simple: practically all, if not all, of the species require pantothenic acid, and the majority require biotin; a few species require either thiamine or para-aminobenzoic acid in addition. Optimum temperature, 30° C. Found in dairy products, especially hard cheeses. The type species is Propionibacterhim freudenreichn van Niel. Key to the species of genus Propionibacterium. I. Little surface growth on yeast-agar-lactate stab. Short chains of small, spherical cells occur in yeast e.xtract-glucose media. Sucrose and maltose not fermented. A. Lactose not fermented. L Propionibacterium freudenreichii. B. Lactose fermented. 2. Propionibacterium shermanii. n. Distinct surface growth on yeast-agar-lactate stab. Growth in yeast e.xtract-glucose media does not occur in the form of small, spherical cells in short chains. Sucrose and maltose are fermented. A. Typical short rods of diphtheroid appearance are produced in yeast extract-glucose media. 1. Brownish red growth in yeast-agar-lactate stab. a. Raffinose and mannitol, but not sorbitol, are fermented. 3. Propionibacterium rubrum. aa. Sorbitol, but not raffinose or mannitol, is fermented. 4. Propionibacterium thoenii. 2. Cream-colored growth in yeast-agar-lactate stab. a. Cream-colored surface growth. 5. Propionibacterium zeae. aa. Yellow to orange surface growth. b. Dextrin, glycogen and starch fermented. 6. Propio7iibacterium technicum. bb. Dextrin, glycogen and starch not fermented. c. Cellobiose fermented. 7. Propionibacterium raffinosaceum. cc. Cellobiose not fermented. d. Growth in liquid media flocculent, as if agglutinated. Acid from salicin. 8. Propionibacterium peter ssonii. dd. Growth in liquid media dispersed and smooth. No acid from salicin. 9. Propionibacterium jensenii. B. Growth in yeast extract-glucose media occurs as highly irregular cells which give the appearance of involution forms. 1. Involution forms occur as large, swollen spheres. Surface growth on yeast-agar- lactate stab is orange-yellow. Xylose and rhamnose not fermented. 10. Propionibacterium arabinosum. Involution forms occur as long, irregular rods. Surface growth on yeast-agar- lactate stab is cream-colored. Xylose and rhamnose are fermented. 11. Propionibacterium pentosaceum. 1. Propionibacterium freudenreichii The Propionic Acid Bacteria, Haarlem, van Niel, 1928. {Bacteriiim acidi propionici 1928, 162.) a, von Freudenreich and Orla-Jensen, Cent. freu.den.reich'i.i. M.L. gen. noun freu- f. Bakt., II Abt., 17, 1906, 532; van Niel, denreichii of Freudenreich; named for Ed-
