8°, 1824, 51; Coccobacterium Schmidt aud Weis, Die Bakterien, 1902, 10; Erythrobacillus Fortineau, Compt. rend. Soc. Biol., Paris, 58, 1905, 104; Dicrohadrum Enderlein, Sitzber. Gesell. Naturf. Freunde, Berlin, 1917, 309; Breed and Breed, Cent. f. Bakt., II Abt., 71, 1927, 435.) Ser.ra'ti.a. M.L. fem.n. Serratia named for Serafino Serrati, the Italian physicist who invented a steam boat at Florence before 1787. Small, peritrichous rods. Gram-negative. Produce characteristic red pigments; white to rose-red strains that lack brilliant colors are common. Gelatin is rapidly liquefied. Milk is coagulated and digested. Typical species produce CO2 and frequently H2 from glucose and other sugars; acetic, formic, succinic and lactic acids, acetylmethylcarbinol and 2,3- butylene glycol are also produced. Coagulated blood serum is liquefied. Nitrates are re- duced. Aerobic. Saprophytic on decaying plant or even animal materials. A characteristic feature of the species belonging to Serratia is the variability of their pigmentation. Heavily pigmented to white cultures probably occur in all species. In some cultures of Serratia marcescens Bizio, white strains arise so readily that growth on agar slants may appear as a mosaic of white and orange-red. Colonies are frequently white as they first appear, usually becoming orange-red later. In the work done by Breed and Breed (loc. cit.) it was found that pigment developed better on the Bacto-Peptone, Liebig meat- extract media then generally used than on new, improved media containing casein digest, yeast extract and similar growth-promoting substances. Recently Earned (personal com- munication, 1955) found mannitol- and sorbitol-peptone media to give profuse pigmentation while no pigment is produced on improved peptone media containing glucose. Prodigiosin dissolved in alcohol is red in acid and orange in alkaline solutions. Old cultures on agar slants become a deep magenta-red. The red pigment found in the various species of Serratia has been named prodigiosin. A series of chemical studies (see Wrede and Rothhaas, Ztschr. Physiol. Chem., S15, 1933, 67; ibid., 222, 1934, 203; and ibii., 226, 1934, 95 for a bibliography) defined the structure of this compound as a tripj^rrl methene. This pigment has been shown by a number of inves- tigators to be active in vitro against a number of pathogenic protozoa and fungi. It is even indicated by clinical studies (Weir, Egeberg, Lack and Leiby, Amer. Jour. Med. Sci., 22^, 1952, 70) that this compound has some promise as a therapeutic agent in the treatment of disseminated coccidioidomycosis, and methods have been devised for securing appreciable quantities of this pigment by submerged culture (Harned, Applied Microbiol., 2, 1954, 365). There is, however, indication that prodigiosin is not a single pigment as there is much variation in the amount and type of pigment produced in various cultures belonging to the same species. Some cultures are heavily pigmented, and in these cases there is some diffusion of pigment through the agar indicating some water solubility. Other heavily pigmented strains quite regularly show a fuchsin-like sheen on the surface, a character not found in the case of other cultures. Other cultures never produce the common orange- red pigment that turns to a magenta-red in old cultures. These less heavily pigmented strains maintain a constant rose-red color while still other strains produce no chromogene- sis whatever. Some of the non-chromogenic strains that are known to have been derived from chromogenic strains have failed to produce color on any medium or condition of growth that has been tried. It has been noted that when the color of normally pigmented strains is extracted in alcohol, the residual cells still show a rose-red color suggesting that there is a rose-red pigment present that is not soluble in alcohol (Breed). The type species is Serratia marcescens Bizio. Key to the species of genus Serratia. I. Pigment not especially water-soluble, readily soluble in alcohol. A. No visible gas from glucose. 1. Inconspicuous pellicle, if any, on plain gelatin. 1. Serratia marcescens.
