A probiotic is defined as ‘a preparation of or a product containing viable, defined micro-organisms in sufficient numbers, which alter the micorbiota (by implantation or colonisation) in a compartment of the host and by the exert beneficial health effects in this host.’

Havenaar R, Huis In’t Veld MJH. 1992. Probiotics: a general overview. Lactic acid Bacteria in Health an Disease, Vol. 1, (ed. BJB Wood, pp. 151-170, Elsevier Applied Science Publishers, Amsterdam.

or

Probiotics are defined as ‘microbial cell preparations or components of microbial cells that have a beneficial effect on the health and well-being of the host.’

Salminen S, Bouley C, Boutron-Ruault MC, Cummings JH, Franck A, Gibson GR, Isolauri E, Moreau MC, Roberfroid MB, Rowland I. 1988. Functional food science and gastrointestinal physiology and function.British Journal of Nutrition 80, Suppl. 1, S147-S171.