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<p><strong>Probiotics</strong> are dietary supplements of live microorganisms thought to be healthy for the host organism. According to the currently adopted definition by FAO/WHO, probiotics are: &quot;Live microorganisms which when administered in adequate amounts confer a health benefit on the host&quot;.<sup id="cite_ref-FAO.2FWHO_0-0" class="reference"><font size="2"><span>[</span>1<span>]</span></font></sup> Lactic acid bacteria (LAB) and bifidobacteria are the most common types of microbes used as probiotics; but also certain yeasts and bacilli are available.</p>

<p>At first, probiotics were thought to beneficially affect the host by improving its intestinal microbial balance, thus inhibiting pathogens and toxin producing bacteria. Today specific health effects are being investigated and documented including alleviation of chronic intestinal inflammatory diseases <sup id="cite_ref-Mach_1-0" class="reference"><font size="2"><span>[</span>2<span>]</span></font></sup>, prevention and treatment of pathogen-induced diarrhea <sup id="cite_ref-Yan_2-0" class="reference"><font size="2"><span>[</span>3<span>]</span></font></sup>, urogenital infections <sup id="cite_ref-Reid_3-0" class="reference"><font size="2"><span>[</span>4<span>]</span></font></sup>, and atopic diseases<sup id="cite_ref-Vanderhoof_4-0" class="reference"><font size="2"><span>[</span>5<span>]</span></font></sup>.</p>

<h2><span id="History_of_probiotics" class="mw-headline">History of probiotics</span></h2>

<p>Probiotics, which means, &quot;for life,&quot; have been used for centuries as natural components in health-promoting foods.<sup style="WHITE-SPACE: nowrap" class="noprint Template-Fact" title="This claim needs references to reliable sources from January 2009"><font size="2">[<em>citation needed</em>]</font></sup> The original observation of the positive role played by certain bacteria was first introduced by Russian scientist and Nobel laureate Eli Metchnikoff, who in the beginning of the 20th century suggested that it would be possible to modify the gut flora and to replace harmful microbes by useful microbes.<sup id="cite_ref-Metchnikoff_5-0" class="reference"><font size="2"><span>[</span>6<span>]</span></font></sup> Metchnikoff, at that time a professor at the Pasteur Institute in Paris, produced the notion that the aging process results from the activity of putrefactive (proteolytic) microbes producing toxic substances in the large bowel. Proteolytic bacteria such as clostridia, which are part of the normal gut flora, produce toxic substances including phenols, indols and ammonia from the digestion of proteins. According to Metchnikoff these compounds were responsible for what he called &quot;intestinal auto-intoxication&quot;, which caused the physical changes associated with old age. It was at that time known that milk fermented with lactic-acid bacteria inhibits the growth of proteolytic bacteria because of the low pH produced by the fermentation of lactose. Metchnikoff had also observed that certain rural populations in Europe, for example in Bulgaria and the Russian Steppes who lived largely on milk fermented by lactic-acid bacteria were exceptionally long lived. Based on these facts, Metchnikoff proposed that consumption of fermented milk would &quot;seed&quot; the intestine with harmless lactic-acid bacteria and decrease the intestinal pH and that this would suppress the growth of proteolytic bacteria. Metchnikoff himself introduced in his diet sour milk fermented with the bacteria he called &quot;Bulgarian Bacillus&quot; and found his health benefited. Friends in Paris soon followed his example and physicians began prescribing the sour milk diet for their patients.<sup id="cite_ref-Vaughan_6-0" class="reference"><font size="2"><span>[</span>7<span>]</span></font></sup></p>

<h2><span id="External_links" class="mw-headline">External links</span></h2>

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