JudithC, I would recommend you go with the more popular strains such as Lactobacillus and the more well studied ones (such as in the product Align).
I take two probiotics, one is called
Align which has several clinical trials associated with the bacteria used in it (Bifantis).
Also, to get my lactobacillus, the same bacteria strain found in yogurt, I prefer
acidophilus pearls . The reason these are superior are because they are enclosed in a little shell that protects them from your stomach acid so the bacteria dont die before they reach your intestines. The pearls contain a combo of 2 seperate popular strains, lactobacillus and bifidobacterium longum (another popular one).
The align is about $20 per 30ct, and the pearls are about $20 per 90ct, so if you are only gonna go with one, Id go with the pearls. By far the cheapest place to get these (as is most vitamins and supplements) is amazon.com. Places like walgreens I believe have their own generic version of the "pearls".
Regardless which probiotic you go with, I would personally avoid the ones that claim like 20 different strains of bacteria, or 80+ BILLION!! bacteria. More isnt necessarily better - its all about the quality of the strain, if any of them survive, and the delivery method; dont be fooled by marketing claims. 1 billion is more then enough.
Some can actually cause problems including bacteria overgrowth and can actually cause infection if they penetrate the intestinal wall and get into your bloodstream. I think its best to keep it simple with a basic lactobacillus since its been well studied and is the most popular. Some of the strains used in certain brands are these really weird, unstudied strains that are sort of "superbugs" and resistant to antibiotics.
Also, try to keep your probiotics at room temperature or less. Temperature can kill the bacteria. Some brands even require them to be refrigerated (however the pearls dont need to be and are designed to be resistant to temperature, moisture, and light)