I was able to find this on the internet in less than 30 minutes. They all come from peer reviewed research journals. All of the articles show colonization of probiotics in the gut.
"L. reuteri persisted at reduced but detectable levels 4 weeks after the end of intake"
Not sure if you know what persisted means. I have no idea if English is your first language. Here's the definition.
Persist - 1) To continue in existence; last, 2) to continue to exist or occur without interruption, 3) to continue to exist especially past a usual, expected, or normal time.
The quote you gave shows that probiotic bacteria CAN colonize the digestive tract, and persist even 4 weeks after stopping the supplementation.
The problem with those who are stating that probiotics cannot colonize the digestive tract is that they seem to be grouping all probiotics together. They assume that if one probiotic cannot adhere to the intestinal wall, then it must be true for all.
The fact is, you cannot lump all probiotics together, you have to look at each one individually. I am including a list of just some of the different probiotic strains that have been researched. This is in no way a complete list of probiotic strains. I'm also not saying that these strains of probiotics listed can or cannot adhere to the intestinal wall, i'm merely posting this to clarify the point i'm making that probiotics are diverse and should be talked about individually instead of collectively.
As you can see, there are numerous strains of probiotic bacteria which have all been researched individually, not as a group. When you say that probiotics can't colonize the gut, you are oversimplifying a very complex area of study.
To hunt3r: You quoted text from a research journal (American Journal of Clinical Nutrition). Out of that whole article, the authors only listed three probiotic strains that had been found not to colonize the human intestinal tract. One was a variant of Bifidobacterium sp that could be distinguished from indigenous bifidobacteria in the fecal flora. The second was a commercially available bifidobacteria (they did not list the specific strain), and the third was Lactobacillus GG.
The one about Lactobacillus GG was interesting. If you actually read the research article the authors quoted (found at PubMed) it shows that after 7 days Lactobacillus GG was not found in 67% of the volunteers. So, what about the remaining 33% of volunteers? Assumedly they still had Lactobacillus GG present in their feces. Unfortunately, the authors of the PubMed research did not go farther than 7 days (at least not what I can see from the abstract) so we can't come to a conclusion. Also, the authors from the American Journal of Clinical Nutrition said that the same was true in premature infants in regards to Lactobacillus GG not colonizing the intestinal tract, and quoted another research article (found at Archives of Disease in Children). They clearly made an error because if you read the abstract from the Archives of Disease in Children it says "Orally administered Lactobacillus GG was well tolerated and did colonise the bowel of premature infants. However, colonisation with Lactobacillus GG did not reduce the faecal reservoir of potential pathogens and there was no evidence that colonisation had any positive clinical benefit for this particular group of infants." However, there was no information as to how long the colonization lasted, so again, we can't come to any conclusion.
Next you gave me a link to some other website (http://whyfiles.org/302gut_flora/index.php?g=3.txt). It's not from a research journal or from a medical or science publication, so i'm not gonna debate it.
To Cheshire77: You offered no links to back up your opinion. You asked for me to provide you links that show probiotic colonization after 2 months. I'm afraid I haven't found any current studies that have looked at colonization at those lengths of time. But here are some links of in vivo studies showing certain strains of probiotics and their ability to colonize the intestinal tract for shorter periods of time.
"During the posttreatment period, the number of CFU detected decreased as a function of time after Lcr35 administration was discontinued (median, 4.8 log10CFU/g; range, 3.5 to 6.8 log10 CFU/g at day 14, i.e., 7 days after the last intake). After a 3-week period without any intake, hybridization-positive CFU were still detected in the subjects' feces, at levels similar to those observed at the end of period 1. This suggests that the proliferation of Lactobacillus populations induced by the oral absorption of Lcr35 had a prolonged effect on the level of the bacteria within the GI tracts of the volunteers. During test period 2, the number increased, but not to a significant degree, and the levels of CFU per gram of feces remained high (Fig. (Fig.22).