1. Rhotlospirillum rubrum (von Esmarch, 1887) Molisch, 1907. (Spirillum rubrum von Esmarch, Cent. f. Bakt., 1, 1887, 225; Rhodospirillum giganteum Molisch, Die Purpurbakterien, Jena, 1907, 24; Molisch, ibid., 25.)
rub′rum. L. adj. ruber red.
Cells characteristically spiral-shaped, but size of elements variable within wide limits, depending upon environmental conditions during growth. Width of cells from 0.5 to 1.5 microns; length from 2 to 50 microns, and over; even in a single culture such differences may be found. Also the shape and size of the spiral coil varies much; it usually ranges between 1 to 4 microns in width, and from 1.5 to 7 microns in length. In alanine media the majority of the cells occur in the form of half-circles to complete rings; malate media tend to produce much flattened spirals. In old cultures involution forms appear, straightened spirals and irregularly swollen cells, the latter common in media with higher fatty acids. Such cells stain irregularly, contain fatty inclusions, and are occasionally branched. Young cultures show active motility, due to polar flagella. Gram-negative.
Mucus is not produced. In calcium-deficient media the growth is flocculent, as if agglutinated. With an adequate calcium supply the growth in liquid media is homogeneous, suspended and consists of single cells.
Gelatin is not liquefied; the amino acids alanine, asparagine, aspartic and glutamic acids are satisfactory oxidizable compounds.
Color: Ordinarily deep and dark red, without any brownish tinge. In ethanol media lighter, and a characteristic pink. Pigment production markedly influenced by oxygen and light. Slants incubated in darkness present a pale grayish surface growth with a faint reddish hue, while often showing deep-red cell masses in the region between glass wall and agar surface where development proceeds at low oxygen tension. The color is due to bacteriochlorophyll and carotenoid pigments. Among the latter, spirilloxanthin is quantitatively predominant and is responsible for the characteristic absorption band at 550 millimicrons. Water-soluble, diffusible pigments are not produced.
Development possible over a pH range of at least 6 to 8.5, although, as in other cases, the combination of an acid reaction and the presence of fatty acids may prevent growth.
Cultures produce a distinctive odor, reminiscent of slightly putrid yeast.
In general, grow well with fatty acids as the chief oxidizable substrate; however, are prevented from growing by 0.2 per cent propionate in a neutral medium. Most substituted acids are equally satisfactory, with the exception of tartrate, gluconate and citrate. In a concentration of 0.2 per cent, ethanol is a suitable substrate, whereas the carbohydrates and their corresponding polyalcohols are not utilized.
Thiosulfate is not oxidized; molecular hydrogen can be used by some strains.
Rather microaerophilic; many strains, upon initial isolation, incapable of growth at atmospheric oxygen tension. Subsequent adaptation can be induced, but even such adapted cultures exhibit negative chemotaxis to air.
Capable of strictly anaerobic development in illuminated cultures on the basis of a photosynthetic metabolism.
Biotin is required for growth (Hutner).
Optimum temperature generally between 30° and 37° C.
Distinctive characters: The most important characteristics of the species are the spiral shape, combined with the ability to produce a red pigment with a definite absorption maximum at 550 millimicrons in the intact cells. Diagnostically useful are the good growth in media with 0.2 per cent ethanol, alanine, asparagine, aspartate or glutamate and the inadequacy of similar concentrations of carbohydrates and thiosulfate as substrates.
Habitat: Regularly present in stagnant bodies of water and in mud.
Illustrations: Molisch, ibid., Plate I, fig. 5-7; van Niel, Bact. Rev., 8, 1944, fig. 9-10, p. 19; fig. 11-16, p. 24; fig. 67-75, p. 103; fig. 76-84, p. 104; fig. 85-90, p. 106; fig. 91-96, p. 107.
2. Rhodospirillum fulvum van Niel, 1944. (Bact. Rev., 8, 1944, 108.)
ful′vum. L. adj. fulvus deep or reddish yellow, tawny.
Characteristic for the species is the very
