yeast infections. It is further employed as part of the treatment for intestinal yeast overgrowth and the many symptoms that this may generate. These bacteria also help in the production of some B vitamins and vitamin K and in the breakdown of various foods.
Yogurt or acidophilus milk, sometimes with L. bulgaricus as well, is often used to provide some stimulus to the colon, though the live bacteria count is not very high in these products. Yogurt can also be used by people with lactose intolerance due to lactase enzyme deficiency, because the bacteria change or ferment the lactose sugar and produce lactic acid. Many people have also described yogurt or, more important, acidophilus as helpful for stomach and digestive upset, for intestinal gas, and even for inflammatory problems of the gastrointestinal tract, but these reports are more anecdotal than proved by research. The further suggestions that acidophilus improves immunity, produces its own antibiotics, helps allergies (particularly to foods), improves skin health, is a benefit in herpes infections, reduces cholesterol levels, and lessens cancer risk (especially colon cancer) are also yet unproved, though current research at several universities for one product looks very promising in regard to these possibilities.
Lactobacillus bifidus has become part of intestinal bioculture treatment, often along with acidophilus. The bifidus culture is more prevalent in infants, often as their first organism, but can also be an important part of the adult gastrointestinal tract. Like acidophilus, it helps in the synthesis of B vitamins, in food digestion, and in inhibiting the growth of the coliform bacteria and possibly more pathogenic colon bacteria, such as salmonellae.
Streptococcus faecium is another important colon bacterium that has received recent attention. Its actions are similar to those of acidophilus. It is important in B vitamin biosynthesis, aids the digestion of foods, likely by producing certain enzymes, and inhibits other, more toxic bacteria; thus supplementation with S. faecium may help in some cases of diarrhea. Strep faecalis, a potentially pathogenic bacterium, has been listed by mistake instead of S. faecium on some bacterial replacement products.
These three bacteria may be taken individually and alternated weekly or every couple of weeks. They can also be taken all together (there are some products that contain all three) on a regular basis when used to balance the effect of one or more courses of antibiotics. There is a possibility that the combination of bacteria works better to rebalance colon health than the individual organisms.
The count of live bacteria in products containing these bacteria is in the millions and billions daily per dose. There has been some question as to whether these bacteria are killed by the acidic stomach juices, but when taken in sufficient quantities, some organisms do make it down to the colon. I believe that these bacteria should not be taken regularly, but rather in specific courses to repopulate the colon with these "friendly" bacteria after antibiotic use or to treat intestinal yeast overgrowth; otherwise, I recommend them for one to two weeks once or twice a year, or when traveling to underdeveloped countries with higher risks of intestinal contamination from infectious organisms, for which the acidophilus bacteria offer some protection.