ABSTRACT
Bifidobacteria were first characterized by their unusual shape. They were originally described in 1899 by Tissier, as Gram-positive, curved or bifid, rod-shaped bacteria, which varied in shape when growing in different media. The size of bifidobacteria, like other bacteria, is dependent on growth rate. Bacteria from the log phase of growth may be both larger and more variable in size than those in the experimental stationary phase. The genus Bifidobacterium has a pleomorphic cellular morphology that is dependent not only on the individual species, but on the media in which it is grown. B. bifidum is a Gram-positive, curved rod with a bifurcated end, giving a characteristic Y shape when present in the stool of the breast-fed infant. The cytoplasmic membrane of bifidobacteria conforms to the model of other cell membranes, a fluid phospholipid bilayer with globular protein molecules embedded in it.
