Safety of probiotics: comparison of two popular strains

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Abstract
Background The human intestine is home to more than a trillion live bacteria from about 400 species. The average adult body contains about 20 times more bacteria than it does cells.1 In the natural environment a delicate symbiosis evolves between these endogenous bacteria and their host. The vital contribution of natural flora to normal intestinal development is underscored by studies of animals raised in a germ-free environment. These animals have hypoplastic intestinal epithelia, reduced gut immunity, and impaired peristalsis—all of which improve only when normal gut flora are introduced.2 Exogenous probiotics are given therapeutically in situations where this naturally beneficial symbiosis has been disturbed, in an attempt to restore normal flora, as in the above animal model. A 2004 Cochrane review reported probiotic therapy to be effective in reducing diarrhoea,3 and a recent report summarising 185 studies found that probiotics successfully treated 68 different conditions in widely diverse populations.4 Sources and selection criteria This review is based on the conclusions of recent randomised clinical trials, Cochrane controlled trials, and European Society of Paediatric Gastroenterology, Hepatitis, and Nutrition reviews of probiotic therapy. We also searched Medline using the keywords “probiotics”, “Lactobacillus”, “Bifidobacterium”, “sepsis”, and “bacteremia”. We reviewed prospective randomised trials of probiotic therapy, looking for reports of intercurrent sepsis, as well as case reports of potentially probiotic associated infections not related to ongoing studies and retrospective population based reviews. We gave preference to articles published after 1995.