In his book, the editor, Roy Fuller stresses the need for a probiotic to be viable when he redefines probiotics in 1989 as "A live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance." Not all probiotic products in the market are viable by the time they reach the shelves, and thus do not have the same potency and desired effects.

Fuller goes on to list the wide range of lactobacilli generally used in probiotics , "Currently available probiotic preparations contain L. delbreuckii subsp. bugaricus, L. acidophilus, L. casei, L. fermentum. L. plantarum, L. brevis, L. cellobiosus, L. lactis and L. reuteri."

In 1905, Tissier showed that bifidobacteria were the dominant bacteria in the gut of brest-fed infants, and from his work stems the use of bifidobacteria used as probiotics, which are, "Bifidobacterium adolescentis, Bif. animalis, Bif. bifidum, Bif. infantis, Bif. longum, and Bif. thermophilum."

In sour milk and yogurts we find the first use of streptococci as probiotics, with the yogurt starter S. salivarius subsp. thermophilus currently being a common probiotic organism.