466 APPENDIX 



latter determines the color of the cultures, which range from purplish 

 to numerous shades of red and brown. 



The four currently accepted species, Rps. palustris, Rps. sphe- 

 roides, Rps. capsulatus, and Rps. gelatinosa, are distinguished on the 

 basis of minor morphological and physiological differences, including 

 specific B-vitamin requirements. 



All strains thus far examined can grow anaerobically in the light 

 as well as aerobically in darkness; in dark-grown cultures the pro- 

 duction of pigments is inhibited. The presence of oxygen has little or 

 no effect on growth in light, except in the case of certain carotenoidless 

 mutants, which are blue-green in color. These are rapidly killed, by a 

 bacteriochlorophyll-sensitized photoxidation, when they are exposed 

 to light and oxygen simultaneously (17), 



Rhodospirillutn. 



The spiral-shaped nonsulfur purple bacteria are classified as 

 Rho do spirillum, with the species R. nibrum, R. fidvutn, R. molischi- 

 anum, and R. photometriciim. Species differentiation rests on cell size 

 and pigmentation. 



The species of Rhodospirillum are distinct from those of Thio- 

 spirillum primarily by the absence of sulfur globules in the cells of 

 the former, even if they are exposed to H2S, 



R. rubrum has a specific requirement for biotin as the only vitamin; 

 the other species also require growth factors, but it has not yet been 

 determined what they are. 



Like the Rhodopseudomonas species, R. rubrum can grow both 

 anaerobically in the light and aerobically in darkness. But the other 

 three Rhodospirillum species appear to be strict anaerobes, and hence 

 can be cultivated only in the light. 



Rhodo microbium . 



Some years ago, Duchow and Douglas (18) described a morphologi- 

 cally unique species of purple bacteria. Because its mode of develop- 

 ment bears a strong resemblance to that of the colorless Hypho- 

 microbium , it was given the name of Rhodomicrobium .^ 



It grows in the form of branched colonies, in which the pear-shaped 

 cells are connected by slender tubes. These are produced by the pear- 

 shaped cells in the course of their development, and represent a sort 

 of germ tube, at the end of which a bud is formed. The latter grows 



1 In the seventh edition of Bergey's Manual of Determinative Bacteriology (1957) 

 this genus is not included among the Athiorhodaceae (p. 53-61), which, as 

 polarly flagellated bacteria that multiply by fission, are placed in the order 

 Pseudomonadales. Instead, because of its unusual morphology, it is grouped 

 with Hyplioii/icrohiuni in the order Hyphomicrobiales, family Hyphomicro- 

 biaceae (p. 276-78). 



