Live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.
This is the definition that was proposed by a panel of experts convened by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) in 2001.
The term, however, has an extensive history going back to 1965 when Lilly and Stillwell published a paper where they defined probiotics as “growth promoting factors produced by microorganisms”. This definition refers only to substances produced by one microbe that promote the growth of others. A definition including live microbes came later by Parker, defining probiotics as “organisms and substances which contribute to intestinal microbial balance”. Since then, multiple definitions have been proposed to cover or underline specific characteristics of probiotics, including those covering beneficial effects beyond the intestinal microbiota or even those that covered dead bacteria.
The FAO/WHO definition has been widely accepted since it was proposed and fits well with our current concept of probiotics.
One important consideration to take into account is that probiotics are strain-specific. This means that a particular strain in one species is a probiotic if it meets the definition: it provides a health benefit to the host. The beneficial effect needs to be scientifically proven, but it cannot be extrapolated to other strains. So, while a particular strain will be a probiotic, other strains in the same species won’t.