1. Rhodopseudonionas palustris (Molisch, 1907) van Niel, 1944. (Rhodobacillus palustris Molisch, Rhodobacterium capsulatum Molisch and Rhodovibrio parvus Molisch, Die Purpurbakterien, Jena, 1907, 14, 18 and 21; van Niel, Bact. Rev., 8, 1944, 89.)
pa.lu.s′tris. L. adj. paluster marshy, swampy.
Cells usually distinctly rod-shaped, though in young cultures very short, lightly curved rods may often predominate. Size variable, even for the same strain, and strongly influenced by age of culture and composition of medium. Rather consistently short cells in young cultures in yeast extract, especially when incubated anaerobically in the light or in anaerobic cultures with substrates, such as malonate, which permit only a slow and scant development. Dimensions in such cultures 0.6 to 0.8 by 1.2 to 2 microns. More often, especially in older cultures, cells are much longer, up to 10 microns. Highly characteristic is the pronounced tendency to the formation of irregularly shaped, bent and crooked long rods, occasionally swollen at one or both extremities, and frequently suggesting branching. Such cells usually form clusters reminiscent of Corynebacterium and Mycobacterium cultures.
Cells in young cultures actively motile by means of polar flagella; irregular and long cells as a rule non-motile. Gram-negative.
Growth in liquid media never mucoid; sediment in older cultures homogeneous and smooth, readily redispersible.
Color varies considerably, depending upon the medium, and especially in anaerobic illuminated cultures. Where development is slight (as in malonate, thiosulfate and, usually, glycerol media), the color is a light pink; in media containing fatty acids, more nearly dark reddish brown. Color due to bacteriochlorophyll and a number of different carotenoid pigments; most strains produce, in addition, a water-soluble, non-carotenoid, bluish red pigment which diffuses into the culture medium.
In yeast extract cultures growth is possible over the range pH 6 to 8.5. With certain substrates, especially fatty acids, the combined effect of low pH and a substrate concentration of 0.1 to 0.2 per cent may prevent growth. No characteristic odors save that old cultures may develop a distinct ionone-like fragrance. Gelatin is not liquefied; leucine is generally utilized as a substrate.
Most strains are able to grow on the surface of agar plates or slants; a few, especially when first isolated, appear more sensitive to oxygen and develop only in stabs in which the upper region may remain free of growth. Generally such strains can be adapted to grow at full atmospheric oxygen tension.
Most fatty acids and hydroxy acids are adequate oxidation substrates. All cultures can grow at the expense of thiosulfate and produce rapid and profuse growth in glutarate and ethanol media. No development in media containing, as the chief oxidation substrate, 0.2 per cent sorbitol, glucose or mannose, even though these substances are not inhibitory. Molecular hydrogen can be oxidized.
All cultures can develop anaerobically in illuminated cultures by photosynthesis.
p-amino-benzoic acid is required for growth (Hutner).
Optimum temperature generally rather high, good development being possible up to 37° C. However, certain strains exhibit a lower optimum temperature.
Distinctive characters: Morphological resemblance to species of Mycobacterium in old cultures, ability to grow with thiosulfate as the chief oxidizable substrate, and failure to develop in media which contain carbohydrates or sugar alcohols in a concentration of 0.2 per cent as the main oxidizable compounds.
Habitat: Regularly found in mud and stagnant bodies of water.
Illustrations: Molisch, op. cit., 1907, Plate I, fig. 1, 2; Plate II, fig. 10; van Niel, op. cit., 1944, fig. 1-3, p. 18, and fig. 18-26, p. 90.
2. Rhodopseudomonas gelatinosa (Molisch, 1907) van Niel, 1944. (Rhodocystis gelatinosa Molisch, Die Purpurbakterien, Jena, 1907, 22; van Niel, Bact. Rev., 8, 1944, 98.)
ge.la.ti.no′sa. L. part. adj. gelatus frozen, stiffened; M.L. gelatinum gelatin, that which
