One of the major problems that oral probiotics face is how to ensure survival of the microbe during the passage from the mouth to the colon. Indeed, microbial strains used as probiotics must be both acid- and bile-resistant. Whether it is necessary for probiotic bacteria to have the ability to colonize the colon for long term survival is not known. For example, common commercial strains such as Lactobacillus bulgaricus and Lactobacillus acidophilus are not adhesive in humans. However, convincing mucosal adhesiveness has been shown for Lactobacillus plantarum strains 299 and 299V, Lactobacillus rhamnosus strains GG and 271, and recently, L acidophilus strain LA1, Lactobacillus salivarius, and Bifidobacterium longum infantis. It must be remembered that when using preparations of microbes that are unable to adhere to the colonic mucosa, continuous consumption is necessary to maintain any beneficial effects. However, even strains such as L rhamnosus, which does adhere to mucosa, gradually disappear by approximately two weeks after the end of administration of the bacteria. However, it must also be noted that adherent probiotics can persist on colonic mucosa even after the strain is no longer detectable in fecal samples. Thus, testing for the presence of a probiotic by testing fecal samples alone may severely underestimate the levels present within the colon.