**1. Introduction**

324 Atopic Dermatitis – Disease Etiology and Clinical Management

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The term "probiotic" was only coined starting from 1953 and means "for life" ("pro = for" and "bios = life") as opposed to "antibiotics". Yet, the related concept dates back to the early 1900s when Tissier and Metchnikoff pioneered the notion that not all bacteria are detrimental and that some may even be ingested for benefit of health and longevity (Tissier, 1906; Metchnikoff, 1908). Since then, over 9000 articles and 700 clinical trials have been devoted to document and unravel the beneficial effects attributed to probiotic strains and the mechanisms of action that may be involved. The health benefits attributed to probiotics are numerous (Nomoto, 2005; Nova et al., 2007; O'Hara and Shanahan, 2007; Parvez et al., 2006; Salminen et al., 2005; Sanders, 2008; Santosa et al., 2006) but the level of proof supporting them is highly variable depending on the benefit and more importantly on the studied strain. In view of the large diversity and the high number of probiotic candidate strains it is important to stress that most observations are strictly strain specific which means that data obtained on a given strain may not be extrapolated to all strains belonging to the corresponding bacterial species and genus. This renders a global analysis of the field quite difficult and general conclusions often lack accuracy.
