small size of the individual cells. These are not over 0.5 micron wide and generally not longer than 2.5 microns. The most common shape consists of a complete turn of about 1 by 1.5 microns. In media with fatty acids as a substrate the spirals appear somewhat steeper than in fumarate, succinate or malate cultures. Swollen individuals resembling vibrios are encountered in cultures which do not appear quite healthy. Formation of mucus or clumping has not been observed.
Gelatin is not liquefied; aspartate has been the only amino acid capable of inducing growth. Thiosulfate is not oxidized.
Color: Quite distinct from that of Rhodospirillum rubrum; colonies and stab cultures are a reddish brown while liquid cultures often appear brownish orange. The color is due to bacteriochlorophyll and carotenoid pigments; among the latter spirilloxanthin, as evidenced by the absence of an absorption maximum at 550 millimicrons, is not represented as a major constituent. Does not produce water-soluble, diffusible pigments.
Capable of strictly anaerobic development in illuminated cultures, due to photosynthetic metabolism.
Fatty acids and the four-carbon dicarboxylic acids are uniformly good substrates; glutarate is not used. Ethanol and glucose, in a concentration of 0.2 per cent, have yielded satisfactory cultures; other carbohydrates, as well as the corresponding polyalcohols, have given negative results.
Little information available concerning pH and temperature relations. Behaves generally as a strict anaerobe; adaptation to microaerophilic conditions has not been achieved. Negative aerotaxis very pronounced.
Distinctive characters: The small size and the color of the cultures serve as adequate criteria for its differentiation from Rhodospirillum rubrum. The strictly anaerobic nature and the failure to grow with glutarate and various amino acids except aspartate can probably be used as supplementary specific properties.
Habitat: Bodies of stagnant water and mud.
Illustrations: Van Niel, ibid., fig. 97-102, p. 109; Giesberger, Jour. Microb. and Serol., 13, 1947, fig. 1-2, p. 141.
3. Rhodospirillum molischianum Giesberger, 1947. (Jour. Microbiol, and Serol., 13, 1947, 137.)
mo.li.schi.an′um. M.L. adj. molischianus pertaining to Molisch.
Cells characteristically spiral-shaped, moderately large, 0.7 to 0.9 by 5 to 10 microns. Mostly with one or two complete turns which vary in width from 1.3 to 2 microns and in length from 4 to 6 microns; this depends upon environmental conditions.
Color: Distinctly reddish brown due to the presence of bacteriochlorophyll and carotenoids, the latter responsible for the absorption maxima at 520 and 485 millimicrons. Spirilloxanthin absent.
Capable of development under strictly anaerobic conditions in media containing ethanol or fatty- or hydroxy-acids as oxidizable substrates. Citrate can also be utilized in this manner, but not glycerol, glucose, hydrogen sulfide or thiosulfates. Tends to be strictly anaerobic, and hence capable of development only in illuminated cultures.
Does not liquefy gelatin.
Distinctive characters: The absence of an absorption band at 550 millimicrons and the ability to utilize citrate serve to distinguish this type from Rhodospirillum rubrum, which it closely resembles in size and shape. The individual cell size differentiates this species from R. fulvum and R. photometricum.
Habitat: Widely distributed. Regularly present in stagnant water and mud; can be found in abundance in anaerobic cultures of hay extract inoculated with such materials and incubated in light.
Illustrations: Giesberger, ibid., fig. 3-5, p. 141.
4. Rhodospirillum photometricum Molisch, 1907. (Die Purpurbakterien, Jena, 1907, 24.)
pho.to.me′tri.cum. Gr. noun phos light; Gr. adj. metricus measured; M.L. adj. photometricus light-measured.
Cells large, stout, spiral-shaped. Mostly with one or two complete turns whose wave
