Of the estimated 400 species of bacteria living within the intestinal tract, the most important to human health belong to two genera: lactobacillus and bifidobacteria. Lactobacillus organisms reside mainly in the small intestine, bifidobacteria in the large. The lactobacillus genus contains approximately 60 species, including such organisms as L. acidophilus, L. plantarum, L. casei and L. rhamnosus. Members of the bifidobacteria genus include B. longum, B. bifidum and B. infantis. Many lactobacillus species used to be classified simply as L. acidophilus, thus "acidophilus" has become almost synonymous with probiotic organisms. Similarly, bifidobacteria used to be collectively referred to as Lactobacillus bifidus, but with improved microbiological differentiation, they have since been assigned their own genus. Probiotic organisms can also be found in several other genera, including enterococcus, bacillus and even streptococcus.