In general, you want to buy high-quality probiotics, and finding the good ones isn't easy mainly because of the huge amount of astroturfing from manufacturers.
So here goes (note that these work for me, IANA doctor, YMMV yada yada):
VSL#3 and Therelac are the best broad-spectrum probiotics. Both have very high concentrations of bacteria, and require refrigeration. These are expensive, and typically used if you're trying to fix some condition. I make sure I always have a bottle of either stocked, and take it only if I feel I need to.
For day-to-day use, the following work great: Culturelle and Jarrow Formulas S. Boulardii. They're reasonably priced, don't require refrigeration, and the effects are noticeable in a day. I typically take a capsule of each twice a day (morning/night.)
Finally, make sure you get digestive enzyme supplements and take them religiously with meals that are hard on the digestive system (e.g., meat.) Twinlab Super Enzymes are what I typically get.
In general, the key effects I've noticed are: much improved digestion, lower anxiety, corrects GI issues, improvements in skin and oral health, and simply a better overall sense of well-being.
What's particularly cool is if you take them with foods that you love but typically upset you (burgers, pizza, other greasy stuff), extra doses of these probiotics right after the meals (along with the digestive enzymes) completely eliminate negative aftereffects.
Good probiotics are incredible, and I wish more doctors better understood them. Meanwhile, I highly recommend trying the above, experimenting (with good judgement), and finding what works for you.
http://www.generalbiotics.com/
It didn't seem to do anything at all for me, but I'm already fairly healthy.
This paper is questionable for other reasons.
E. coli is a little more complicated: some E. coli spp. produce toxins, and other species are enteroinvasive and actually can outcompete the gut flora (confusingly enough, other species of E. coli are also in the normal gut flora).
The main reason that some species of E. coli can make it to your gut and cause disease are that they have a variety of pathogenic factors that definitely would not be found in bacteria in 'probiotic' foods.
(and I see who you are replying to just edited their response to add 'bad' E. coli)